Astronomy and Astrophysics – Astrophysics
Scientific paper
Jan 2000
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000a%26a...353..276k&link_type=abstract
Astronomy and Astrophysics, v.353, p.276-286 (2000)
Astronomy and Astrophysics
Astrophysics
34
Molecular Processes, Stars: Circumstellar Matter, Stars: Individual: Vega, Beta Pictoris, Stars: Planetary Systems
Scientific paper
Vega and beta 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 beta 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 beta 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 mu m spherical grains, the dust temperature in the beta 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.
Bertoldi Frank
Kamp Inga
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