Ultraviolet Observations of the Hydrogen Deficient Variable Star Mv-Sagittarii

Abstract

IUE observations of the hydrogen-deficient irregular variable star MV Sgr obtained in 1980 June-October and also in 1979 November are discussed. These observations show a prominent λ 2200 absorption feature. A value ofE(B — V) = 0.55 is deduced from the strength of λ 2200 band assuming that this absorption is caused by interstellar medium. The dereddened continuum obtained at different times can be fitted to a theoretical energy distribution of a helium star model with T_eff = 18000 K and log g = 2.5, similar to that of BD + 10^‡ 2179. This theoretical energy distribution, after applying interstellar extinction, givesV = 12.7 mag, agreeing with the observed visual magnitude of ≃ 13 in 1979 November and 1980 June-October. Even though there was no change in the continuum flux, the ultraviolet line-spectrum shows variations. The IUE spectra of 1980 October show enhanced (circumstellar) absorption lines of Fe II, Si II, O I, C I and others along with the absorption lines of a B star. In view of the similarity of the spectroscopic phenomena of MV Sgr with that of α Sco system, a model is proposed in which a cool companion star, surrounded by dust, occasionally blows gas towards the hotter hydrogen-poor B star. This model explains the irregular light variations and the spectroscopic phenomena.