bystander wrote:A Contracting White Dwarf?
astrobites | 2017 Nov 17Matthew Green wrote:
Today’s paper takes another look at a binary system, HD 49798, which has been puzzling scientists for some time. HD 49798 is an X-ray binary, a type of binary in which matter is transferred onto a central, compact star (a white dwarf, neutron star or black hole) from a donor star. The authors suggest that many of the unusual attributes of the system could be explained if one of the stars is a young, half-formed white dwarf which is still in the process of contracting. ...
A Young Contracting White Dwarf in the Peculiar Binary HD 49798/RX J0648.0–4418 ? - S.B. Popov et al
Monthly Notices of the RAS (online 13 Nov 2017) DOI: 10.1093/mnras/stx2910
arXiv.org > astro-ph > arXiv:1711.02449 > 07 Nov 2017
Very interesting. I note that the donor star is an O6 star and very blue, with a -0.27 Johnson B-V index, which is blue indeed, not least given the distance to the binary of about 2,700 light-years.
If the compact star is a contracting white dwarf, some two million years old, then the progenitor must have been an O-type star too, and massive enough to die after a few million years. And yet, this massive star didn't turn into a neutron star, much less into a black hole, but into a humble white dwarf, albeit a massive one as white dwarfs go.
It seems strange to me that an O6-type star would be the surviving member of a binary system whose more-massive star has already died, and yet it hasn't turned into a neutron star. Admittedly though, the O6-type star just might be a blue straggler, which picked up mass from its swollen, dying sibling. Maybe, in fact, the progenitor of the compact star dumped most of its mass on its companion, Algol style. So maybe the progenitor of the compact star wasn't quite as massive as we might think it was, at least not when it died.
I guess it hasn't been proved beyond a doubt that the compact object is in fact a white dwarf. But this is hugely interesting, all the same.