Azimuthal structures in the wind and chromosphere of the Herbig Ae star AB Aurigae. Results from the MUSICOS 1992 campaign^*

¹ European Southern Observatory, Karl-Schwarzschildstr. 2, 85748 Garching, Germany
² Département d'Astrophysique Extragalactique et de Cosmologie et Unité de Recherche associée au CNRS 173 et à l'Université Paris 7, Observatoire de Paris, Section de Meudon, 92195 Meudon Principal Cedex, France
³ Laboratoire d'Astrophysique et Unité de Recherche associée au CNRS 285, Observatoire Midi-Pyrénées, France
⁴ Pennsylvania State University, Department of Astronomy and Astrophysics, 525 Davey Laboratory, University Park, PA 16802, U.S.A.
⁵ Armagh Observatory, Armagh BT61 9DG, N. Ireland, UK
⁶ University of New South Wales, Australia
⁷ School of Physics & Astronomy, University of St Andrews, North Haugh, St Andrews, Fife, Scotland KY16 9SS
⁸ Solar System Division, ESA Space Science Department, ESTEC (SO), postbus 299, NL-2200 AG, Noordwijk, The Netherlands
⁹ Indian Institute of Astrophysics, Vainu Bappu Observatory, Kavalur, Alangayam 635701, Tamilnadu, India
¹⁰ Beijing Astronomical Observatory, China
¹¹ CSIRO Division of Radiophysics, Epping, NSW 2121, Australia
¹² Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu HI 96822, U.S.A.
¹³ LAEFF/INTA, P.O. Box 50727, 28080 Madrid, Spain

Send offprint request to: T. Böhm (German address)

Received: 22 February 1996
Accepted: 2 May 1996

Abstract

The observations of the MUSICOS 1992 campaign concerned three scientific programs, among which was the study of azimuthal structures in the wind and chromosphere of the pre-main sequence Herbig Ae star AB Aur. The He I 5876 Åline of AB Aur, which is formed in the expanding chromosphere of this star, most probably in the innermost parts of its wind, was continuously monitored at a spectral resolution of 30000 for about 4 days. The line was discovered to be variable in a spectacular way, the profile changing from pure emission to a composite profile including a deep absorption component in the course of a few hours. In this paper we present strong clues that suggest a rotational modulation of the line, with a period corresponding to the stars rotation period of 32 hrs. We confirmed the covariation of several non-photospheric spectral lines, formed at very different radial distances of the star, which strengthens the idea that the variability is the signature of azimuthal structures in the wind of AB Aur, most probably due to magnetically confined streams or loops emanating from the stellar surface. We present the data collected during the MUSICOS 1992 campaign, but also a data set of the He I D3 line obtained with FOE at KPNO during the years 1991–1994 and a series of IUE spectra containing the Mg II h & k UV lines obtained one month prior to the campaign. A thorough discussion of the possible interpretations of the spectacular variations of the He I 5876 Å line is presented.