A&a Manuscript No. Astronomy and Astrophysics Co in the Circumstellar Disks of Vega and Pictoris

Vega and 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 ux and to derive the dust temperature from radiative equilibrium. Detailed modelling of the chemistry in the disks around Vega and 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 Pictoris disk, but inclusion of an isotropic interstellar radiation eld can { depending on the disk mass { lead to signiicant CO underabundances in this disk. (3) Assuming 3 m spherical grains, the dust temperature in the 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 innuenced 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.