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Astron. Astrophys. 345, 884-904 (1999)

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1. Introduction

The Herbig Ae/Be stars are pre-main sequence objects with masses ranging from 2 to 5 [FORMULA]. A very significant fraction of them show conspicuous signs of strong stellar winds, such as P Cygni profiles at H[FORMULA] and Mg II h & k, and of chromospheric activity, such as the presence of C IV and Si IV resonance lines, emission in Ca II K & the infrared triplet, and in the He I 5876 Å lines (Catala et al. 1986a).

AB Aurigae is the brightest Herbig Ae star of the northern hemisphere, and is often considered as the prototype of the whole class. A detailed analysis of its line profiles and continua led to a model of its outer layers, including a wind with a mass loss rate of [FORMULA] [FORMULA] yr-1, and an extended chromosphere with a maximum temperature of 17,000 K overlying a photosphere at 10,000 K (Catala & Kunasz 1987).

This wind is certainly not spherically symmetric. Indeed, the rotational modulation of lines formed in the wind (Mg II resonance lines, Ca II K) was interpreted as due to co-rotating streams, organized in an azimuthal structure controlled by a surface magnetic field, by analogy with the structure of the solar wind (Praderie et al. 1986; Catala et al. 1986b).

Besides, the changes of the H[FORMULA] line profile from a type II P Cygni profile to a single emission was interpreted by Pogodin (1992) in terms of an equatorial wind model, in which the opening angle of the wind is variable and controlled by a magnetic field.

The picture which emerges from these previous analyses is that of a complex wind, with both a latitudinal and azimuthal structure, probably controlled by a magnetic field. However, this model is far from being well established and needs observational confirmation. Furthermore, no magnetic detection of AB Aur has been reported so far, in spite of several attempts.

The MUSICOS 96 campaign on AB Aur constituted a major effort to better understand the photosphere and wind of this star, and a further attempt at detecting directly a surface magnetic field. The main goal of the AB Aur observations during the MUSICOS 96 campaign was the monitoring of photospheric lines, and a selection of chromospheric and wind lines, such as the He I D3 lines, and the H[FORMULA] line.

The present paper deals with the short-term variability of these lines. In Sect. 2, we review the conclusions of previous related work. The observations and data reduction procedures are presented in Sect. 3. The observed variability is described in Sect. 4, and discussed in Sect. 5 in terms of a photosphere-wind model involving localized outflows affecting the formation of photospheric lines and He I D3 lines. A general conclusion is given in Sect. 6.

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© European Southern Observatory (ESO) 1999

Online publication: April 28, 1999