000005382 001__ 5382
000005382 005__ 20210716113335.0
000005382 0247_ $$2DOI$$a10.1051/0004-6361/202038275
000005382 037__ $$aSCART-2021-0103
000005382 100__ $$aShenar, T.  
000005382 245__ $$aThe "hidden" companion in LB-1 unveiled by spectral disentangling
000005382 260__ $$c2020
000005382 520__ $$aContext. The intriguing binary LS V +22 25 (LB-1) has drawn much attention following claims of it being a single-lined spectroscopic binary with a 79-day orbit comprising a B-type star and a ≈70 M⊙ black hole - the most massive stellar black hole reported to date. Subsequent studies demonstrated a lack of evidence for a companion of such great mass. Recent analyses have implied that the primary star is a stripped He-rich star with peculiar sub-solar abundances of heavy elements, such as Mg and Fe. However, the nature of the secondary, which was proposed to be a black hole, a neutron star, or a main sequence star, remains unknown.  Aims: Based on 26 newly acquired spectroscopic observations secured with the HERMES and FEROS spectrographs covering the orbit of the system, we perform an orbital analysis and spectral disentangling of LB-1 to elucidate the nature of the system.  Methods: To derive the radial velocity semi-amplitude K2 of the secondary and extract the spectra of the two components, we used two independent disentangling methods: the shift-and-add technique and Fourier disentangling with FDBinary. We used atmosphere models to constrain the surface properties and abundances.  Results: Our disentangling and spectral analysis shows that LB-1 contains two components of comparable brightness in the optical. The narrow-lined primary, which we estimate to contribute ≈55% in the optical, has spectral properties that suggest that it is a stripped star: it has a small spectroscopic mass (≈1 M⊙) for a B-type star and it is He- and N-rich. Unlike previous reports, the abundances of heavy elements are found to be solar. The "hidden" secondary, which contributes about 45% of the optical flux, is a rapidly rotating (vsini ≈ 300 km s-1) B3 V star with a decretion disk - a Be star. As a result of its rapid rotation and dilution, the photospheric absorption lines of the secondary are not readily apparent in the individual observations. We measure a semi-amplitude for this star of K2 = 11.2 ± 1.0 km s-1 and adopting a mass of M2 = 7 ± 2 M⊙ typical for B3 V stars, we derive an orbital mass for the stripped primary of M1 = 1.5 ± 0.4 M⊙. The orbital inclination of 39 ± 4° implies a near-critical rotation for the Be secondary (veq ≈ 470 km s-1).  Conclusions: LB-1 does not contain a compact object. Instead, it is a rare Be binary system consisting of a stripped star (the former mass donor) and a Be star rotating at near its critical velocity (the former mass accretor). This system is a clear example that binary interactions play a decisive role in the production of rapid stellar rotators and Be stars.
000005382 594__ $$aNO
000005382 700__ $$aBodensteiner, J. 
000005382 700__ $$aAbdul-Masih, M. 
000005382 700__ $$aFabry, M.
000005382 700__ $$aMahy, L. 
000005382 700__ $$aMarchant, P. 
000005382 700__ $$aBanyard, G.
000005382 700__ $$aBowman, D. M.  
000005382 700__ $$aDsilva, K.
000005382 700__ $$aHawcroft, C.
000005382 700__ $$aReggiani, M.
000005382 700__ $$aSana, H.
000005382 773__ $$c7$$nL6$$pAstronomy & Astrophysics$$v639$$y2020
000005382 8560_ $$flaurent.mahy@ksb-orb.be
000005382 905__ $$apublished in
000005382 980__ $$aREFERD