Changes to AAS Governance

The American Astronomical Society is about to change its governance structure for the first time in 50 years in response to a task force report on the topic. Here is the task force chair’s description of the process, and here are the executive officer’s thoughts.

Here is a summary of the changes (from the executive summary):

To improve responsiveness and assure timely action on important matters, and to move toward a model consistent with best practice, we replace the AAS Council with a Board of eleven members that meets monthly. To assure effective communication between this Board and the entities in which our members work to advance the interests of the Society and our profession — our Committees and Divisions — we provide a Board liaison for each. Most importantly, each Committee and Division Chair is scheduled to attend from two to four Board meetings per year in which their issues are pre-ordained agenda items.

To enhance inclusivity in the governance of the Society, Committee members and chairs will no longer be appointed by the Board (nee’ Council); members will be derived from volunteers and selected by the existing committees, and the committee members will, in most cases, elect their own Chairs.

To further involve a broader community in setting the strategic directions of the Society, the AAS Council will be reconstituted as a body — the Strategic Assembly — including the Board and the Committee Chairs of the eleven Standing Committees as well as Division representation. The Assembly will meet twice a year at the Society’s scientific meetings 1) to foster collaboration among committees and with the Board, 2) to improve communication, and 3) to guide the strategic thinking of the Society.

The many changes, large and small, outlined in the attached document have been carefully designed to promote inclusivity, foster communication, embrace creativity, and maximize transparency — in short, to enhance the functioning of the Society so that it will be the welcoming and natural home for the many people who, in a variety of different capacities throughout their careers, work to enhance and share humanity’s scientific understanding of the Universe.

Full details in the task force report.

The AAS would like feedback from its members specific to the drafting of the bylaws to be sent to the task force chair, David Helfand, and general comments can be emailed to AAS President Christine Jones.

Comments can also be sent anonymously here.  Comments will be appreciated before May 12.

There will be a town hall about these governance changes on 7 June at 12:45pm at the summer AAS meeting

When to Violate Confidentiality of Peer Review

Peer-review is an important part of scientific communication. It is not a panacea and is quite fallible, but it serves several functions well:

  1. It gives journals a “filter” against unprofessional work (including crankery and its cousins)
  2. It gives authors and editors the opportunity to receive unfiltered feedback on how a paper will be received. The “unfiltered” part is ensured in part by the anonymity of the referee (which the referee can waive).
  3. It provides a fresh set of eyes from an expert who can help spot errors and mistakes
  4. Ideally, it gives authors constructive feedback to improve their work
  5. It does all of this confidentially, so when science is finally presented to the world it has already been vetted and mistakes have been fixed (this step is optional for authors—they can post preprints whenever they like).

Refereeing papers is an unpaid service to the community that scientists provide. I think of it like jury duty: I rely on good referees to fairly judge and improve my papers, and so I try to write at least as many referee reports as I receive, and be the sort of referee I hope I’ll get on my papers (I have to referee more than I am refereed because some scientists are very bad referees (slow, delinquent, unduly harsh, sloppy, etc.) and the rest of us need to pick up their slack.

The confidentiality of the process is important to the journals, and to some authors and referees. For the authors, this means they do not need to worry that their work will be scooped, that their mistakes have a chance to be fixed before they are made public, and that they have a chance to respond to any unfair criticisms in a report. For journals, this confidential review process is one of the primary values they add to papers (some of the others being editing, typesetting, publicity, and curation). For referees, it means they can frankly help improve the literature without being concerned about the reaction of the paper’s authors.

There are other ways to do this; there are good arguments for entirely open peer review, as well.

One interesting issue is whether the referee should be bound by confidentiality. There are good reasons why a referee might feel they should publish their review. This is especially true in instances where there is evidence of fraud, for instance. The case is made here:

The argument is that there can be good reasons to publish your report (as open peer review shows; read the article), and because the referee holds the copyright to the review.

But in the AAS astronomy journals, confidentiality of referees report is explicitly part of the journal ethics statement that binds referees and authors. Referees thus promise to keep the reports confidential.

Of course, this could lead to an ethical dilemma if publishing the referees report would improve science. But I think there are many perfectly ethical routes one can take to avoid that dilemma:

  1. Polish the relevant parts of your referee’s report, make sure it only refers to the published paper, and not the (strictly confidential) manuscript, and publish that on the arXiv as a “comment”.
  2. If the content of the manuscript needs to be mentioned, go directly to the editor. The purpose of confidentiality is to preserve scientific integrity; the editor has the authority to waive confidentiality, or take other actions to rectify the problem.

For instance, a climate science journal Environmental Science Letters rejected a paper by a climate skeptic, Lennart Bengtsson, on the basis of two very unfavorable referees reports that heavily criticized the science. The reports even offered suggestions for how to improve the paper and make it publishable.

Prof. Bengtsson cherry-picked a few words from one of the reports and (violating confidentiality) complained to the media that the paper had been rejected because of its conclusions for political reasons.

The journal, in this case published a rebuttal that included the entire referees reports, exposing the mendacity of Prof. Bengtsson’s accusations. Review confidentiality is at the discretion of the editors, so they can (and should) waive it when it is not serving its purpose (preserving scientific integrity).

If, for some reason, the editors were to enforce the confidentiality to the detriment of scientific integrity, then a referee may need to violate the rules and publish their review.  In this case, I recommend checking (confidentially) with others familiar with the issues, especially an ethicist, to make sure you are in the right.  After you post your report, the journals may complain, and you may face repercussions from your professional society, but if on balance you are behaving ethically you will have a good defense.

Astronomy and “Meta-Astronomy”: An Allegory

My blog is about astronomy and “meta-astronomy.” By the latter I mean the stuff that isn’t strictly astronomy research but is necessary for, or relevant to, its practice. I think both astronomy and meta-astronomy are appropriate topics for journals, talks, conferences, blogs, and research, especially since the line between them is not sharp. Here is an allegory I thought of this morning to illustrate this point.

This story is pure fiction. The allegory is purposeful, but literal similarities to real people, events, and fields of study—while perhaps not entirely coincidental—are not intentional or relevant.

Joe is an exoplanet astronomer at Research Center. Like many exoplanetary astronomers, his PhD thesis was on planetary science, which gave him a firm foundation for studying exoplanets, their composition, internal structure, and their potential habitability.

He goes to an internal departmental lunch talk by Diamond. Diamond is another exoplanet astronomer at Research Center. Like many exoplanetary astronomers, Diamond’s PhD thesis was in stellar astronomy. Her talk is about the complications of deriving exoplanetary properties from observations: starspots, convection models, mixing length assumptions, and photometric errors.

On the way back to his office, Joe and another colleague with a planetary science background, Jack, banter about how it feels to attend these “stars” talks: they agree that they appreciate stellar astronomy is important for their work, and that they should know that stuff better than they do, but they’re glad to get back to their offices so they can focus on their part of the science.

Joe is organizing a conference on exoplanets focusing on exoplanets’ composition, internal structure, and habitability. It’s going to be an important meeting where exciting new results will be presented and discussed for the first time, and he wants the best exoplanet astronomers to attend so it will be maximally successful.

He heads over to Diamond’s office and asks if she will please attend, since she is one of the best exoplanet scientists. She points out that all of the planned sessions titles contain geology jargon, and there don’t seem to be any talks about stars. Perhaps she could recommend some speakers for a panel on that?

Joe explains that the success of the conference will depend on it staying focused on its primary topic, and he doesn’t want to “dilute” the science with “stars stuff.” She points out that “stars stuff” is actually very relevant to exoplanets. Joe hastily agrees, but reemphasizes that he wants *this* conference to stay focused on geophysical aspects of planets. Will she please attend?

Diamond points out that she has limited time and can’t accept every conference invitation. She says it looks like Joe is giving only lip service to the relevance of stellar astronomy to his work, and that his actions indicate he doesn’t really think it’s very important at all. She points out that the “focus” of his conference excludes much of what she spends much of her professional time thinking about. Disappointed, Joe says thanks and goes back to his office.

He is annoyed. He would like to work more closely with Diamond, since they are both exoplanet astronomers in the same department, and they generally get along well, but it seems like she wants to drag stars into every discussion. He wishes she could compartmentalize better.

Diamond is annoyed. Joe professes to want to work with her, but can’t seem to appreciate that stars are integral to her work. She finds many planetary scientists are like that: they seem to imagine that they can divorce planets from the stars they orbit. Of course, that’s true in a purely theoretical, pedantic sense: many planetary interiors are more-or-less insensitive to the most of the precise properties of the star they orbit. But in practice you can’t measure anything about those planets without observing the stars and knowing their properties. She finds Joe’s myopia on this point frustrating.

Back in his office, Joe is glad to be able to get back to work on his geophysics. He looks at his panels: his science organizing committee has done a good job of getting many of the best speakers and presenters, even though many people had to decline.

Diamond looks at the preliminary program of Joe’s conference. All of the panel speakers come from a planetary science background. She finds a single stellar astronomer on the registration list. She thinks about going to Joe’s office to point this out, but she’s had this conversation with him before. He’ll say that he tried to get stellar astronomers to attend—he even tried to get her to attend! But, he’ll say, they all declined, so what does she want him to do?

I hope my astronomer friends will not be like Joe. When a fellow astronomer tells you a subject you find to be “meta” is important to their work and needs to be part of your science, your conference, or your work, accept that, embrace that, and act on that. Don’t nod and then go on treating it as a tangent or a distraction to your work. And when you look at the composition of panels and find homogeneity, don’t let “they all declined” be an excuse. Ask “what is it about my panel that made only certain kinds of scientists end up on it?”

Przybylski’s Star IV: Or…

Part IV of III.  Part I is here.

A coda: Howard Bond correctly points out that my three explanations are only necessary if a very plausible and less interesting explanation is wrong (a caveat that I had in an early draft of my posts but edited out unintentionally.)

The identification of short-lived actinides could be a mistake! The Gopka et al. identification of these lines was made in a journal I had not heard of, Kinematics and Physics of Celestial Bodies, apparently originally in Russian. As far as I can tell, the paper has been cited exactly once, by the Dzuba et al. paper that proposed the metastable heavy isotope.

The journal and language of the Gopka et al. paper aren’t necessarily problems, of course, but they do raise eyebrows. The fact that it has not been cited could mean that the paper was simply not read (not surprising, given the journal), or that everyone who studies the star that saw the paper decided it was not worth citing, even to refute it.

