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Paper: Short-Term Spectral Variability in the Binary FS CMa-Type Object MWC 728
Volume: 508, The B[e] Phenomenom: Forty Years of Studies
Page: 295
Authors: Zharikov, S. V.; Miroshnichenko, A. S.
Abstract: We report the results of a long-term spectroscopic monitoring of the FS CM–type object MWC 728. We found that it is a binary system with a B5 ve (T1,eff = 14000±1000 K) primary and a G8 III (T2,eff ∼ 5000 K) secondary. Absorption lines of the cool star show regular variations with a semi-amplitude of ∼20 km s-1 and a period of 27.5 days. The system mass function is 2.3×10–2 M, and its orbital plane is ∼ 13–15° tilted from the plane of the sky. The hot star has a projected rotational velocity of ∼110 km s-1 which implies a nearly breakup rotation at the equator. We detected strong variations of the Balmer and He I emission-line profiles on timescales from days to years. This points out to a variable stellar wind of the hot star 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 hot star contributes ∼60% of the V–band flux, the disk contributes ∼30%, and the cool star ∼10%.The binary system parameters, along with the interstellar extinction, suggest a distance of ∼1 kpc, that the cool star radius (∼8 R) is smaller than its Roche lobe, and that the companions' mass ratio is q ∼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.
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