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Statistical properties of a sample of periodically variable B-type supergiants. Evidence for opacity-driven gravity-mode oscillations
Aims.We have studied a sample of 28 periodically variable B-typesupergiants selected from the HIPPARCOS mission and 12 comparison starscovering the whole B-type spectral range. Our goal is to test if theirvariability is compatible with opacity-driven non-radialoscillations. Methods: .We have used the NLTE atmosphere codeFASTWIND to derive the atmospheric and wind parameters of the completesample through line profile fitting. We applied the method to selectedH, He, and Si line profiles, measured with the high resolution CESspectrograph attached to the ESO CAT telescope in La Silla, Chile.Results: .From the location of the stars in the (log T_eff, log g)diagram, we suggest that variability of our sample supergiants is indeeddue to the gravity modes resulting from the opacity mechanism. We findnine of the comparison stars to be periodically variable as well, andsuggest them to be new α Cyg variables. We find marginal evidenceof a correlation between the amplitude of the photometric variabilityand the wind density. We investigate the wind momentum-luminosityrelation for the whole range of B spectral type supergiants, and findthat the later types (>B5) perfectly follow the relation for Asupergiants. Additionally, we provide a new spectral type - T_effcalibration for B supergiants. Conclusions: .Our results imply thepossibility of probing internal structure models of massive stars ofspectral type B through seismic tuning of gravity modes.Figures of the spectral line fits and discussion of the individualobjects, Appendices A, B and Table 6 are only available in electronicform at http://www.aanda.org
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