[Edit: Steinn is much better at this than I am.  He points me to a 2003 AAS abstract by Crowley et al. supporting the existence of short-lived isotopes, a topic Howard Bond also mentioned on Facebook to me. Steinn also finds this paper and this one which I think I missed because I didn’t realize that promethium, a lanthanide, has no isotopes with half-lives longer than 20 years.

The Mkrtichian paper I linked to in the last post mentions Bidelman et al. PASPC, 336, 309, as supporting the short-lived isotope interpretation, and conference proceedings by Yushchenko, Gopka, & Goriely that ADS doesn’t know. Goriely discusses mechanisms here.

So the claim is stronger than I originally hedged in this post.  It’s put best in this followup paper by Crowley it al., originally shown to be by Brian Davis (but which I only just found again, now that I’m thinking of Pm): “The spectroscopic evidence is strong enough that we would declare promethium to be present without hesitation, if any of its isotopes were stable.”  In their other words, it’s only the strong prior against finding unstable isotopes that makes them hedge.]

The mystery of Przybylski’s Star is still a very good one if there are no short-lived actinides isotopes in the spectrum—the identification of the stable lanthanides seems quite secure and fascinating and it remains the most peculiar of the peculiar A stars—but it would mean that it is much more plausible that technical but mundane explanations for the star exist.

[P.P.S. There is now a part V/III about prior art by Drake and Shklovskii.]

Przybylski’s Star III: Neutron Stars, Unbinilium, and aliens

Part I is here.

Last time I promised three solutions to the problem of short-lived actinides in the atmosphere of Przybylski’s Star.  Here they are:

1) Neutron Stars

In 2008, shortly after identifying the “impossible” elements in Przybylski’s Star, Gopka et al. proposed a solution: the star has a neutron star companion.  Neutron stars have strong winds of positrons and electrons that bombard the heavy elements in the atmosphere of the star, transmuting them to the elements we see.

The big problem with this is that these are sharp lines, so we can measure radial velocities to Przybylski’s Star, and it does not have a short period neutron star companion.  Which is great, because the last two solutions are even more fun.

2) Flerovium, Unbinilium, Unbihexium

A few days ago I saw this from William Keel on Twitter:

Following his link, I found a delightful proposal for Przybylski’s Star.

Atomic physicists have long sought to fill out the periodic table of the elements.  Since the discovery of Francium in 1939, all additions to the periodic table have come from elements synthesized through nuclear reactions.  Every few years you’ll see a news item about one of the teams around the world that has finally proven that they have produced a tiny, fleeting sample of some heavy element, by detecting its presence before it decays away in seconds (or less!).

There is reason to believe, though, that there might be longer-lived elements higher up the table, in an “island of stability” that experimenters have yet to reach.  This is a region of the Table of the Isotopes that might have unusually stable members because they contain a “magic number” of neutrons and protons.  According to Wikipedia:

Many physicists think [these isotopes’ half-lives] are relatively short, on the order of minutes or days.[2] Some theoretical calculations indicate that their half-lives may be long, on the order of 109 years.[14]

Enter Dzuba, Flambaum, and Webb, who propose that the source of the short-lived actinides in Przybylski’s Star is one of these isotopes! As the isotope decays, its daughter products—all less massive than it but still actinides—are visible in the star before they decay away. There would be some steady-state concentration dictated by the lifetime of the isotope. They propose the parent isotope could be 298Fl, 304Ubn, or 310Ubh.

If this is right then it means that we can discover a new, important isotope the old fashioned way—in nature! It would not be a first element to be found first in a star, though—helium is so named because it was first discovered in the Sun.

But where would it come from?  Dzuba et al. suggest that it might be the product of a supernova explosion, like other neutron-heavy elements. Its half life could be short enough that it would be present in a young A star but very rare on the Earth—or perhaps you need a certain kind of supernova to make it, and one of those wasn’t in the mix that generated the elements that make the Earth.  If so, it could be common in other stars and planets, but just very hard to detect in anything other than an Ap star with levitation.

Very cool!

3) Aliens

The last of the three solutions I’m aware of, whispered but never published, is that it’s the product of artificial nuclear fusion.

Here on Earth, people sometimes propose to dispose of our nuclear waste by throwing it into the Sun (in one case, literally throwing it:)

(This is a terrible idea, by the way. I mean putting nuclear material on top of giant towers filled with rocket fuel and igniting them—but Superman IV too.)

In fact, 7 years before Superman thought of the idea, Whitmire & Wright (not me, I was only 3 in 1980) proposed that alien civilizations might use their stars as depositories for their fissile waste (because of course alien civilizations would use 20th century nuclear technology for their energy needs…but I digress). They even pointed out that the most likely stars we would find such pollution in would be… A stars! (And not just any A stars, late A stars, which is what Przybylski’s Star is). In fact, back in 1966, Sagan and Shklovskii in their book Intelligent Life in the Universe proposed aliens might “salt” their stars with obviously artificial elements to attract attention.

So short lived, obviously artificial elements in A stars are in fact a prediction of artifact SETI!

This just goes to show that artifact SETI is hard. When people stick their necks out and make bold, silly-sounding predictions about unambiguous technosignatures like this (or like megastructures), I suspect they usually don’t actually expect them to come true. And then when they do come true (as in Przybylski’s Star, KIC 12557548, or Boyajian’s Star) not only are their prediction papers rarely cited (which is, I think, inappropriate), but there’s always immediately a flurry of perfectly natural explanations that arrive to explain things without aliens (which is, I think, totally appropriate).

I think the answer to SETI will ultimately come, if it comes at all, from communication SETI because the signals it seeks are pretty unambiguous, but who knows? If that narrow band microwave carrier wave is ever found and we can’t decode it, maybe some plausible natural maser emission source will be hypothesized to explain it away, too?

We should keep trying though, because even when artifact SETI finds no aliens, it finds interesting things. After all, regardless of what the solution to Przybylski’s Star is, it’s bound to be fascinating!

Anyway, that’s the end of this series. I know that Tabby’s Star is supposed to be The Most Mysterious Star in Our Galaxy, but I think Przybylski’s Star gives it a run for its money.

Edit: One more part: a caveat I had meant to include earlier but inadvertently edited out.

Przybylski’s Star II: Abundance Anomalies

Last time we talked about Ap stars in general. Now, let’s get to the really weird part.

In cool stars (hotter than type M), most of the lines by number are from the element iron. This is because of two “accidents”. The first is that due to the rules of quantum mechanics, iron’s six(!) outer (“active”) electrons have a lot of ways they can be excited. In fact, the 3d shell alone has 120 substates that form 25 distinct spectroscopic states, each with its own energy level. The second is that due to the rules of nuclear physics, iron is the end product of runaway fusion in stars, and so gets spewed out in great quantities in supernova explosions. The result is a large number of lines with relatively large strengths. In fact, until Cecilia Payne’s PhD thesis showed otherwise (Best. Thesis. Ever, by the way) this was sometimes taken as evidence that the Sun was mostly iron (it’s mostly hydrogen in fact, no matter what “iron sun guy” tells you.  Stellar astronomers know who I mean.)

We might expect, then, that most of the lines in Ap stars would also be iron, though perhaps of the ionized variety (since they’re too hot for neutral iron).  Not so; in fact for a long time Antoni Przybylski wondered if his eponymous star even had any iron; its abundance is down by a factor of at least an order of magnitude from the Sun’s.

Instead, Przybylski found lots of other elements in his weird star: strontium, lanthanum, cerium, praseodymium, neodymium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium… stuff you rarely hear mentioned outside of a Tom Lehrer song.

Now, these things should be only present in the tiniest of abundances, not the most easily seen lines in the atmosphere! What’s going on?

The answer seems to be levitation, although it appears we still don’t have a great model for it.

Here’s the idea, as I roughly understand it: The strong fields, high temperatures, and still atmospheres of Ap stars combine to create a situation where the atoms in the atmosphere, being mostly ionized, get stuck to the field lines and sort themselves out by element. This is because they can only move in one direction, along the field lines, and the strong radiation from the hot star provides a sort of upward force on the atoms, depending on the specifics of how they absorb photons (which depends on how many electrons and protons they have—that is to say, what element they are). I’m fuzzy on the details (I think maybe everyone is), but the bottom line is that the ions of a certain element get concentrated in a thin (or at least high) layer in the atmosphere.  If this layer is high enough, we see it in absorption and the element looks much more abundant than it really is (because we usually assume the atmosphere is well mixed, not stratified). And if this layer is low enough, we might not see the element at all!

So that’s apparently what puts the “p” in “Ap,” the bulk star does not have weird abundances, but its upper atmosphere does because the upper layers of the star are differentiated and stratified!

But that’s not what’s so weird about Przybylski’s star.  No, that star doesn’t just have weird abundance patterns; it has apparently impossible abundance patterns.  In 2008 Gopka et al. reported the identification of short-lived actinides in the spectrum. This means radioactive elements with half-lives of order thousands of years (or in the case of actinim, decades) are in the atmosphere. 

What?! The only way that could be true is if these products of nuclear reactions are being replenished on that timescale, which means… what exactly?  What sorts of nuclear reactions could be going on near the surface of this star?

There are three proposed solutions I’m aware of.  The first is about 8 years old; the second is brand new and a “huge if true” sort of exciting idea. The last is quite fanciful and has never, so far as I can tell, gotten past a journal referee (if anyone’s even tried to publish it), but sort of dials the “huge if true” up to 11.

More on these next time.

Przybylski’s Star I: What’s that?

OK, a new slow blogging.  This one in three parts.

Przybylski’s Star is my favorite astrophysical enigma (this coming from the guy notorious for making Tabby’s Star famous!)  It is occasionally mentioned as a SETI target, but usually in private conversations or irreverent asides on social media. I’m not sure when I first heard about it, but it may have been when looking at the Wikipedia page for stars named after people while doing research for that other star.

Przybylski’s Star is famous for having bizarre abundance patterns.  Not like: “oh look, the carbon-to-oxygen ratio is greater than one”; more like this star has more praseodymium than iron. Yeah.  How could that be?!

First off, let’s start with the basics.  Przybylski’s star is an Ap star.  That’s not “app”, that’s “Ay-Pee”, as in spectral type “A”, with a note “p” meaning “peculiar” (which is an astrophysical understatement.)

Normally, A stars are pretty boring, spectrally.  Cool stars, like the sun, a G star, have convecting envelopes (and atmospheres) like a boiling pot of water; this drives a magnetic dynamo that gives the Sun and other cool stars their magnetic fields. Hot stars (spectral types mid-F and hotter, including A stars) have radiative envelopes, meaning that most of the outer layers are very still, like the water in a bathtub left untouched for days. This means no dynamo.

