Modeling the gas reservoir of circumstellar disks around young G-type stars

¹ Leiden Observatory, PO Box 9513, 2300 RA Leiden, The Netherlands
² Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA e-mail: kamp@stsci.edu

Received: 27 August 2003
Accepted: 27 July 2004

Abstract

Interpretation of molecular line observations in tenuous circumstellar disks around young G-type stars in terms of a disk mass is difficult without a model that describes the chemical structure of these disks. This paper discusses the chemistry in tenuous disks around young solar-type stars based on disk models that take into account the presence of a stellar chromosphere. The example of the disk around a 70 Myr old solar-type star shows that the dissociating radiation from the chromosphere is stronger than the interstellar ultraviolet radiation field up to a distance of ~400 AU from the star. Similar to other studies in this research field, it is found that, due to photodissociation, the CO-to-H₂ ratio is far from the canonical value of 10^-4 for molecular clouds. Moreover, the dust-to-gas mass ratio as well as the dust grain size plays an important role for the H₂ abundance in these disks.