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

Abstract

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${}_{\mathrm{eff}}$ = 14000$\pm$1000 K) primary and a G8 III type (T${}_{\mathrm{eff}}\sim$ 5000 K) secondary. Absorption line positions of the secondary vary with a semi-amplitude of $\sim$20 km/s and a period of 27.5 days. The system's mass function is 2.3$\times 10^{-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.

13 pages, 16 figures, 5 tables, accepted in The Astrophysical Journal

References in corpus (4)

• Effects of gravitational darkening on the determination of fundamental parameters in fast rotating B-type stars
• Results of the ROTOR-program. II. The long-term photometric variability of weak-line T Tauri stars
• Spin-up and hot spots can drive mass out of a binary
• First detections of FS Canis Majoris stars in clusters. Evolutionary state as constrained by coeval massive stars

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