Apparently, this figure from the Wikipedia page for magnetic braking of stars is supposed to illustrate how it works.

As stars form from collapsing clouds of gas, they spin up, going faster and faster.  When they finally “turn on” they can be spinning so fast they they are near break-up speed: the centrifugal force of the spinning is enough to make them oblate.  Cool stars have magnetic fields, and as the star’s ionized outer atmosphere escapes in a wind, the wind particles get stuck on these field lines and fly down them like beads on a spinning wire.  The fields, being anchored to the star’s surface, impart some of the star’s angular momentum onto the particles, and the star slows down ever so slowly. Over billions of years, the star spins down to rotation speeds of more like once per month.

But hot stars lack this field! As a result, they never spin down. The rapid rotation greatly Doppler broadens the lines: the light from the approaching limb is moving towards us very fast, so its light is blueshifted (just as the receding limb is redshifted). The result is that the “missing colors” that characterize the elements in the star’s atmosphere (the absorption lines) get smeared out a lot.  In fact, the only lines broad enough not to be smeared out beyond recognition are usually the hydrogen lines (the Balmer lines), which are particularly strong in A stars which are hot enough to excite hydrogen but not hot enough to completely ionize it.  (In fact, A stars are so named because in Williamina Fleming’s stellar classification scheme they came first, having the strongest Balmer lines.)

But Ap stars break all the rules.  They have intensely strong magnetic fields, and as a result they don’t rotate fast (presumably having slowed down long ago), and as a result they have very narrow lines, and as a result you can see all of the spectral features of the elements in their atmosphere.  Why?

I’ve never seen a good answer as to why Ap stars have strong fields.  They could be primordial or generated from a dynamo, says Wikipedia, which is fine but misses the weird part: regardless of where the field comes from, why do only a minority of A stars have such fields? What’s different about them?

And here’s the even weirder thing: the abundances of the elements that we get to see thanks to the slow rotation are way off of the abundance patterns we see elsewhere in the universe.  Why?

Next time, I’ll discuss likely answers, and then get to the weirdest member of this already weird class: Przybylski’s Star.

Oh, two last notes before moving on.  First, that name. It’s Polish, and despite what a phonetic Polish pronunciation guide might imply, it’s apparently pronounced “shi-BILL-skee”.1  Not as hard as it looks. (And not as hard as some Polish words are to say. Incidentally, first-exoplanet-discover and Penn State Professor Alexander Wolszczan’s name is pronounced “VOLSH-chan”.  Mentally replace the z’s with h’s and you’ll be fine.)

Secondly, the seminal paper on Przybylski’s star gets fewer citations than it should because, as far as I can tell, Nature misspelled his name!  You can find it here in ADS, but not if you search by author! [Edit: The folks at ADS noticed my Tweet about this, and they have fixed it! It is still wrong on Nature’s website, though.]

1Apparently the initial “P” is not completely silent, but it’s so subtle that to untrained US/UK ears it might as well be. I think it’s roughly approximated by having your lips closed when you start the “sh”: the slight plosive as your lips pop open is all it takes, as in the interjection “pshaw“. It’s not “puh-shi-BILL-skee”. Thanks Andrew Przybylski‏ (@ShuhBillSkee) and Jackie Monkiewicz (@jmonkiew) for setting me straight.

Trump and Fascism

To my friends and followers who disagree with or don’t understand the strong reactions Trump gets from across the political spectrum, I offer this:

After WWII, there emerged a liberal consensus in America that has dominated politics and journalism. By “liberal” I don’t mean “Democratic”, I mean the broader understanding that tolerance of differences among Americans was an essential component of the American fabric. Until recently, even the most conservative Republicans understood that speaking out in explicit contradiction of this consensus was political suicide. When liberals warn of the dangers of “normalizing Trump”, they aren’t just talking about his behavior and tone; they also mean his regular, even gleeful violations of this consensus.

These values were imparted to me and many of my generation (and the next) by my parents’ generation, and onto them by the American reaction to the rise and fall of fascism, especially its most evil and visible manifestation in Europe. Certainly some of this came from the inevitable vilification of our wartime enemies, but it was largely justified by the horrors revealed by the post-war occupation of Germany. Family lore is that my maternal grandfather was one of the officers that led American forces through Germany after VE day, and gave ordinary Germans tours of the concentration camps.

This video has been making the rounds, and I encourage you to watch it with this mindset. It is a propaganda video produced by the US government in 1943 in an attempt to inoculate America against the tactics the Nazis used to take control of Germany. Its parallels to recent events are eerie (so eerie I at first suspected that it was an elaborate hoax, but it appears to be genuine, having been used as part of the effort to desegregate the armed forces.)

Today’s left will find much to fault in the video (it is, after all, almost 75 years old) but I’m not offering it as a guide to how you should think. I’m offering it as a good example for the foundational values that many Americans find violated by Trump, and why the analogies to fascist Europe are not made hyperbolically. Trump’s rhetoric and actions are literally, exactly what we were raised to be on guard against.

(A)eon, era, epoch, age

Because I’m a recreational (and occasionally professional) pedant, I was curious if I was using the word “epoch” right in a recent paper. I dug around and got my answer.  For my own and others’ future reference, here’s what I found:

In general or popular writing, the terms era, epoch, and age can be used, without error, synonymously to refer to a particular span of time.

Writers who wish to respect their more precise and historical meanings or who need to respect jargon, can follow these rules:

“Eon” is the preferred spelling; “aeon” is a variant that may appear affected.

In general, an eon is a very long time, comparable to the age of the universe.

An epoch is a fixed point in time (like the zero date of a calendar, or the moment a world-changing event occurred), especially one that marks the beginning of a new era. One can “make an epoch” by doing something that changes things forever.

An era follows an epoch and is defined by it. For instance, the “Christian Era” is the time since AD 1, with Christ’s birth (roughly) making the epoch.

An age is basically an era, but seems to be a bit more general, not necessarily needing an epoch. “The Tudor Age” would then just refer to “the time when the Tudors were in charge,” while “The Victorian Era” carries the connotations of the many things that distinguished the time when Queen Victoria was alive.  This distinction might be too subtle to honor.

In geology jargon, time is divided into eons, then further divided into eras, periods, epochs, and finally stages.

In astronomy jargon, an epoch is the moment of an observation. It most commonly comes up in ephemerides, giving the moment in time that a certain object had or will have certain coordinates or orbital parameters. Not to be confused with “equinox” which specifies the Celestial coordinate system one is using.  “Age” should probably be avoided except to refer to how old something is, to avoid confusion.

And now you know.

AstroWright Science at #AAS229

It’s the “Superbowl of Astronomy” again, this time in Grapevine, Texas, at the 229th meeting of the American Astronomical Society.  You can follow the fun on social media at #AAS229.

AstroWright group members are there in force.  Here they are:

Wednesday, January 4

Jacob as an undergrad at THE Ohio State University

Jacob as an undergrad at THE Ohio State University

Poster #146.30: NSF graduate research fellow Jacob Luhn presents his work connecting flicker with jitter, a project he’s doing under the supervision of Hubble Fellow (and soon-to-be PSU faculty member) Fabienne Bastien.  Back in 2005, I published a paper trying to quantify the amount of radial velocity noise intrinsic to a star with that star’s properties: evolved stars, quickly spinning stars, and young stars all have more of this noise, which is called “jitter”.  More recently, Fabienne published one showing that photometric noise also correlates with evolution—she dubbed this noise “flicker”. Soon after, I was a co-author on one of her papers showing a good correlation between flicker and jitter.

Now, Jacob is expanding the sample way beyond the 10 stars we used (4 were upper limits!) to make this relation quantitative and explore its relationship with stellar properties.  This will allow us to predict, in advance, which Kepler stars will make good radial velocity follow-up targets.

Thursday, January 5

Kim Cartier

Kimberly M. S. Cartier a.k.a. @AstroKimCartier

Texas A 10:30am 202.04D: Kimberly M. S. Cartier (a.k.a. Astrolady, a.k.a. @AstroKimCartier) presents her ***DISSERTATION TALK!*** on the Exo-Atmosphere of WASP-103b. Kim has nominally been my student, but her primary research advisers have been Ron Gilliland(!), Ming Zhao, and Thomas Beatty. She has worked an a range of projects, the theme being exoplanets with space telescopes, with a brief digression into SETI with me. She’s done a lot of projects, so she’ll have a lot to talk about, probably sticking to the science part of her thesis. Her final science project will be using MINERVA with Tom to do precise differential colors with photometry—”chromometry” Tom’s calling it—you’ll have to go to the talk to learn more! For the other side of her thesis (this mostly with me) you’ll have to visit her poster on Friday (see below).

Posters #245.25, 26, 27: Brendan Miller, Winonah Ojanen, and Spencer Miller present their work on Swift and Chandra data studying M dwarf coronae and high-energy photons’ effects on planetary atmospheres, something I’ve been working on with Brendan for a while, now.

Friday, January 6

Texas D 2:10pm 320.02: Jason Eastman talks about our spectroscopic commissioning results with MINERVA

Kim at her poster last year

Kim at her poster last year

Poster #335.01: The other piece of Kimberly M. S. Cartier‘s thesis: “Multimedia Astronomy Communication”. Kim has been applying best practices in social media outreach and online and print journalism to astronomy research. Together, we’ve written a couple of articles (there’s more to come!) and she’s been working with Penn State’s media relations folks on press releases. Last time she presented her “meta-poster” on good poster design, and this time she’s giving a much broader overview of effective communication of astronomy research to different audiences, and in different media.

Journalists: Kim is pursuing a career in science journalism with her PhD in astrophysics. I encourage you to stop by her poster to chat!

Saturday, January 7


Dr. Bastien as a graduate student at Vanderbilt.

Texas D 10:10am 403.02: Hubble Fellow Fabienne Bastien takes her work on flicker (see Jacob’s poster above) to new domains—in particular those F-ing stars those stubborn F stars and stars observed by TESS and K2.  Those missions will measure flicker, but in a different way than Kepler did, and also target a different typical sort of star from Kepler.

She will also briefly present the recommendations of the RV community on how to reach 10 cm/s RV precision from her recent Aspen Center for Physics workshop. If you couldn’t make the workshop, be sure to stop by. Also, while you’re there, congratulate her on her new tenure-line faculty position at Penn State :)



Texas A 10:50am 401.05: In a talk in a competing session (you’ll have to hustle over after Fabienne’s talk) CEHW postdoctoral fellow Thomas Beatty discusses thermal inversions in hot Jupiters.

