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标题：Wind properties of variable B supergiants
其他标题：Evidence of pulsations connected with mass-loss episodes★
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作者：M. Haucke ; L. S. Cidale ; R. O. J. Venero 等
期刊名称：Astronomy & Astrophysics
印刷版ISSN：0004-6361
电子版ISSN：1432-0746
出版年度：2018
卷号：614
DOI：10.1051/0004-6361/201731678
语种：English
出版社：EDP Sciences
摘要：Context.Variable B supergiants (BSGs) constitute a heterogeneous group of stars with complex photometric and spectroscopic behaviours. They exhibit mass-loss variations and experience different types of oscillation modes, and there is growing evidence that variable stellar winds and photospheric pulsations are closely related.Aims.To discuss the wind properties and variability of evolved B-type stars, we derive new stellar and wind parameters for a sample of 19 Galactic BSGs by fitting theoretical line profiles of H, He, and Si to the observed ones and compare them with previous determinations.Methods.The synthetic line profiles are computed with the non-local thermodynamic equilibrium (NLTE) atmosphere code FASTWIND, with aβ-law for hydrodynamics.Results.The mass-loss rate of three stars has been obtained for the first time. The global properties of stellar winds of mid/late B supergiants are well represented by aβ-law withβ> 2. All stars follow the known empirical wind momentum–luminosity relationships, and the late BSGs show the trend of the mid BSGs. HD 75149 and HD 99953 display significant changes in the shape and intensity of the Hαline (from a pure absorption to a P Cygni profile, and vice versa). These stars have mass-loss variations of almost a factor of 2.8. A comparison among mass-loss rates from the literature reveals discrepancies of a factor of 1 to 7. This large variation is a consequence of the uncertainties in the determination of the stellar radius. Therefore, for a reliable comparison of these values we used the invariant parameterQr. Based on this parameter, we find an empirical relationship that associates the amplitude of mass-loss variations with photometric/spectroscopic variability on timescales of tens of days. We find that stars located on the cool side of the bi-stability jump show a decrease in the ratioV∞∕Vesc, while their corresponding mass-loss rates are similar to or lower than the values found for stars on the hot side. Particularly, for those variable stars a decrease inV∞∕Vescis accompanied by a decrease inṀ.Conclusions.Our results also suggest that radial pulsation modes with periods longer than 6 days might be responsible for the wind variability in the mid/late-type. These radial modes might be identified with strange modes, which are known to facilitate (enhanced) mass loss. On the other hand, we propose that the wind behaviour of stars on the cool side of the bi-stability jump could fit with predictions of theδ−slow hydrodynamics solution for radiation-driven winds with highly variable ionization.
关键词：Key wordsenstars: early-typesupergiantsstars: mass-lossstars: winds, outflows