EDP Sciences
Free access
Volume 418, Number 2, May I 2004
Page(s) 727 - 736
Section Stellar atmospheres
DOI http://dx.doi.org/10.1051/0004-6361:20035638

A&A 418, 727-736 (2004)
DOI: 10.1051/0004-6361:20035638

Photospheric and stellar wind variability in  $\epsilon$ Ori (B0 Ia)

R. K. Prinja1, Th. Rivinius2, O. Stahl2, A. Kaufer3, B. H. Foing4, J. Cami5 and S. Orlando6

1  Department of Physics & Astronomy, University College London, Gower Street, London WC1E 6BT, UK
2  Landessternwarte Heidelberg, Königstuhl 12, 69117 Heidelberg, Germany
3  European Southern Observatory, Alonso de Cordova 3107, Casilla 19001, Santiago 19, Chile
4  Research Support Division, ESA RSSD, ESTEC/SCI-SR postbus 299, 2200 AG Noordwijk, The Netherlands
5  NASA Ames Research Center, Mail Stop 245-6, Moffett Field, CA 94035-1000, USA
6  INAF-Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, Palermo, Italy

(Received 6 November 2003 / Accepted 17 January 2004)

We provide direct observational evidence for a link between photospheric activity and perturbations in the dense inner-most stellar wind regions of the B supergiant star $\epsilon$ Ori. The results, which are relevant to our understanding of the origin of wind structure, are based on a multi-spectral line analysis of optical time-series data secured in 1998 using the HEROS spectrograph on the ESO Dutch 0.9-m telescope in La Silla. A period of ~1.9 days is consistently identified in Balmer, He I absorption, and weak metal lines such as Si III and C II. The primary characteristic is a large-amplitude swaying of the central absorption trough of the line, with differential velocities in lines formed at varying depths in the atmosphere. The variance resulting from the "S-wave" velocity behaviour of the lines is constrained within  $\pm$ the projected rotation velocity (~80 km s -1) in the weakest absorption lines, but extends blue-ward to over -200 km s -1 in H $\alpha$. A second (superimposed) 1.9 day signal is present at more extended blue-ward velocities (to ~-300 km s -1) in lines containing stronger circumstellar components. Inspection of archival optical data from 1996 provides evidence that this modulation signal has persisted for at least 2.5 years. Non-radial pulsational modelling is carried out in an attempt to reproduce the key observational characteristics of the line profile variability. Only limited success is obtained with prograde ( m=-1) modes. The principal S-wave pattern cannot be matched by these models and remains enigmatic.

Key words: stars: early-type -- stars: individual: $\epsilon$ Ori -- stars: mass-loss -- stars: oscillations

Offprint request: R. K. Prinja, rkp@star.ucl.ac.uk

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