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Astron. Astrophys. 353, 276-286 (2000)


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CO in the circumstellar disks of Vega and [FORMULA] Pictoris

Inga Kamp 1 and Frank Bertoldi 2

1 Institut für Theoretische Physik und Astrophysik, Universität Kiel, 24098 Kiel, Germany
2 Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany

Received 24 February 1999 / Accepted 12 October 1999

Abstract

Vega and [FORMULA] Pictoris are the most prominent A stars surrounded by dust disks. However up to now CO radio observations have not revealed any gas in the circumstellar environment. We present thin hydrostatic equilibrium models for the disks around these stars using ATLAS9 photospheric models (Kurucz 1992) to determine the stellar UV flux and to derive the dust temperature from radiative equilibrium.

Detailed modelling of the chemistry in the disks around Vega and [FORMULA] Pictoris leads to the following results: (1) Photodissociation by stellar UV photons is capable of destroying CO molecules in the circumstellar environment of Vega. The amount of dissociation strongly depends on the disk mass. (2) Photodissociation by stellar UV radiation alone cannot explain the extremely low CO content in the [FORMULA] Pictoris disk, but inclusion of an isotropic interstellar radiation field can - depending on the disk mass - lead to significant CO underabundances in this disk. (3) Assuming 3 µm spherical grains, the dust temperature in the [FORMULA] Pictoris disk drops below 50 K, the critical temperature at which CO molecules start to freeze out on the surfaces of dust grains. This process is strongly influenced by the dust grain size and its chemical composition.

Hence the non-detection of CO radio lines in these disks does not necessarily imply a general gas depletion.

Key words: molecular processes – stars: circumstellar matter – stars: individual: Vega, fi Pictoris – stars: planetary systems

Send offprint requests to: Inga Kamp (kamp@astrophysik.uni-kiel.de)

© European Southern Observatory (ESO) 2000

Online publication: December 8, 1999

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