Photospheric and stellar wind variability in ϵ Ori (B0 Ia) (original) (raw)

A&A 418, 727-736 (2004)

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

Corresponding author: R. K. Prinja, rkp@star.ucl.ac.uk

Received: 6 November 2003
Accepted: 17 January 2004

Abstract

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 ϵ 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 ± 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_α_. 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: ϵ Ori / stars: mass-loss / stars: oscillations

Based on observations obtained as part of the MUSICOS 98 campaign from ESO La Silla, Chile.