Have fun at the meeting!

Metzger, Shen, and Stone

The next round of WTF star papers continues.  Brian Metzger (whom I know from grad school), Ken Shen, and Nicholas Stone have submitted a paper to MNRAS exploring in detail the idea that that Boyajian’s Star is dimming secularly because it recently “ate” a companion, and it’s still processing the energy from the merger, which is slowly “dribbling” out as an excess of luminosity.   

Steinn and I explored this in our rundown of possible explanations.  It appeared as hypothesis 13 in my blog post as “post-merger return to normal” and in our paper in section 11.3.

We had two primary objetctions: it would not really explain the dips, and the timescales are all wrong.  We wrote:

One issue here is that the dimming is too fast.  When confronted with big changes in energy content or flux, stars evolve on the Kelvin-Helmholz timescale, roughly the time it takes for all of the energy in the star at a given moment to finally escape the surface (while being constantly replenished by the fusion in the star’s core). For Boyajian’s Star this timescale is about 1 million years. This means that if the entire star is processing a big change in internal energy or luminosity, it takes around 1 million years to complete the adjustment.  Changing by 15% in 100 years is therefore about 10,000 times too fast.

But, the star’s radiative envelope is not very massive, so perhaps the energy never made it deep into the star? In that case the Kelvin-Helmholz timescale is a bit shorter, so maybe we’re off by only 1,000 times. It’s an order of magnitude argument, so maybe we’re being too pessimistic by a factor of 10, so we’re only off by 100 times. It’s possible that a detailed simulation of such a merger will reveal shorter timescale events, perhaps even things that might produce the dips.

So, I’m intrigued, and I like the idea despite the timescale argument not working out. It’s possible that there are other ways to temporarily brighten a star we haven’t thought of.  I’d like to hear from people who model these things before I commit to a plausibility level, so I’ll say:

Subjective verdict: unclear.

Well, Brian Metzger and company have come through.  In their paper, they look at the same mechanism Neslušan and Budaj explored to put material on highly eccentric, “cometary” orbits around the Boyajian’s Star.  The idea is that the close companion (which is presumably bound to Boyajian’s Star) interacts with material (anything from comets, planets, brown dwarfs to other stars) and slowly perturbs it into a highly eccentric orbit.  Then, if it’s comets, it outgasses when it gets close and you get the big dusty comae that might cause the dips.

Metgter et al. invoke the same mechanism to put a heavier object on an eccentric orbit, then have that object merge with Boyajian’s Star.  They deposit the energy into the envelope of the star, then run a stellar structure and evolution code called MESA to see how the energy is processed.

Their key result is their Figure 2:

screen-shot-2017-01-03-at-8-51-53-amIt shows, on the top, the total brightness of Boyajian’s star after merging with four fiducial objects of very different sizes.  The extra energy here is coming from the object’s orbital kinetic energy, which gets dissipated as heat when the two objects merge and eventually comes out as starlight.  Bigger objects have more energy to deposit, and deposit it at different levels.

The bottom plot shows the fractional change in the star’s luminosity with time (it’s the time derivative of the top plot divided by the top plot).  Zero means the star is not changing brightness, -0.01 means that the star changes its brightness by one e-folding (a factor of 2.7) in about 1/0.01 = 100 years.  The grey bands are the long-term dimmings seen by the DASCH plates over the last 100 years (top) and by Kepler over 4 years(bottom).

Steinn and I argued that the values you get in this scenario are more like -10-6, so way too small to notice.  What Metzger et al. have shown is that most of the energy does indeed end up in the envelope—the top millionth of the star’s mass—so time timescales are correspondingly shorter.  Our order of magnitude estimate was way way off, and so the hypothesis may be plausible after all (we recognized this could be the case in our paper, which is why we declined to give this scenario a plausibility).

So there are regions of the graph where all four curves cross the secular dimming levels seen.  This means that the model does not have to commit to what merged with Boyajian’s Star to explain the dimming.

So where does this scenario rank now? There are still several details to be worked out:

First, there’s the dips.  Metzger et al. point out that the same mechanism that sent the object into the star could also send other material there — a big planet could have lost its moons during a merger, or a planet could have been ripped apart, or something similar. This is essentially Boyajian et al.’s original hypothesis of material on a cometary orbit due to a single disruption event. The big difference is that there was originally a lot more material in the form of something that fell into the star.

Then, there’s the lack of infrared flux.  Again, the highly eccentric orbits save the hypothesis, and Metzger et al. point out that stellar radiation will blow sufficiently small dust out of the system, where it would no longer be warm and radiate.

The next is the details of the Montet & Simon light curve.  It changes slope pretty dramatically, and overall is steeper than the Schaefer dimming. What does this imply? I don’t see similar changes in slope in the Metzger et al. models, but presumably they’re invoking multiple ingestion events. Is this a problem for the model, or does it perhaps tell us the timings and masses of the mergings?

The next is the luminosity. The European Gaia spacecraft will measure the distance to the WTF star very precisely. This, combined with its apparent brightness, will give us the total luminosity of the star quite precisely.  This should give constraints on the merger history of the star.  Combined with the various secular dimmings, this should constrain the model—or prove inexplicable.  It would be nice to know what Gaia weill tell us, if anything, about this model. It would be especially nice if this model turned out to make a falsifiable prediction for the parallax.

Finally—and Metzger et al. acknowledge this is a major flaw in the model—there’s the issue of how likely this is to happen.  Steinn and I have argued that whatever the explanation for Boyajian’s Star, it’s got to be an unlikely one because it’s unique among 200,000+ stars Kepler has observed.  But this scenario turns out to be really unlikely—like Kepler had all but zero chance of seeing such a thing happen.  The effects of these merging events don’t last very long, so you need to stare a long time to have any chance of catching it happen. You would need practically every F star to have planetary material ready to go on eccentric orbits and merge, and even then you need a lot of planetary material.

I’m glad to see this scenario fleshed out so well. I suspect that there are ways to save the model by finding ways to make sort of event occur more frequently—perhaps by making the merging/dips more frequent by getting a chain of material from a single massive object—so I’m optimistic there’s more to this.  I’d say this paper has moved the “post-merger return to normal” scenario from “unclear” plausibility to something like “less plausable,” or even higher.

As I wrote last time:

This is how Tabby’s Star will be solved: a vague and qualitative hypothesis will get turned into a simple, quantitative model like this one, and that model’s success will inspire further work on more complex quantitative models. Eventually, these models will explain all of the data well and make some sort of prediction that will be confirmed by observations. Then we’ll say we have a good model for the system.

Thoughts on Neslušan and Budaj

A lot of folks want to know my opinion about the two new Tabby’s Star papers out this week:

Mohammed A. Sheikh, Richard L. Weaver, and Karin A. Dahmen
Phys. Rev. Lett. 117, 261101
With commentary by Steinn Sigurdsson here:

On this one, my opinion closely matches Steinn’s (because I asked him to explain it to me!). From what I understand of the paper, certain statistics of the dips follow a power law, and so-called “avalanche” models of certain phenomena associated with phase transitions follow a similar power law.  The authors suggest that this means that the processes causing the dips are internal to the star, and represent some sort of transition it is undergoing, like a global magnetic field flip.

That’s interesting, but I don’t know what it really means. It may provide a way for physical models to try to reproduce the data, by asking if the dips they predict follow the same power law.

Neslušan & Budaj  A&A accepted:

In this paper, the authors model four of the deeper dip complexes with a relatively simple but physically motivated model of massive objects (small planets) with very large, extended dust shrouds moving on highly eccentric orbits.

The physical motivation for this model is the same as for Boyajian et al.’s invocation of “comets”: eccentric orbits mean that the material will only spend a brief time near the star where it occults it, and so only be warm briefly.  This concentration in time explains the lack of IR excess at other epochs, and the lack of repetition of the dips.  Like Boyajian et al., Neslušan & Budaj posit that the bodies are the result of a single break-up event, and at least approximately share an orbit.

What Neslušan & Budaj add to this is a rough physical model for the clouds of material.  They assume it appears around 4AU (around the time comets get their comae) and that the dust particles are orbiting the planet (not how I imagined they would behave—I would have guessed like a collisional gas).  They have a variety of models for the initial conditions of these orbits, including some that generate spherical clouds and rings.  They then modeled the gravitational interactions of the star and massless dust particles, and included PR drag (radiation pressure from the star and the dust particle re-emission).  They used MERCURY to do the integrations.

They tried lots of variations of these parameters, and found a few that gave good fits.  Here are some of their efforts:

Neslušan & Budaj


The green lines (very hard to see: don’t use bright green on white, and use heavier weight for your lines, people!) are good qualitative fits to the data.  The right hand side shows the evolution of the dust cloud (lots of colors) and the orbit of the planet (red line) about the star (not shown, but at (0,0) and the focus of the red line).

In all cases what seems to be happening from the right hand side is that a dust cloud is released from the body, and then radiation pressure blows it away from the progenitor body near periapse.  They get similar results and some better fits with other models, including the ring model.

The authors have not really addressed the long term dimming seen (they mention it but have only a hand-wavey explanation that it’s accumulated dust in the orbits), nor the lack if IR excess (qualitatively, these things are only IR-bright when they are close, but the long-term dimming demands significant dust all the time, so, one would think, IR emission all the time).  As Steinn and I wrote, the “comets” explanation is “plausible for the dips, [but] very unlikely for the secular dimming.”  I think that still stands.

Keep in mind that the reason Neslušan & Budaj get good results for the “comets” hypothesis while Bodman & Qullien had more equivocal results is that they have more free parameters to play with from a more sophisticated model.  I don’t think they can do it with much less mass, but they can do it with fewer objects because their objects are bigger.  They still need one object per dip complex.

I’m also surprised that they want to model dust like massless particles in orbit around the planet—I would have guessed that dust cloud would be better modeled with a gas dynamics code than an n-body code. I’d like to hear from someone who studies cometary tails or atmospheric escape about how physically plausible these initial conditions and equations of motion are.

But this is a great next step.  This is how Tabby’s Star will be solved: a vague and qualitative hypothesis will get turned into a simple, quantitative model like this one, and that model’s success will inspire further work on more complex quantitative models. Eventually, these models will explain all of the data well and make some sort of prediction that will be confirmed by observations. Then we’ll say we have a good model for the system, and, if that model includes interesting features with wider applicability, we will use it to understand the Universe better.

