Context. It has recently been proposed that LB-1 is a binary system at 4 kpc consisting of a B-type star of 8 M⊙ and a massive stellar black hole (BH) of 70 M⊙ . This finding challenges our current theories of massive star evolution and formation of BHs at solar metallicity. Aims. Our objective is to derive the effective temperature, surface gravity, and chemical composition of the B-type component in order to determine its nature and evolutionary status and, indirectly, to constrain the mass of the BH. Methods. We use the non-LTE stellar atmosphere code FASTWIND to analyze new and archival high-resolution data. Results. We determine (Teff, log g) values of (14 000 ± 500 K, 3.50 ± 0.15 dex) that, combined with the Gaia parallax, imply a spectroscopic mass, from log g, of 3.2+2.1-1.9 M⊙ and an evolutionary mass, assuming single star evolution, of 5.2+0.3-0.6 M⊙ . We determine an upper limit of 8 km s-1 for the projected rotational velocity and derive the surface abundances; we find the star to have a silicon abundance below solar, and to be significantly enhanced in nitrogen and iron and depleted in carbon and magnesium. Complementary evidence derived from a photometric extinction analysis and Gaia yields similar results for Teff and log g and a consistent distance around 2 kpc. Conclusions. We propose that the B-type star is a slightly evolved main sequence star of 3-5 M⊙ with surface abundances reminiscent of diffusion in late B/A chemically peculiar stars with low rotational velocities. There is also evidence for CN-processed material in its atmosphere. These conclusions rely critically on the distance inferred from the Gaia parallax. The goodness of fit of the Gaia astrometry also favors a high-inclination orbit. If the orbit is edge-on and the B-type star has a mass of 3-5 M⊙ , the mass of the dark companion would be 4-5 M⊙ , which would be easier to explain with our current stellar evolutionary models.
Bibliographical noteFunding Information:
Acknowledgements. We thank R.-P. Kudritzki for his interesting suggestions for improvement of the first version of this Letter. We thank J. Hernández for his help with Gaia DR2 data. S-SD, DJL, AdB, AHD and JMA acknowledges support from the Spanish Government Ministerio de Ciencia, Innovación y Uni-versidades through grants PGC2018-095 049-B-C22 and PGC-2018-091 3741-B-C22. JIGH acknowledges financial support from the Spanish Ministry of Science, Innovation and Universities (MICIU) under the 2013 Ramón y Cajal program RYC-2013-14875. JIGH and CAP acknowledge financial support from the Spanish Ministry project MICIU AYA2017-86389-P. SJS acknowledges funding from STFC Grant ST/P000312/1. Based on observations made with the Telescopio Nationale Galileo (TNG) and the Gran Telescopio Canarias (GTC), installed at the Spanish Observatorio del Roque de los Muchachos of the Insti-tuto de Astrofs´ica de Canarias, in the island of La Palma. This research has made use of the Keck Observatory Archive (KOA), which is operated by the W. M. Keck Observatory and the NASA Exoplanet Science Institute (NExScI), under
contract with the National Aeronautics and Space Administration. Guoshoujing Telescope (the Large Sky Area Multi-Object Fiber Spectroscopic Telescope LAMOST) is a National Major Scientific Project built by the Chinese Academy of Sciences. Funding for the project has been provided by the National Development and Reform Commission. LAMOST is operated and managed by the National Astronomical Observatories, Chinese Academy of Sciences.
© ESO 2020.
Copyright 2020 Elsevier B.V., All rights reserved.
- Binaries: spectroscopic
- Stars: abundances
- Stars: black holes
- Stars: early-type
- Stars: fundamental parameters
- Techniques: spectroscopic
ASJC Scopus subject areas
- Astronomy and Astrophysics
- Space and Planetary Science