Context. The intriguing binary
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.
Astronomy and Astrophysics
- Pub Date:
- July 2020
- stars: early-type;
- stars: emission-line;
- binaries: spectroscopic;
- binaries: close;
- stars: individual: LS V+22 25;
- Astrophysics - Solar and Stellar Astrophysics
- Accepted to A&