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

TL;DR

This study presents long-term spectroscopic observations of the MWC 728 binary system, revealing its components, orbital parameters, and spectral variability, supporting a non-conservative mass transfer model in a close binary system.

Contribution

First detailed spectroscopic analysis of MWC 728 revealing its binary nature, orbital parameters, and spectral variability, advancing understanding of FS CMa type objects.

Findings

MWC 728 is a binary with a B5 Ve primary and G8 III secondary.

The secondary's absorption lines vary with a 27.5-day period.

The primary rotates near breakup velocity and shows variable stellar wind.

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$_{\rm eff}$ = 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.