EDP Sciences
Free access
Volume 457, Number 3, October III 2006
Page(s) 987 - 994
Section Stellar structure and evolution
DOI http://dx.doi.org/10.1051/0004-6361:20065114

A&A 457, 987-994 (2006)
DOI: 10.1051/0004-6361:20065114

The superimposed photospheric and stellar wind variability of the O-type supergiant $\mathsf{\alpha}$ Camelopardalis

R. K. Prinja1, N. Markova2, S. Scuderi3 and H. Markov2

1  Department of Physics & Astronomy, University College London, Gower Street, London WC1E 6BT, UK
    e-mail: rkp@star.ucl.ac.uk
2  Institute of Astronomy, Bulgarian National Astronomical Observatory, PO Box 136, 4700 Smoljan, Bulgaria
    e-mail: [nmarkova;hmarkov]@astro.bas.bg
3  INAF - Osservatorio Astrofisico di Catania, via S. Sofia 78, 95123 Catania, Italy
    e-mail: scuderi@oact.inaf.it

(Received 28 February 2006 / Accepted 14 June 2006)

Aims.This study seeks to provide empirical constraints on the different physical components that can yield temporal variability in predominantly or partially wind-formed optical lines of luminous OB stars, and thus potentially affect the reliable determination of fundamental parameters, including mass-loss rates via clumped winds.
Methods.Using time-series spectroscopy from epochs spread over ~4 years, we present a case study of the O9.5 supergiant $\alpha$ Cam. We demonstrate that the HeI $\lambda$5876 (23P0-33D) line is an important diagnostic for photospheric and wind variability in this star. The actions of large radial velocity shifts (up to ~30 km s-1) in the photospheric absorption lines can also affect the morphology of the H $_{\rm\alpha}$ line profile, which is commonly used for measuring mass-loss rates in massive stars.
Results.We identify a 0.36-day period in subtle absorption profile changes in HeI $\lambda$5876, which likely reveals the photospheric structure, perhaps due to low-order non-radial pulsations. This signal persists over ~2 months, but it is not present 2 years later (November 2004); it is also not seen in the stellar wind components of the line profiles. Using a pure H $_{\rm\alpha}$ line-synthesis code we interpret maximum changes in the redward and peak emission of $\alpha$ Cam in terms of mass-loss rate differences in the range ~5.1 $\times$ 10-6 to 6.5 $\times$ 10 $^{-6}~M_\odot$ yr-1. However, the models generally fail to reproduce the morphology of blueward (possibly absorptive) regions of the profiles.
Conclusions.The optical line profiles of $\alpha$ Cam are affected by (i) deep-seated fluctuations close to, or at, the photosphere, (ii) atmospheric velocity gradients, and (iii) large-scale stellar wind structure. This study provides new empirical perspectives on accurate line-synthesis modelling of stellar wind signatures in massive luminous stars.

Key words: stars: early-type -- stars: mass-loss -- stars: individual: $\alpha$ Camelopardalis

© ESO 2006