Finally, Neslušan & Budaj conclude with “with such physical models at hand, at present, there is no need to invoke alien mega-structures into the explanation of these light-curves.” My thoughts on the propriety of the ETI hypothesis are well documented at this point, but let me say that I don’t think this paper takes the comets hypothesis across any critical threshold that we can say that we now have a good physical model for the system.  They’ve shown that one can get the sorts of complex structures we see in four the dips from a very simple model that is still missing a lot of physics — but a spline is also a very simple model that will fit the data well, which shows that simple models that fit are not by themselves enough to close the book here.  We still have a lot of work to do!

Some notes on the term “comets” here:

  • Boyajian et al. invoked giant comets, meaning bodies on highly eccentric orbits with extended clouds of gas and dust, but much bigger than Solar System comets.
  • Now, Neslušan and Budaj have taken exactly that idea and built a physical model for it.  It seems to be able to generate the dips.
  • Makarov & Goldin invoked interstellar “comets,” but that term usually refers to comets that have been ejected from the Solar System: such objects would not have comae, and so would not be big enough to cause Tabby’s Star to dim. Like Boyajian et al. and now Neslušan & Budaj, they mean things with big clouds of material around them, although it’s unclear why they would emit big clouds in interstellar space.
  • In all cases, we’re not really talking about comets like in the Solar System.  Both Boyajian et al. and Neslušan & Budaj are invoking a single break-up event that generates things that have orbits like comets and, like comets, generate big comae and tails when they get close to the star.  But these are not the primordial “dirty snowballs” of our comets.

Jargon update: “dispositive null”

Back when Sharon Wang published her first paper with me, we struggled to title it. A big result in the paper was that our team had shown that the planet did not transit: that is, we had ruled out transits.

But calling this a “null detection” of a planet seemed to send the wrong message: we had not failed to find a transit, we had succeeded in showing that the planet had failed to transit! I mulled over this in this space here and here.

Eventually, we coined some jargon in the paper: “dispositive null detection”.  Such a result “disposes” of the question of whether an effect exists. You use it when you are ruling out an effect with a known minimum amplitude (like non-grazing transits of a giant planet, or the effects of the ether in the Michelson-Morley experiment) and you do so definitively.  You don’t use it when you don’t find anything, but there could still be something beneath your detection threshold: that’s an ordinary “null detection”.

I wondered if it would catch on.  So far, I count six additional instances of its use, only 3 of which are my own uses of the term!

  1. Greg Henry used the term in text I contributed to his paper on HD 38529.
  2. I used it in our first Ĝ paper.
  3. Natalie Hinkel, at my suggestion, included it in the abstract of our paper on HD 130322.
  4. Natalie later included the term in the body of her paper on HD 6434.
  5. Danielle Piscorz and Heather Knutson defined and used the term in their “Cold Friends of Hot Jupiters” series.
  6. And just last month Andrew Howard and BJ Fulton used the term in their big RV limits paper.

So it hasn’t exactly taken off, but it is getting more use than I thought it would.  We’ll see!

A New Scale for SETI interest

I spent way too much time today trying my hand at another version of the Rio scale, one that does not assume that you understand the signal you see or know where it comes from, exactly.  I think it’s a little more useful down at the bottom (unlikely to be aliens signals) and a bit compressed at the top (very little discrimination among truly interesting signals). I also don’t think I have the weighting quite right yet.

Anyway, here it is.  It applies only to astrophysical anomalies for which the ETI hypothesis has some explanatory power.

It’s a 10-point scale (anything scoring below 1 is uninteresting). I conceived it as a log scale, so it roughly tracks as the product of three probabilities calculated as the sum of three numbers: A+B+C.

The idea is that you find where a given “detection” is in each category (maybe interpolating a bit). I give characteristic examples in brackets for each category, citing some famous (non-) examples of signals (n.b. “first pulsars” means the state of the field when the first pulsars were discovered, and there was not yet any theoretical explanation for their existence).

Note that these three categories are not orthogonal: the routine-ness of detection can influence likelihood of instrumental effects and astrophysical nature of anomaly.

A: Is the detection confirmed?

0: Unclear if detection is real [Poor metadata, possible transcription error, corrupted data]

1: One off: one event, one site [Wow! signal]

2: Verified: simultaneous observations at multiple sites OR multiple events at a single site [FRBs when they were still only seen at Parkes; something that survives strict coincidence detection]

3: Repeated: (multi-site AND multiple events) OR absolutely-no-doubt multiple events at single site [Tabby’s Star, especially with dimming]

4: Routine: Phenomenon regularly detected at different sites by independent groups [long-delayed echoes; FRBs today]

B: Could it be instrumental?

-7. Looks like a known instrumental effect

0. Does not look like a known instrumental effect, but too little metadata or understanding of data to assess signal reality; instrument builders / users not consulted [RATAN-600, SDSS lasers]

1. Data not likely to be instrumental, but instrumental or non-astrophysical cause cannot be ruled out [perytons, cosmic rays that look like lasers, early days of FRBs]

2. Data vetted by instrument builders/users; probably a real signal [Wow! signal]

3. Data thoroughly vetted with control; unambiguously astrophysical [FRBs today, Tabby’s Star’s dips]

C: Could the anomaly detection be natural or terrestrial?

-7: Obvious natural explanations exist [pulsars today, quasars today, RFI]

-4: Rare or somewhat ad-hoc natural explanations exist [J1407 rings, evaporating planets, GHAT III galaxies, FRBs today]

-1: Lack of study makes it unclear if any natural explanations could exist [Przybylski’s Star,SDSS lasers]

0. Only the rarest or most unlikely natural explanations make sense [Tabby’s Star]

1. No known natural cause, but new natural phenomena could be at play [first pulsars, first FRBs]

2. Not obviously intelligent communication or manufacture, but no natural explanations make sense [successful artifact SETI: complete Dyson sphere in the field, short-lived actinides in a spectrum]

3. ETI hypothesis is only plausible explanation [monolith on the Moon; communication: narrow band radio signals, pulsed optical, prime numbers, etc.]

Here’s how I interpret the outcome of a calculation:

Below 1: No interest warranted
1–5: SETI interest potentially warranted; no press interest warranted
6–8: SETI interest definitely warranted; technical popular press interest warranted; fun, off-beat news item for general press, with appropriate caveats. If not aliens, still very interesting.
9: Significant mainstream press interest warranted. Could be aliens
10: Aliens

OK, so how do our favorite candidates compare?

SDSS lasers: 2 + 0 – 1 = 1
GHAT III galaxies: 4 + 3 – 4 = 3
RATAN-600: 1 + 0 + 3 = 4
Przybylski’s Star: 4 + 3 – 1 = 6
FRBs before they were understood or seen at other telescopes: 2 + 3 + 1 = 6
Wow! signal: 1 + 2 + 3 = 6
Tabby’s Star: 3 + 4 + 0 = 7
First quasars (before understanding of their nature): 4 + 3 + 1 = 8
First Pulsars (before understanding of their nature): 4 + 3 + 1 = 8

This roughly (but imperfectly) tracks my impression of the SETI consensus on these “detections”: some signals are really interesting and only explicable as new phenomena (FRB’s, pulsars, quasars, maaayyybe Tabby’s Star, and Wow! signal, if it’s real) and they rightly rank high.

Some anomalies, like Przybylski’s Star are less compelling, but still intriguing and maybe deserve more attention than they get.  Then there’s a gap to things that will almost certainly not pan out on closer inspection.

Anyway, I’m not entirely sure it works well enough to be useful, but that’s my day’s-work contribution to the conversation about improving the Rio scale. Let me know what you think.

When the Press Should Pay Attention to a Possible SETI Detection

How should the media react to news of a claim of detection of alien intelligence? Claims come with various degrees of credit—there is some threshold that needs to be crossed for it to be worth repeating.

With earthquakes, the media can get a quick sense of importance from the Richter scale, which reduces a lot of qualities into a single number that the press has more-or-less learned how to use. Combined with location, even a non-expert can quickly use the scale to make a good guess about whether the earthquake warrants a call to see how loved ones are doing.

The Torino Scale

There is an analogous scale for asteroid threats.

It’s not very often that we have one, but when one is reported there’s generally a range of reactions in the media from totally ignoring it to overhyping how the world will end soon.  Fortunately, responsible journalists can look to the Torino scale to see how newsworthy a potentially hazardous asteroid is.

Visual representation of the Torino scale, from Wikipedia

Visual representation of the Torino scale, from Wikipedia

In short, the worry of an impact is some combination of the likelihood of impact and the damage it would cause if it hit.  A number above 3 is newsworthy, a number above 5 is genuine cause for concern and deliberation, and anything above 7 is call-Bruce-Willis territory.

In the history of the scale, only there have only ever been 2 asteroids that even reached Torino 2, and only one of those was (briefly) upgraded to Torino 4. Both were quickly downgraded to zero upon further measurements.  The media can, if they care to be responsible about this, generally ignore anything that is Torino 0, except maybe as a hook for a think-piece on planetary protection or something.

The Rio Scale

What about SETI? SETI has the Rio scale, set up by the International Academy of Astronautics at a conference in Rio to help the public understand how excited to get about “signals” from space.  Seth Shostak has a nice formula to help one calculate it here, and there is a calculator here.  It’s not perfect — it assumes that you understand the nature and origin of the signal you’ve detected — but it does help put things in perspective.

The Rio scale is a 10 point scale that multiplies the credibility of a report (a 5-point scale from 0-4) with the sum of scores from the class of phenomenon (from 1-6), type of discovery (from 1-5) and the distance to the source (from 1-4, important since it goes to whether two-way contact might occur).

Let’s take two famous examples to get a sense of scale: the Wow! signal and Tabby’s Star.

The Wow! Signal

The Wow signal is the most famous unexplained, potentially alien radio signal from space, detected 13 days after I was born in 1977 by Jerry Ehman (there have been many such “one-off” signals detected).

The class of phenomenon here is unclear—there’s no reason to think it was intended for Earth or could be interpreted by us.  This gives it 1 or 2/6 on this portion of the scale.

The discovery type is also low — a one-off signal.  This gives it a 1 or 2 /5 on the second portion.

The distance is unknown because we only have a general direction that it came from— so we only get the minimum 1 point out of 4 here (I think).

Finally, there’s the credibility: professional radio astronomers who understand their equipment well believed they found something.  In my mind, that puts it up to a 3 or 4/4 on the reliability scale.

Following the formula, I get somewhere from 1 to 3 on the Rio scale for this signal — not bad, and commensurate with how it’s talked about.  It’s a neat signal!

