Ref: SCART-2021-0103

The "hidden" companion in LB-1 unveiled by spectral disentangling

Shenar, T. ; Bodensteiner, J. ; Abdul-Masih, M. ; Fabry, M. ; Mahy, L. ; Marchant, P. ; Banyard, G. ; Bowman, D. M. ; Dsilva, K. ; Hawcroft, C. ; Reggiani, M. ; Sana, H.

published in Astronomy & Astrophysics, 639 issue L6, pp. 7 (2020)

Abstract: Context. 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.

DOI: 10.1051/0004-6361/202038275

The record appears in these collections:
Royal Observatory of Belgium > Astronomy & Astrophysics
Science Articles > Peer Reviewed Articles

 Record created 2021-07-16, last modified 2021-07-16