Astron. Astrophys. 359, 1011-1024 (2000)

NLTE calculations of neutral helium lines in the wind of the Herbig Ae star AB Aurigae

J.-C. Bouret ^1,2 and C. Catala <sup>1

1</sup> Laboratoire d'Astrophysique de Toulouse, CNRS UMR 5572, Observatoire Midi-Pyrénées, 14, avenue Edouard Belin, 31400 Toulouse, France
² NASA/Goddard Space Flight Center, Code 685, Greenbelt, MD 20771, USA

Received 17 January 2000 / Accepted 25 May 2000

Abstract

In order to improve our knowledge about the structure of the winds of the Herbig Ae/Be stars and the mechanisms that drive them, we performed a NLTE analysis of the formation of helium lines in the wind of AB Aur, the prototype of the whole class. These lines are indeed expected to form in the densest layers of the envelope, at the very base of the wind, and therefore to provide constraints on these regions where the wind originates. A large observational data set is now available which we have extensively used to deduce strong constraints on the physics that prevail in the inner part of the wind. Among these data, we particularly used those from the MUSICOS 1996 campaign, which show that the He I D3 ( Å) line is made of two distincts components: a blue-shifted emission component, and a redshifted component alternatively seen as an absorption or an emission.

The classical model for AB Aur's wind (Bouret & Catala 1998; Catala & Kunasz 1987) fails to reproduce the observed profile. We find that the observations can be explained by a two component model in which the base of the wind includes dense and hot clumps of outflowing material, while the bulk of the wind is still described by the model by Bouret & Catala (1998). This clumpy structure can result from the presence of co-rotating interaction regions which form where fast streams in the wind overtake slow streams. Our results show that a blue-shifted

emission is formed in such conditions, and can be rotationally modulated as observed, due to fast and slow streams alternating on the line of sight as the star rotates.

Our model can also explain the occasional presence of an absorption component near rest wavelength, as formed in the bulk of the extended chromosphere. Concerning the red emission component, which is occasionally observed instead of the absorption one, we argue that it may arise from polar downflows.

Key words: line: formation line: profiles stars: atmospheres stars: chromospheres stars: mass-loss stars: pre-main sequence

Send offprint requests to: J.-C. Bouret (bouret@winds.gsfc.nasa.gov)

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Contents

• 1. Introduction
• 2. The He I lines in AB Aur
• 3. Modelling the He I - He II spectrum
  • 3.1. Wind model and radiative transfer
  • 3.2. The atomic model
• 4. Results
  • 4.1. The standard BC model
  • 4.2. Departures from the standard BC model
    • 4.2.1. Results with a hot (T 140,000 K) region
    • 4.2.2. Formation of a blue emission component in the He I D3 line
    • 4.2.3. Constraints from the He I 4471 Å line
  • 4.3. A two-component model
  • 4.4. The red emission: a three-component model
• 5. Conclusion
• Acknowledgements
• References

© European Southern Observatory (ESO) 2000

Online publication: July 13, 2000
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