Toward Understanding The B[e] Phenomenon: V. Nature and Spectral Variations of the MWC 728 Binary System

• Published in: arXiv.org
• Main Authors: Miroshnichenko, A S, Zharikov, S V, Danford, S, Manset, N, Korčáková, D, Kříček, R, Šlechta, M, Omarov, Ch T, Kusakin, A V, Kuratov, K S, Grankin, K N
• Format: Paper Journal Article
• Language: English
• Published: Ithaca Cornell University Library, arXiv.org 05.08.2015
• Subjects: Absorption; Binary stars; Binary system; Interstellar extinction; Near infrared radiation; Physics - Solar and Stellar Astrophysics; Stellar winds
• ISSN: 2331-8422
• DOI: 10.48550/arxiv.1508.00950

We report the results of a long-term spectroscopic monitoring of the FS\,CMa type object MWC\,728. We found that it is a binary system with a B5 Ve (T\(_{\rm eff}\) = 14000\(\pm\)1000 K) primary and a G8 III type (T\(_{\rm eff} \sim\) 5000 K) secondary. Absorption line positions of the secondary vary with a semi-amplitude of \(\sim\)20 km/s and a period of 27.5 days. The system's mass function is 2.3\(\times10^{-2}\) M\(_\odot\), and its orbital plane is \(13^{\circ}-15^{\circ}\) tilted from the plane of the sky. The primary's \(v \sin i \sim\)110 km/s combined with this tilt implies that it rotates at a nearly breakup velocity. We detected strong variations of the Balmer and He I emission-line profiles on timescales from days to years. This points to a variable stellar wind of the primary in addition to the presence of a circum-primary gaseous disk. The strength of the absorption-line spectrum along with the optical and near-IR continuum suggest that the primary contributes \(\sim\)60% of the \(V\)--band flux, the disk contributes \(\sim\)30%, and the secondary \(\sim\)10%. The system parameters, along with the interstellar extinction, suggest a distance of \(\sim\)1 kpc, that the secondary does not fill its Roche lobe, and that the companions' mass ratio is \(q \sim\)0.5. Overall, the observed spectral variability and the presence of a strong IR-excess are in agreement with a model of a close binary system that has undergone a non-conservative mass-transfer.