Photospheric and stellar wind variability in ɛ Ori (B0 Ia)

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 ɛ 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 (m=-1) modes. The principal S-wave pattern cannot be matched by these models and remains enigmatic.

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