Line emission from circumstellar disks around A stars

The nature of the tenuous disks around A stars has raised considerable controversy in the literature during the past decade. The debate whether or not the disk around beta Pictoris contains gaseous molecular hydrogen is only the most recent example. Since CO is in general a poor tracer for the gas content of these low mass disks, we discuss here detailed emission line calculations for alternative tracers like C and C+, based on recent optically thin disk models by Kamp & van Zadelhoff (2001). The [C II] 157.7 mum line was searched toward Vega and beta Pictoris - the most prominent A stars with disks - using ISO LWS data, and a tentative detection is reported toward the latter object. From a comparison with emission line observations as well as absorption line studies of both stars, the gas-to-dust ratio is constrained to lie between 0.5 and 9 for beta Pictoris. For Vega the [C II] observations indicate an upper limit of 0.2 M+ for the disk gas mass. Predicted line intensities of C+ and C are presented for a range of models and appear promising species to trace the gas content in the disks around A stars with future instrumental capabilities (SOFIA, Herschel, APEX and ALMA). Searches for CO emission should focus on the J = 3-2 line.