Photospheric and stellar wind variability in ϵ Ori (B0 Ia) ^*

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 Hα. 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 () modes. The principal S-wave pattern cannot be matched by these models and remains enigmatic.