Tabby’s Star

What about Tabby’s Star? Plugging in the numbers again:

Class of phenomenon is low: there is no communication here, just possible evidence of astroengineering: 1/6

Discovery type is unclear, but since Kepler saw the dips for years and we’ve ruled out instrumental effects, let’s be very generous and give it the full 5/5.

Distance is well-known: 1,500 light years or so.  So we get 2/4 on this one.

Credibility on the report is high, but at some point we also to include our confidence that the source was not natural, which is low. We still have natural explanations to rule out. Let’s say 2/4 (“Possible, but should be verified before being taken seriously”)

Plugging everything together, we optimistically get a 3 on the Rio scale, which is pretty high as these things go (which is why it gets attention from the SETI community). But this optimistic number is somewhat subjective, so it’s clear why many astronomers give this explanation no credence.  A reasonable person might give much lower numbers to these categories and give it a 1 or 0.  The point is, it’s analogous to those near-miss asteroids: interesting enough for a popular article (or blog post) for lay aficionados, but anything more than that is unwarranted hype.

Unconfirmed Signals

Now, let’s imagine a hypothetical discovery of “something weird” in some archival data of some distant stars. The “something weird” matches their pet theory for how aliens might communicate among each other, but the discoverers are not the people who took the data, and have not consulted with such experts to see if it could be an instrumental effect. They have also not gotten verification that the signal is seen with other instruments.  So:

Type of phenomenon: intercepting undecipherable communication: 2/6

Discovery type: from archival data, no verification: 1/5

Distance: in the galaxy 2/4

Credibility: Instrument experts have not weighed in yet: 1/6 (“very uncertain, worthy of verification”).  Also, “something weird” does not rule out natural explanations, so lower it down again to maybe 0.5/6.

Plugging things in, we get 0.4 on the Rio Scale.  Most astronomers can ignore it, the press should definitely ignore it, but someone should probably follow up, just in case.

Another perspective

I’m not a big fan of the Rio scale because, as you can see if you try it yourself, if you have a signal you don’t understand, it’s hard to put on the scale.  The possibility it might be natural or instrumental is not really a parameter, so I’ve forced it into the “credibility” and “discovery type” categories in my examples above.

Rather, I prefer this sort of sequence for “something strange” detections. Every step of the way a detection gets more interesting from a SETI perspective:

  1. Point to a prediction of such a signal in a SETI context.  Not every weird thing is interesting from a SETI perspective.
  2. Rule out instrumental effects. This can be done by using a different instrument to detect the same signal, or thorough analysis of the data by experts in that instrument.
  3. Rule out obvious natural or terrestrial explanations. This means showing that any natural explanation is extremely unlikely or physically impossible.

Most “signals” discovered in SETI top out on this sequence at steps #1 or #2.  Optical and radio searches find “interesting” “signals” all the time, but they expect a few statistical flukes and equipment hiccups from time to time (astronomers like to blame “cosmic rays”, which are real examples of so-called “gremlins” that sometimes cause electronics glitches). Those that are probably real usually don’t repeat, and so it’s hard to rule out natural or terrestrial phenomena that we don’t understand if we can’t see it happen again (like the recent RATAN-600 “detection”).

But if you do get this far— the signal repeats, it’s definitely seen with other instruments— it’s very interesting!  You’re in the same space now as the discovery of pulsars, GRBs, FRBs, and quasars. Note that all four of those discoveries were very interesting natural phenomena, but the SETI community was rightly interested in them until they were solved. There have probably been many other “discoveries” that got this far and turned out to be not so interesting as those four game-changers (Tabby’s Star will probably end up in that category).

At this point, if all you have is “something weird” and not a clear, deliberate signal, you still have work to do before claiming a “probable” discovery of aliens, but you are at the point that you should tell communication SETI people that they should consider pointing their instruments at your target, just in case.  Then:

  1. See what the reaction is from the community of astronomers by getting the discovery published in a peer-reviewed journal. This will get more eyes on the data, and help determine if you’ve missed anything. At this point, the press may be rightfully interested—you need to point out to them that it’s probably not aliens.
  2. Learn more about your targets. Brainstorm less-likely natural explanations. Organize campaigns to follow it up

At this point you might be a 3 on the Rio scale.  Beyond that, you’re in terra incognito, and if the source resists natural explanation you might creep up higher on the Rio scale, slowly.

If you are sure that you have something that looks like communication from alien intelligence, then you can calculate a proper Rio scale number, but you also go farther along this sequence, following the generally accepted protocols for an ETI signal detection:

  1. Rule out all forms of terrestrial and natural origin for your signal, including hoaxes.
  2. Get confirmation from independent researchers of the signal using different equipment, and set up continuous monitoring

In other words, only if you find and confirm extraterrestrial signals of unambiguously intelligent origin—impossibly narrow-band radio signals or impossibly-brief laser pulses, for instance (impossible for a natural source, I mean)—do you claim any level of certainty of the detection of alien life. At this point a researcher can contact national authorities, the international astronomical community, and start planning for how to manage the press.

So, the press should certainly publish when a researcher gets to the sixth step of this sequence, but anything at an earlier stage is a curiosity for those interested about how the process works and what SETI researchers are interested in these days. Anything before step 2 is just the usual background rate of (presumably) false positives, and responsible journalists should generally ignore it.

[Edit: I have another attempt at a scale, so I’ve tweaked the description of my sequence in this post to avoid confusion with that one.]


What Could Be Going on with Boyajian’s Star? Part X: Wrapup and Gaia’s Promise

If you’re looking for a guide to this series, click here.

Last time we finished up our roundup of hypotheses for Boyajian’s Star.  Let’s summarize them:

Plausible Hypotheses

  • #3 Small-Scale ISM structure: This will find support if future dimmings are accompanied by increased reddening and sodium absorption.
  • #4 An Intervening Bok globule: In addition to the above, this would be supported if we discover a cloud of neutral gas near Boyajian’s Star.
  • #7 Comets and other circumstellar material: Plausible for the dips. Would be supported if future dips are accompanied by significant infrared flux and absorption features consistent with cometary or other circumstellar material.

Less Plausible Hypotheses

  • #5 An Intervening Black Hole Disk: Would find support if we can detect the central object or if the dips repeat in reverse order, as with J1407, or if more rigorous analysis shows this to be more likely than we have estimated.

Hypotheses with Unclear Plausibility

  • #2 A Solar System Cloud: Would have plausibility if such a cloud is theoretically realistic, if the cloud were found by other means, or if the dimmings are accompanied by absorption features characteristic of Solar System ices.
  • #9 Alien Megastructures: Would find support if all natural hypotheses are ruled out, we detect signals, or if star suffers significant achromatic extinction.
  • #13 Post-Merger Return to Normal: Would have plausibility if Boyajian’s Star is found by Gaia to be overluminous, and if simulations show this could explain the dips and dimming timescale.

Not Likely Hypotheses

Very Unlikely Hypotheses

And here’s a handy guide for what the Gaia parallax, which will determine the distance to Boyajian’s Star and is due out on September 14, will tell us:

  • Within 400pc: Means that ISM extinction cannot explain current level of dimming, so favors non-ISM and non-dust explanations.  These are #9 ,11, and 12.  Could also imply that the cause is very opaque dust blocking part of the stellar disk, so #5, 6, 7, and 8 might still be OK
  • Around 450pc: Means that all of the secular dimming can explained by the dust we observe along the line of sight.  Favors #2, 3, 4, 5, 6, 7,  and 8.
  • Beyond 500pc: Means that the star is more luminous than we expected from the reddening.  Favors #13: return to normal brightness.
  • No answer: Star does not have a good astrometric solution.  Unclear how this could happen given the lack of RV variation, but could that the 2″ companion is much bluer than we think, which would be annoying and not tell us very much except that it’s not an M dwarf, maybe boosting #5 (if it’s really the central black hole).  Might imply that there is another star nearby that we haven’t accounted for, suggesting new hypotheses.

You can vote on what you think Gaia will find here.

The paper is Wright & Sigurdsson, on the arXiv here.  Thanks for reading, and hopefully we’ll know a lot bit more in 11 days!

Update: Jason Curtis points out that the first Gaia release will not actually have the precision to distinguish among these cases!  So unless the parallax comes out really for from 450, we’ll have to wait longer than Sep. 14.

What Could Be Going on with Boyajian’s Star? Part IX: Intrinsic Variations

If you’re looking for a guide to this series, click here.

Last time we finished up circumstellar scenarios with a discussion of alien megastructures, the reason this star got so much press in the first place.  This installment: intrinsic variations.

I think Robin Ciardullo was the first to mention this possibility to me, though it may have been Richard Wade. The idea is that somehow the star itself is getting slowly dimmer, with occasional dips.

Hypothesis 10) Starspots and Magnetic Cycles

The most natural way stars change brightness over years and days is from stellar magnetic activity.  The Sun at the maximum part of its cycle is about 0.1% brighter than usual, and sunspots and plage coming into and out of view can also modulate its brightness by a few tenths of a percent.  More magnetically active stars might have even greater variations.

But the timescales and amplitudes are all wrong for Boyajian’s Star.  Activity cycles take decades, not centuries, and Boyajian’s Star rotates about once every 0.88 day, not many days.  If the dips were caused by spots, they should come and go every 0.88 day, but instead they last many days or longer.  Finally, in order to cause the big changes in brightness we see, the spots or cycles would have to be 10–100 times larger than the strongest effects known in other stars.

Another problem here is that Boyajian’s Star is decidedly inactive, being a hot star, not a cool star.  Unlike the Sun, it has a radiative outer layer (not a convective one), so it does not have the same dynamos that produce the Sun’s magnetic fields.  Hot stars like this have very little in the way of surface features (“stellar dermatology”) and no activity cycles (that we know of).  There are hot stars with strong magnetic fields and spots, but they are almost always slow rotators (their fields slow them down), and are generally hotter than Boyajian’s Star.

So, subjective verdict: very unlikely.

Nonetheless, something weird an unexpected is going on, so maybe we should not dismiss this entire line of reasoning entirely.  This bring us to

Hypothesis 11) Polar Spots

Montet & Simon point out that a polar spot could be doing this.  Some very cool stars are known to have large, dark, polar starspots.  This explanation nicely explains why the spots don’t come and go with a 0.88 day period: since they are at the pole, we always see them, and so at best their irregular shapes get modulated. This would explain why we see a 0.88 day photometric signature at all, on what we might expect would be a featureless star. Then, the dimmings could be due to the ebb and flow of this spot’s size and darkness on days- and century-long timescales.

