text:   companions",13,"['New paper! We study two “mass gap” black hole candidates in binaries with red giant stars, “the Unicorn” and “the Giraffe”. 1/n <LINK> <LINK>', 'We used spectral disentangling to search for possible luminous companions (as opposed to BHs) to the giants. We found them! 2/n https://t.co/j5WPzU3ikL', 'What that means, roughly, is that two luminous stars do (much) a better job fitting the spectra than one. https://t.co/bogv9ie8nu', 'In both systems, the disentangled spectra of the companions look like subgiant stars (i.e., cooler and larger than main-sequence stars; warmer and smaller than giants). 4/n https://t.co/2cdMaFwDn2', 'Because these subgiants are cooler than main-sequence stars of similar mass, they are faint in the UV and consistent with the observed spectral energy distributions and UV limits. 5/n https://t.co/S20JlBiznj', 'We can measure the masses of the giants from the observed ellipsoidal variation. In both systems, they are 0.3-0.5 Msun. This is very low for a giant, and implies most of their initial mass was stripped off by a companion. 6/n https://t.co/jpx3Pk4kkh', ""We can also measure the masses of the subgiants dynamically. The dynamically-inferred values (1.8 and 2.8 Msun) are in reasonably good agreement with what we'd estimate from their temperature and luminosity. 7/n https://t.co/KSLzPBlDdm"", ""We used binary evolution models to investigate how these systems formed and how they'll evolve in the future. We think the giant are almost completely stripped of their envelopes and will soon contract to become low-mass helium white dwarfs. 8/n https://t.co/NKLLw40ob0"", ""This scenario (and the component masses) is almost identical to how we think Regulus, the ~20th brightest star in the sky, formed. It's a main-sequence star with a helium white dwarf companion in a wide orbit. https://t.co/nk0iFv0E9K 9/n"", 'The fact that the companions are subgiants (i.e, off the main sequence) implies that either the initial mass ratio was very close to 1 (like, q&gt;0.99), or that the companions are temporarily inflated due to rapid accretion. 9/', 'The second possibility is particularly exciting, but it will take more work (ideally a population model of interacting giant binaries) to test it.', 'The Unicorn and Giraffe join a growing population of mass-transfer binaries recently observed at various stages of the stripping process. Several of these other objects have also been previously interpreted as BHs. https://t.co/sxKQRKlkSb', 'Summary: stellar-mass BHs are small needles in a very large haystack. But they haystack contains a lot of other interesting stuff! n/n']",22,03,2420