EDP Sciences
Free access
Volume 382, Number 3, February II 2002
Page(s) 1032 - 1041
Section Stellar atmospheres
DOI http://dx.doi.org/10.1051/0004-6361:20011649

A&A 382, 1032-1041 (2002)
DOI: 10.1051/0004-6361:20011649

Evidence for a connection between photospheric and wind structure in HD 64760

A. Kaufer1, R. K. Prinja2 and O. Stahl3

1  European Southern Observatory, Alonso de Cordova 3107, Casilla 19001, Santiago 19, Chile
2  Department of Physics & Astronomy, University College London, Gower Street, London WC1E 6BT, UK
3  Landessternwarte Heidelberg, Königstuhl 12, 69117 Heidelberg, Germany

(Received 2 August 2001 / Accepted 16 November 2001 )

We report on the results of an extended optical spectroscopic monitoring campaign on the early-type B supergiant HD 64760 (B0.5 Ib). The study is based on high-resolution echelle spectra obtained with the Landessternwarte Heidelberg's HEROS instrument at ESO La Silla. Ninety-nine spectra were collected over 103 nights between January 19 and May 1, 1996. The H $\alpha$ line shows a characteristic profile with a central photospheric absorption superimposed by symmetrically blue- and red-shifted wind-emission humps. The time-averaged line profile is well described by a differentially rotating and expanding radiation-driven wind: the redistribution of the wind emission flux into a double peak profile is interpreted in terms of the resonance zone effect in rotating winds as first described by Petrenz & Puls  (1996). Detailed time-series analyses of the line profile variations across the H $\alpha$ profile reveal for the first time in an optical data set of HD 64760 a periodic 2.4-day modulation of the inner and outer flanks of the H $\alpha$ emission humps. The stronger modulations of the inner flanks of the emission humps at photospheric velocities are due to complex width variations of the underlying photospheric H $\alpha$ profile. The weaker variations of the outer flanks are in phase and reflect variations at the base of the stellar wind. The detected 2.4-day modulation period together with a second period of 1.2 days (in the red emission hump only) is in excellent agreement with the outer-wind modulation periods as reported by Fullerton et al. (1997) from intensive IUE UV time-series observations in 1993 and 1995. The 2.4-day period is further detected in the photospheric $\ion{He}{i}$  $\lambda4026$ line as prograde traveling (pseudo-)absorption and emission features. The observed variability pattern is indicative for low-order non-radial pulsations in the photosphere of HD 64760 . The non-radial pulsations are identified as the source of persistent, regularly spaced stellar surface structure which is maintained throughout the photosphere - wind transition zone (this work) out into the UV regime of the terminal velocity outflow.

Key words: stars: early-type -- stars: individual ( HD 64760 ) -- stars: mass loss -- stars: rotation -- stars: oscillation

Offprint request: A. Kaufer, akaufer@eso.org

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