Excitation of Molecular Hydrogen in the Circumstellar Environment of
Herbig Ae/Be stars
Authors:
Claire Martin,
Laboratoire d'Astrophysique de Marseille, France
M. Deleuil,
Laboratoire d'Astrophysique de Marseille, France
J-C. Bouret,
Laboratoire d'Astrophysique de Marseille, France
T. Simon,
Institute for Astronomy, University of Hawaii, USA
C. Catala,
Observatoire de Paris-Meudon, Paris, France
J. Le Bourlot,
Observatoire de Paris-Meudon, Paris, France
A. Lecavelier
des Etangs, Institut d'Astrophysique de Paris, Paris, France
A. Vidal-Madjar,
Institut d'Astrophysique de Paris, Paris, France
A. Roberge,
Carnegie Institution of Washington, Washington, DC, USA
P.D. Feldman,
Department of Physics and Astronomy, JHU, Baltimore, MD, USA
Abstract:
We present FUSE
observations of a sample of Herbig Ae/Be stars (HAeBes) spanning the
spectral range A1-B2. HAeBes are the likely precursors of the
Beta-Pictoris and Vega-type stars, but the nature and the evolution of
their close environment remain the subject of considerable controversy.
Molecular hydrogen is the most abundant molecule in the circumstellar
environment of young stars, and the analysis of H2 lines in the FUSE
spectral domain allows measurement of the amount of gas surrounding the
star. Our analysis demonstrates that the excitation of H2 is clearly
different around HAeBes than in the interstellar medium. Moreover, the
characteristics of H2 around Herbig Ae and Be stars gives evidence of
different mechanisms of excitation. Indeed, for the most massive stars
of our sample (B8 to B2 type) the excitation diagrams are similar to
that obtained for photodissociation regions, while those of the Ae
stars (A1 to B9 type) correspond to more collisionally excited media,
very close to the stars. This suggests structural differences between
Herbig Ae and Be stars' environments which could be explained by a
faster evolution of the latter combined with stronger radiation fields.