A stripped helium star in the potential black hole binary LB-1
Abstract
Context. The recently claimed discovery of a massive (MBH = 68-13+11 M⊙) black hole in the Galactic solar neighborhood has led to controversial discussions because it severely challenges our current view of stellar evolution.
Aims: A crucial aspect for the determination of the mass of the unseen black hole is the precise nature of its visible companion, the B-type star LS V+22 25. Because stars of different mass can exhibit B-type spectra during the course of their evolution, it is essential to obtain a comprehensive picture of the star to unravel its nature and, thus, its mass.
Methods: To this end, we study the spectral energy distribution of LS V+22 25 and perform a quantitative spectroscopic analysis that includes the determination of chemical abundances for He, C, N, O, Ne, Mg, Al, Si, S, Ar, and Fe.
Results: Our analysis clearly shows that LS V+22 25 is not an ordinary main sequence B-type star. The derived abundance pattern exhibits heavy imprints of the CNO bi-cycle of hydrogen burning, that is, He and N are strongly enriched at the expense of C and O. Moreover, the elements Mg, Al, Si, S, Ar, and Fe are systematically underabundant when compared to normal main-sequence B-type stars. We suggest that LS V+22 25 is a stripped helium star and discuss two possible formation scenarios. Combining our photometric and spectroscopic results with the Gaia parallax, we infer a stellar mass of 1.1 ± 0.5 M⊙. Based on the binary system's mass function, this yields a minimum mass of 2-3 M⊙ for the compact companion, which implies that it may not necessarily be a black hole but a massive neutron- or main sequence star.
Conclusions: The star LS V+22 25 has become famous for possibly having a very massive black hole companion. However, a closer look reveals that the star itself is a very intriguing object. Further investigations are necessary for complete characterization of this object.
- Publication:
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Astronomy and Astrophysics
- Pub Date:
- January 2020
- DOI:
- 10.1051/0004-6361/201937343
- arXiv:
- arXiv:1912.08338
- Bibcode:
- 2020A&A...633L...5I
- Keywords:
-
- stars: abundances;
- stars: chemically peculiar;
- stars: early-type;
- pulsars: individual: LS V+22 25;
- Astrophysics - Solar and Stellar Astrophysics
- E-Print:
- Accepted for publication in A&