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.