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:
TIL via arxiv: Przybylski's star shows every element in its spectrum from neptunium to einsteinium (93-99). https://t.co/VAeUCgVpFD
— William Keel (@NGC3314) March 14, 2017
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. Some theoretical calculations indicate that their half-lives may be long, on the order of 109 years.
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.
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.