Last time I blogged about our bottom line on Kardashev Type III civilizations.  This time: science of natural sources.

One of the nice things about a search like the one we’re doing is that because we are exploring a new parameter space with a new instrument with orders of magnitude better sensitivity than previous instruments, we’re going to find some strange new things.

First, we found some MIR-bright galaxies that no one had ever noticed before.  They have almost no presence in the literature at all, despite having some of the most extreme MIR colors of any galaxies in the sky.  They are most likely nearby starbursts of various flavors, but until we check more carefully we can’t be sure.

Next, we found a huge nebula around the classical Be shell star 48 Librae.  Now, 48 Librae is very bright, and so is very well known among a certain class of stellar astronomer.  It is a Main Sequence B star (so very hot and massive) that exhibits very low surface gravity features because it is very rapidly rotating — nearly at breakup speeds.  This gives it a highly oblate shape, with the material at the equator nearly flying off.  Indeed, the star is rotating so quickly that it has an excretion disk.  This disk has material that is occasionally seen in absorption as it moves away from the star (the “shells”) and shines brightly in some emission lines (the “e” in “Be”).

48 Librae is known from the IRAS survey to have a midinfrared excess, but that’s not too unusual, since these stars’ excretion disks can have a lot of warm material shining in the MIR.  What is unusual is that the source isn’t the disk, it’s an enormous nebula around it:

The 22 micron Nebula around the Be shell star 48 Librae.

Eric Mamajek. Chalkboard mathy guy and Facebook scientist.

What’s going on?  To find out, I consulted some astronomers.  Mostly by email, but one, in particular, I consulted over Facebook.  Regular readers know that Eric Mamajek helped us figure out the coordinates of the CTIO 1.5m, and taught us astronomers a lot of about the various coordinate systems used to describe the location of things on the Earth.  Eric, true to form, responded to my request to figure out what sort of star 48 Librae is with a lengthy dive into the literature.

So I had to figure out how to cite him, of course.  Fortunately, Facebook has unique URLs for every post, and since my posts are public, that URL serves to point to Eric’s research.  Here’s how the citation looks in the paper:

How to cite a Facebook post

(AAS journals have a “no URLs” rule in the body (and bibliography, I think), but anything goes in the footnotes, so there it goes).

Anyway, we thought at first it must be a reflection nebula.  Lots of B stars, like those in the Pleiades, illuminate the dust around them.  Paul Kalas has a nice paper describing the “Pleiades phenomenon” they discovered around many hot stars in their search for substellar companions with adaptive optics.

In the Pleiades, this dust scatters the blue light of the stars very well, and the hard radiation from these stars both heats and excites the dust, causing it to give of thermal radiation, and the PAHs in the dust fluoresce in the WISE bands (especially W2 and W3).

The problems with this interpretation for 48 Librae are:

• There’s no blue scattered light here.  OK, whatever, maybe you just need more dust to do that?
• There’s no W2 emission, and hardly any W3.  OK, so not much PAH emission, maybe there aren’t many PAHs in the dust?
• The nebula doesn’t look like a randomly illuminated patch of dust. There are arc structures in it.  I fit the arcs by eye to a series of nested ellipses centered on the star, and found that they roughly match up:

I fit the ellipses in by hand using Keynote. They are concentric, centered on the star.

OK, so maybe it’s not illuminated by the star?  The other way to get these nebula is with a bow shock: the star moves faster through the gas than the local sound speed, and shocks it.  The shock heats the gas, which then glows.  The problem here is that 48 Librae isn’t moving very fast. It’s proper motion is quite small, in fact almost entirely due to the Sun’s motion through the Galaxy.  Plus, the nebula doesn’t really look like a bow shock.

So I wondered whether the nebula could be from 48 Librae itself?  The basic physics makes sense:  Be shell stars have excretion disks, and they are known to have winds that carry away material (we see the narrow, blueshifted metal lines in the spectra).  At some distance, this material becomes cool enough to condense into solids (like soot condenses in a flame, I’ve learned from Eric Feigelson). If this happens when the material is still at a sufficiently high density, it will form dust.

This explanation would explain the symmetries of the arcs in the nebula, and provide a mechanism for their emission.  The arcs are centered on the star because they are rings of ejected material, and they might be bright because they have been shocked during collisions either with previously ejected material, or the surrounding ISM.  They dust graints might also might have a size distribution that makes them fluoresce under the light of 48 Librae, but be inefficient at scattering the blue light that makes a reflection nebula.

If this is right, then we would expect the orientation of the ellipses I’ve drawn to correspond to the excretion disk.  I had email consultations with a bunch of Be star experts, including Stan Owocki and Thomas Rivinius, and they pointed me to some of the basics.  Since the disk is seen in absorption, it must be inclined no more than ~25 degrees from edge on, and my ellipses are inclined 20 degrees from edge on.  The actual position angle is known from interferometry to be 50±9 degrees from Štefl et al. 2012, consistent with our rings at 1–2σ, or within the rough precision with which we can define them.  Also, the nebula is not too big to be from the star: at the shell expansion velocity it would only take 30,000 years or so to get that big.

So it all checks out: the arcs are about right to be extensions of the excretion disk.  If this is correct (and I consider it a longshot) then Be stars could be important sites of dust formation, and we should reconsider some of the “Pleiades phenomena” as being something other than a coincidental pairing of stars and dust.

What fun!

Next time: a mysterious cluster of IR sources with no optical counterparts!