Radio and submillimetre observations of ϵ Ori

¹ Royal Observatory of Belgium, Ringlaan 3, 1180 Brussel, Belgium
² Department of Physics & Astronomy, University College London, Gower Street, London WC1E 6BT, UK

Corresponding author: R. Blomme, Ronny.Blomme@oma.be

Received: 16 October 2001
Accepted: 7 November 2001

Abstract

In common with other early-type stars, ϵ Ori (B0 Ia) shows evidence for structure in its stellar wind. Variations in optical and ultraviolet line profiles reveal the presence of large-scale structure in the inner wind. The detection of X-rays and the existence of black troughs in saturated ultraviolet lines are indicative of small-scale structure. The geometric extent of both types of structure is poorly known. In principle, large-scale structure can be detected directly from very high spatial resolution observations that resolve the stellar wind. A simpler technique is to look for the presence of additional flux compared to that expected from a smooth wind. The run of this excess flux as a function of wavelength indicates how fast structure decays in the wind. If there is variability in the excess flux, it shows us that the structure must be large-scale. Such variability is suggested by two previous 6 cm radio observations of ϵ Ori: Abbott et al. ([CITE]) found mJy, while Scuderi et al. ([CITE]) measured only mJy. This could indicate that the large-scale structure persists beyond . To further investigate this variability, we used the Very Large Array (VLA) to monitor ϵ Ori over a 5-day period in February 1999. We supplemented our data with observations from the VLA archive. In an attempt to resolve the stellar wind, we also obtained a series of high spatial resolution observations with the Multi-Element Radio Linked Interferometer Network (MERLIN) during January–March 1999. From this combined material we find no evidence for variability and we conclude that the Abbott et al. ([CITE]) flux determination is in error. The data do show substantial excess flux at millimetre wavelengths, compared to a smooth wind. This excess is confirmed by a submillimetre observation which we obtained with the James Clerk Maxwell Telescope (JCMT). The behaviour of ϵ Ori is therefore similar to what had been found previously for α Cam, δ Ori A, κ Ori and ζ Pup. While the present data do not allow very strong constraints, they show that considerable structure must persist up to at least in the wind of ϵ Ori. The combined radio fluxes are used to derive a mass-loss rate of . This value is in good agreement with the Hα mass-loss rate. The good agreement between Hα and radio mass-loss rates for hot stars in general remains puzzling, as it implies that the same amount of structure is present in very different formation regions.