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.

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