Wind structure of late B supergiants

I. Multi-line analyses of near-surface and wind structure in HD 199 478 (B8 Iae)

¹ Institute of Astronomy, National Astronomical Observatory, Bulgarian Academy of Sciences, PO Box 136, 4700 Smoljan, Bulgaria e-mail: [nmarkova;hmarkov]@astro.bas.bg
² Department of Physics & Astronomy, UCL, Gower Street, London WC1E 6BT, UK e-mail: rkp@star.acl.uk
³ Tartu Observatory, Toravere 61602, Estonia e-mail: indrek@aai.ee
⁴ Ritter Observatory, The University of Toledo, Toledo, OH 43606, USA e-mail: NMORRIS2@uoft02.utoledo.edu
⁵ Department of Astronomy and Astrophysics, University of Toronto, Toronto ON M5S 3H4, Canada e-mail: jpercy@utm.utoronto.ca
⁶ Department of Physics, The Citadel, Charleston SC 29409-0270, USA e-mail: adelmans@citadel.edu

Received: 9 January 2008
Accepted: 21 April 2008

Abstract

Aims. We provide a quantitative analysis of time-variable phenomena in the photospheric, near-star, and outflow regions of the late-B supergiant (SG) HD 199 478. This study aims to provide new perspectives on the nature of outflows in late-B SGs and on the influence of large-scale structures rooted at the stellar surface.

Methods. The analysis is based primarily on optical spectroscopic datasets secured between 1999 and 2000 from the Bulgarian NAO, Tartu, and Ritter Observatories. The acquired time-series samples a wide range of weak metal lines, He i absorption, and both emission and absorption signatures in Hα. Non-LTE line synthesis modelling is conducted using FASTWIND for a strategic set of late-B SGs to constrain and compare their fundamental parameters within the context of extreme behaviour in the Hα lines.

Results. The temporal behaviour of HD 199 478 is characterised by three key empirical properties: (i) systematic central velocity shifts in the photospheric absorption lines, including C ii and He i, over a characteristic time-scale of ~20 days; (ii) extremely strong, variable Hα emission with no clear modulation signal; and (iii) the occurrence in 2000 of a (rare) high-velocity absorption (HVA) event in Hα, which evolved over ~60 days, showing the clear signature of mass infall and outflows. In these properties HD 199 478 resembles few other late-B SGs with peculiar emission and HVAs in Hα (HD 91 619, HD 34 085, HD 96919). Different possibilities accounting for the phenomenon observed are indicated and briefly discussed.

Conclusions. At the cooler temperature edge of B SGs, there are objects whose wind properties, as traced by Hα, are inconsistent with the predictions of the smooth, spherically symmetric wind approximation. This discordance is still not fully understood and may highlight the role of a non-spherical, disk-like, geometry, which may result from magnetically-driven equatorial compression of the gas. Ordered dipole magnetic fields may also lead to confined plasma held above the stellar surface, which ultimately gives rise to transient HVA events.