Reconstruction of the surface features of the T Tauri star V410 Tauri (from Carroll et al., A&A 548, A95). Note the large spot at the north pole of the star. Very young, cool stars have strong magnetic fields that seem to produce spots like this which, if the star is inclined with the spot towards us, we see persistently, with only weak rotational modulation.

Reconstruction of the surface features of the T Tauri star V410 Tauri (from Carroll et al., A&A 548, A95) shown from 4 different vantages. Note the large spot at the top pole of the star. Very young, cool stars have strong magnetic fields that seem to produce spots like this which, if the star is inclined with the spot towards us, we see persistently, with only weak rotational modulation.

This is very clever, but for the reasons I stated above we have good reasons to think this star can’t have spots like this.  (Again, if this were a thing, we should see it all the time!)  Montet & Simon offer the hypothesis very provisionally for this reason.

So my subjective verdict on this one is: not likely.

This hypothesis would find support if Gaia finds the star to be much farther away than we expect from the reddening and sodium absorption.  This would indicate that reddening is not accompanying the dimming we see, so it could be spots.  It would also find support if signs of a polar spot could be identified, such as narrow absorption features from the much cooler atmosphere, perhaps with Zeeman splitting if the spot has strong magnetic fields.

OK, what else could the star be doing?  Well, many forms of variable star are pulsating, so how about:

Hypothesis 12) Stellar Pulsations

Stars have a few characteristic timescales on which they change.  If you poke a star with a regular cadence, there are certain frequencies that will cause it to respond strongly.  Some ways this can happen are:

  1. In the Sun, random convective motions cause it to “ring” with many frequencies, the strongest being around 5 minutes.  These “asteroseismic” modes are generally very, very small: they can never cause variations of order 20%, and they are usually periodic, not random like Boyajian’s Star.  The timescales are wrong for a Main Sequence star, too: these operate on sound-crossing times and their harmonics, not days or centuries.
  2. Many classes of pulsating stars rely on internal instabilities involving the opacity of their constituent gasses.  If these gasses heat up, they ionize, which makes them more opaque, which makes it harder for starlight to emerge from the core to the surface, which causes the star to expand, which makes the gasses cool, which makes them recombine, which lowers their opacity, which makes it easier for starlight to emerge, which makes the star shrink, which heats the gasses… etc. etc.  These timescales can be of order days, but, like asteroseismic modes, they are periodic, not episodic, and don’t cause century-long dimming.  Delta Scuti stars are a kind of star of very similar mass and temperature to Boyajian’s Star, so it’s possible that it has something like this going on, but it’s not clear how that could cause any of the effects that we see.

Basically, all known pulsation modes are regular or semi-regular, not episodic, and none we know of operate over centuries.   If it’s some form of pulsations, it’s one we haven’t seen before or thought of before.

Subjective verdict: not likely

The last one is my favorite, from Steinn:

Hypothesis 13) Post-Merger Return to Normal

What if the star isn’t dimmer than it should be: what if it’s brighter, and we’re seeing it return to normal brightness?  In that case Gaia will reveal that it is much farther away than we think based on its brightness and reddening.  This would imply that the long-term “dimming” we see is the star returning to a normal state after somehow getting way too bright.

But what could that mean?  The best idea we have is some sort of merger: perhaps the star recently coalesced with another star, or a brown dwarf or planet.  This would deposit a lot of orbital and gravitational potential energy into Boyajian’s Star, which would eventually get dissipated as heat and escape as starlight, causing a temporary brightening.

It’s not clear why this would create the dips, but perhaps the merger was quite recent, and there are still hydrodynamical effects going on inside the star as the companion is “digested”, causing the internal structure of Boyajian’s Star to adjust on days-long timescales.

One issue here is that the dimming is too fast.  When confronted with big changes in energy content or flux, stars evolve on the Kelvin-Helmholz timescale, roughly the time it takes for all of the energy in the star at a given moment to finally escape the surface (while being constantly replenished by the fusion in the star’s core). For Boyajian’s Star this timescale is about 1 million years. This means that if the entire star is processing a big change in internal energy or luminosity, it takes around 1 million years to complete the adjustment.  Changing by 15% in 100 years is therefore about 10,000 times too fast.

But, the star’s radiative envelope is not very massive, so perhaps the energy never made it deep into the star? In that case the Kelvin-Helmholz timescale is a bit shorter, so maybe we’re off by only 1,000 times. It’s an order of magnitude argument, so maybe we’re being too pessimistic by a factor of 10, so we’re only off by 100 times. It’s possible that a detailed simulation of such a merger will reveal shorter timescale events, perhaps even things that might produce the dips.

So, I’m intrigued, and I like the idea despite the timescale argument not working out. It’s possible that there are other ways to temporarily brighten a star we haven’t thought of.  I’d like to hear from people who model these things before I commit to a plausibility level, so I’ll say:

Subjective verdict: unclear.

So that’s it!  That’s all I’ve got.  I’ve tried to be comprehensive, but cleverer people than me keep coming up with new ones, so I’m sure I’ve missed something.  I’m still not convinced that the right answer has appeared in the literature, but I’m hopeful that one of the explanations in our paper is correct.  Before wrapping up in the final post, though, I’d like to discuss some nonstarters that keep coming up:

Popular Nonstarters:

  1. Gravity darkening.  This one got a lot of attention for some reason. Basically, stars that spin very fast have equators that are darker than their poles.  A transit or eclipse of a gravity-darkened star can therefore have an unusual shape, potentially being asymmetric.  Since some of the dips are asymmetric, some people declared the system “solved” with this suggestion.  Problem is, Boyajian’s Star is not spinning fast enough to have significant gravity-darkening, and this still doesn’t address the issues of what is causing the dips!
  2. Black hole or other nearby companions.  Boyajian et al. ruled this one out, but just to reiterate their argument: Boyajian’s Star is RV stable, so it does not have any close-orbiting stellar-mass companions.  It’s not any kind of binary system.
  3. Gravitational waves. I’ve heard this one suggested, presumably because they were just discovered by LIGO and so are in the zeitgeist right now.  They have no explanatory power here (i.e. there is no way they could cause what we see).
  4. Asteroids in the Solar System.  We discuss Solar System solutions here.  Anything closer than the outer Solar System would not persist for years, and asteroid occultations have a very different photometric signature than Boyajian’s Star.
  5. It’s an ordinary “dipper”. There are “dipper” stars that have superficially similar light curves to Boyajian’s Star. But these as caused by close-in disks and other circumstellar debris that are revealed by their long-wavelength emission, which Boyajian’s Star lacks. That was the original reason Boyajian’s Star stood out as being so weird: it’s not young, it has no disk, so it’s not a dipper. I don’t understand why this one keeps coming up.
  6. Starkiller Base.  Yes, I saw The Force Awakens, too.  I presume these suggestions were all made facetiously.

OK, next time: summary of hypotheses, and what Gaia will tell us.


What Could Be Going on with Boyajian’s Star? Part VIII: Alien Megastrutures

If you’re looking for a guide to this series, click here.

Last time I tried to wrap up circumstellar solutions, but before moving on to intrinsic variations, it’s clear I need to address the megastruture in the room.

Hypothesis 9) Alien Megastructures

Background on this hypothesis is here, and here:

Now, I can’t give this hypothesis the treatment I’ve given the others, because, even compared to the vague, general families of solutions in the previous hypotheses, this one has very little to hang any physics on. Ancient alien civilizations could be arbitrarily advanced, and so it’s not clear what physics we’re allowed to assume. We don’t know why they would create megastructures (though energy collection seems like a good guess to me) so we have no good reason to expect any particular shape or size for them.

That said, we can build up a straw man model, and see how well it holds up.

But first, let’s dispense with some unnecessarily complicated ideas. Lintott & Simmons made me smile with their article in the Journal of Brief Ideas in which they used the timescale of secular dimming to estimate the construction time for a Dyson sphere. Similarly, Villaroel et al. skeptically mentioned this possibility in their paper.

But we don’t have to imagine building a gigantic sphere in a century to hypothesize how megastructures could explain the dimming.

Imagine that it is advantageous to collect solar energy to be used for some purpose, and that there is enough material to do so, but not an infinite supply.  Imagine that the panels have a range of sizes and orbit the star in a range of orbital periods.  In this case, the cost (in time, energy, and mass) to construct a panel is balanced by the benefit (the energy intercepted, which favors close-in panels, times the efficiency of the panels, which favors far-out panels). If too many such panels are created, the close-in ones will shadow the farther-out ones, reducing their efficiency.

In this toy model, one expects (in a rough order-of-magnitude sense) to end up with a swarm of panels that have an optical depth near 1. That is, they should not capture, say 99.999% of the photons, because the marginal efficiency of the next panel is reduced by a factor of 100,000 compared to the first one.  And they shouldn’t intercept only 0.001% of the light, because 99.999% of the light still free to take with virtually zero efficiency hit. You’d expect somewhere between, say, 10% and 90% of the light to get absorbed.

The smaller panels will appear as a translucent fluid around the star, constantly blocking some fraction between 10-90% of the light, roughly speaking. This fraction will vary as denser parts of the swarm come into and out of view, and as chance alignments of parts of the swarms at different orbital distances align. We might see variations in brightness on scales from hours to centuries. Particularly large panels—even bigger, perhaps, than the star itself—will cause large, discrete dips as they transit, with profiles according to their shape. The timescale for crossing may not be a good indicator of their distance from the star, because they might be so thin that radiation pressure is important (see the appendix here).

This is actually right in line with the observations of Boyajian’s Star: we see a constant dimming of the star, one that erratically has increased by about 15% in the past 115 years, and occasional dips.

OK, so how does the toy model hold up against long-wavelength observations?  Well, not great. The balance of efficiency with orbital distance is trickier, and interacts with the shadowing argument, but you might expect there to be a typical, characteristic temperature that is around 150K (95% maximum efficiency for a star like the Sun—we argued all of this in more detail in our paper here).

The long-wavelength constraints apply almost as well to megastructures as they do to dust. We argued in that paper that it is reasonable to expect nearly all of the energy  collected to be reradiated as waste heat.  As we saw from the long-wavelength limits, it just can’t be that 15% of the starlight is being reprocessed at 150K, or at any other temperature:

Spectral energy distribution of Boyajian's Star, with arrows indicating upper limits. The right-most measurements are from Thompson et al. They rule out big clouds of dust blocking 15% of the star's light in all directions, because the dust would reradiate at levels easy to detect.

Spectral energy distribution of Boyajian’s Star, with arrows indicating upper limits. The right-most measurements are from Thompson et al. They rule out big clouds of megastructures blocking and reradiating 15% of the star’s light in all directions (black line) at 65K, because the heat would be easy to detect.  Other temperatures are actually in worse agreement with the data.

