A&A 390, 1049-1061 (2002)
DOI: 10.1051/0004-6361:20020741

A chromospheric scenario for the activity of  Pictoris, as revealed by FUSE

J.-C. Bouret¹, M. Deleuil^{1, 2}, T. Lanz^{3, 4}, A. Roberge⁵, A. Lecavelier des Etangs⁶ and A. Vidal-Madjar<sup>6

1</sup>  Laboratoire d'Astrophysique de Marseille, Traverse du Siphon, BP 8, 13376 Marseille Cedex 12, France
²  Université de Provence, Centre de Mathématiques et d'Informatique, 39 rue J. Curie, 13453 Marseille Cedex 13, France
    e-mail: Magali.Deleuil@astrsp-mrs.fr
³  Laboratory for Astronomy and Solar Physics, Code 681, NASA Goddard Space Flight Center, Greenbelt MD 20771, USA
⁴  Department of Astronomy, University of Maryland, College Park, MD 20742, USA
    e-mail: lanz@stars.gsfc.nasa.gov
⁵  Department of Physics and Astronomy, Johns Hopkins University, 3400 N. Charles St., Baltimore MD 21218, USA
    e-mail: akir@pha.jhu.edu
⁶  Institut d'Astrophysique de Paris, CNRS, 98bis Bd Arago, 75014 Paris, France
    e-mail: lecaveli@iap.fr
    e-mail: alfred@iap.fr

(Received 5 March 2002 / Accepted 22 April 2002 )

Abstract
We investigate a chromospheric scenario to explain the characteristics of the far-UV emission lines of  Pictoris, revealed by FUSE spectra of this famous circumstellar disk system. The model assumes a thin region heated up to a few 10 ⁵ K located close to the stellar photosphere. The resonance lines of at 977 Å and at 1032-1037 Å, seen in emission, are produced in this chromosphere-transition region complex. Using complementary data in the mid and near UV (specifically, a resonance doublet of at 1548-1550 Å and h & k), we show that the whole dataset is remarquably well reproduced by the model. In addition, we investigate the properties of the * multiplet at 1176 Å and conclude that this line likely forms in a weak warm wind, originating from the prominent circumstellar disk of  Pictoris. Finally, radiative losses have been calculated and have provided estimates of the amount of non radiative energy dissipated in the external atmosphere of  Pictoris, which is a measure of the star's activity level. Such behavior for an A5 V star presents a challenge for both evolution and activity models since the former predict that main-sequence A stars should not be active, while the latter are unable to quantitatively account for the characteristics of the chromospheric heating of  Pictoris.

Key words: line: profiles -- stars: atmospheres -- stars: chromospheres -- stars: activity -- stars: winds, outflows

Offprint request: J.-C. Bouret, Jean-Claude.Bouret@astrsp-mrs.fr

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