The black line is our toy model swarm blocking 15% of the light, and it is strongly inconsistent with the red upper limits from WISE and Thompson et al.

Now, there are a couple of ways out.  First, the aliens might be doing some non-dissipative work with this starlight. Perhaps they are launching interstellar probes. Perhaps they are sending out powerful laser or radio transmissions. Anything that puts all of that stellar luminosity into energy that leaves the system in a low-entropy way would reduce their waste heat luminosity.  Can that do it?

Well, if the work is all done at 65K, then the maximum (Carnot) efficiency allowed by thermodynamics is about 99%. This means that they could reduce their long-wavelength luminosity down from 15% of the star’s, to only 0.15%.  The upper limit at this temperature, as you can see in the figure above, is 0.2% (the purple line).

So… not completely ruled out, yet, but I’d have to say we’re close enough that I’d put this one down as very unlikely, even after normalizing for the unknown chance that there are megastructures there in the first place.

Another way out is that it’s not a spherical swarm. Make it an edge-on ring, and you can further reduce the waste heat. Make them radiate towards their ecliptic poles, and you can reduce it further. Turn the energy into low entropy radiation at high efficiency and do both of the above, and they’d be nigh undetectable.

So you see, the hypothesis is sufficiently flexible that it can fit almost any observations, but I do think we can say this about the vanilla, simplest, strawman scenarios:

  1. IR/mm observations rule out a spherical swarm doing only dissipative work (like humanity does) being responsible for the long-term dimming
  2. We can further rule out a spherical swarm sending the energy off in another way at maximum efficiency with temperatures above ~90K
  3. We can lower this cut-off temperature with more sensitive measurements

So, subjective verdict: unclear, with an extra multiplier of very unlikely for the simplest spherical swarm strawman.

The megastructure hypothesis would find support if Gaia shows us that the star is actually much more extinguished than the reddening suggests (meaning there is a significant optical depth of geometric absorbers: the swarm of solar panels).  Or if, you know, we detect those radio waves that all those panels are producing!

OK, next time: back to the natural explanations, this time looking at intrinsic variability.

What Could Be Going on with Boyajian’s Star? Part VII: Circumstellar Dust

If you’re looking for a guide to this series, click here.

The central puzzle of Boyajian’s Star is that although the dips look a lot like those of dippers, it has no close-in disk.  That is, most stars that show this sort of behavior are young and have close-in disks, and those disks cause their dips.  Not only does Boyajian’s Star have no close-in disk, it isn’t near a star forming region, so there is no reason to think it is young, anyway.

So, if it can’t have close-in material, what could be causing the dimmings?  Well, if the material is circumstellar, it must be very cold, and/or there can’t be very much of it.  Boyajian et al. also showed that the timings and slopes of the dips also put constraints on the material’s proximity to Boyajian’s Star, meaning it needs to be pretty far-out (10 AU or so).  This brings us to:

Hypothesis 7) Comets or other circumstellar debris

tabby-cometBoyajian’s original hypothesis was that the dips were caused by giant swarms of giant comets.  This remains a plausible hypothesis for the dips: the comets would certainly have been warm when near periastron, but their eccentric orbits could bring them very far from the star quite quickly.  Thus, the infrared excess would only be noticeable during a dip.  If the swarm is on a long-period orbit, then we may never see them again!

On the other hand, there doesn’t seem to be any way the comets can explain the long-term dimming, especially without an infrared excess.

So, my subjective verdict on this is: plausible for the dips, very unlikely for the long-term dimming. Occum’s razor argues that if they aren’t causing the long-term dimming, then they probably aren’t causing the dips, either, but your mileage may vary on that one.

OK, what about:

Hypothesis 8) A cool annulus of material

spitzer_stardebris.jpg.CROP.original-originalThis is basically the hypothesis that there is a disk of material, but there is a very large (10 AU, at least) gap between the star and the inner edge of the disk (annulus).  The disk is almost, but not quite, edge-on: then, we could invoke corrugation or other irregularities that occasionally block part of the star for the dips.   This needs to be a real protoplanetary disk, not just a debris disk, because it needs to be optically thick.

There are lots of problems with this:

  1. The Thompson millimeter results rule out very much mass for the disk—there is not a lot of wiggle room between what you need for the dips and the secular dimming and what would have been detected
  2. I’m not aware of any theoretical justification for such ring, except maybe for young stars.
  3. If the inner edge is at 10 AU, the probability of edge-on alignment is roughly 0.1%, so in order to expect to have seen one, we need to have about 1% of all Kepler stars to host such a disk (despite #2, and the fact that the field has no young stars)
  4. The secular dimming is not naturally explained—if the disk is always occulting part of the star the sodium lines would probably not be symmetric, but they are.

So, while an annulus is not ruled out by the data, it doesn’t have a ton of explanatory power, and it has no theoretical justification, and it requires a very unlikely geometry and high frequency of such disks in the field.

Although I know some others still like this one a lot, these are just a bridge to far for me.  Subjective verdict: not likely.

We’ve already discussed the long-wavelength constraints that show it can’t be embedded in a cloud of material, so I won’t even mention that one.

OK, now that we’re done with all of the circumstellar possibilities, in the next installment we’ll continue our conceptual journey from Earth to the star itself and discuss intrinsic variations of the star…

Huh? What’s that? Megastructures? You want a rundown on where that hypothesis is?

Well, I hadn’t planned to talk about it, but since you asked, I suppose we could do a brief detour into some serious speculation before finishing our list.  As luck would have it I happen to have some remarks prepared right here…

All right then. Next time: alien megastructures.


What Could Be Going on with Boyajian’s Star? Part VI: Black Holes!

If you’re looking for a guide to this series, click here.

Last time we covered SINS and Bok globules as potential sources of the dimmings in Boyajian’s Star.  This time we’ll finish up the interstellar solutions and start to look at circumstellar solutions.

Our topic? BLACK HOLES!

Hypothesis 5) An Interstellar Black Hole Disk

Now, a very popular suggested explanation from laypeople has been “a black hole”.  For a long time, I treated this as a total non-starter: black holes are tiny, tiny, tiny, and don’t have the geometric cross-section to block any noticeable amount of light.  Further, they tend to gravitationally lens light, so if there really were a black hole between us and Boyajian’s Star we’d expect a brightening, not dimming. Finally, if it’s close enough to suck up any material, we’d see Boyajian’s Star get brighter from the accretion, we’d see light from radio to X rays from the interactions, and we’d see RV variations to boot.  So, no black hole, right?

So it’s ironic that we found another way to do it (though not what those who suggested it were thinking).  This solution is more analogous to J1407, the “planetary system under construction” discovered and characterized by Eric Mamajek and Matt Kenworthy.

Here’s the idea: the dips really suggest small-scale structure in the intervening material, but the long-term dimming suggests a smooth distribution.  One solution is a gigantic disk with annular features, like a protoplanetary disk with gaps and waves from planets and protoplanets.  The timescales involved (115 years!) mean that it must be HUGE — like 600 AU across.  What could host such a disk, but have totally escaped notice?  After all, Boyajian et al. used adaptive optics to hunt for anything nearby that could do the trick, and only found an M dwarf 2 arcseconds away.

Well, it must be very massive, and it must be dark.  Stellar remnants do the trick: a cold white dwarf, a quiescent neutron star, or a black hole.  We focus on the black hole solution in our paper.

The black hole disk from Interstellar, with the black hole's gravity distorting the image of the disk. Our scenario is much less photogenic: the close-in disk is gone, and only a very wide, very cold disk of dust and debris remains. Also, our black hole is several solar masses, not several million.

The black hole disk from Interstellar, with the black hole’s gravity distorting the image of the disk. Our scenario is much less photogenic: the close-in disk is gone, and only a very wide, very cold disk of dust and debris remains. Also, our black hole is several solar masses, not several million.

The idea is that after a supernova explosion, there will be material that falls back towards the remnant black hole, where conservation of angular momentum requires it to collapse to a disk, and some amount of the material to move outward while a lot of it spirals in and accretes onto the black hole.  Eventually, the black hole finishes its meal, and goes quiescent, leaving behind a big inner gap within a large ring of debris outside.  This ring can now get very cold (the central object is dark! nothing to heat it), thin, and wide.  Perna et al. (2014) describe the process.

So, does it work?  Could there really be a black hole aligned with Boyajian’s Star? Well, the alignment doesn’t have to be very good: the disk needs to span a big chunk of the sky for the dimming to persist over 115 years (a couple of arcseconds, at least).  The black hole itself is small and would be unlikely to actually go between us and Boyajian’s Star (if it did it would lens it, but its Einstein radius is only ~4 mas).

We calculated the volume of space probed by Kepler for objects with that size (angle squared times typical distance cubed), multiplied by the number of stars Kepler observed, and decided you needed about 10 billion disk-bearing black holes in the Milky Way for one to have had a good chance to wonder in front of ~1 such star in the field.  That’s not too far off from the estimated number of black holes in the galaxy!

So, the numbers aren’t too bad!

I really like this one, but there still isn’t any observational evidence that such disks exist, or that they are common enough for Kepler have found one.  We haven’t done a rigorous calculation of the probabilities, so it could still fall apart upon closer inspection.

Given the uncertainties, I give it a subjective verdict of: less plausible.

This hypothesis would find support if the dips repeat in reverse order while the star starts brightening again, like J1407 did.  Alternatively, that so-called “M dwarf” 2″ away could turn out to be the central object, in an almost-but-not-totally quiescent state.  Someone get a spectrum of that thing!

Hypothesis 6) An Orbiting Black Hole Disk

I had hoped that we could make an alignment more likely by putting the black hole in orbit around Boyajian’s Star, but it turns out that makes things much harder.  In addition to the low probability of such a binary companion in the first place, the chances that it would be in a part of its orbit such that we would see it are very low, like 1 in a million low.  Since Kepler only looked at 100,000 stars, and since every star does not have such a companion, this one doesn’t work.

Subjective verdict: not likely.

OK, enough with the black holes. Next time: Circumstellar material.

Update: Commenter Herp McDerp (obviously their real name) points to this Nature article by Alastair G. W. Cameron (Bethe Prize winner and originator of the Giant Impact Hypothesis for the formation of the Moon).  In it, Cameron tries to explain the eclipses of the ε Aur system with our Hypothesis 6!  I’d write “great minds think alike” but I’m totally out of my league on this one, so I’ll just write that we’re in very good company with this hypothesis!