2003A&A...397.1129K

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 β 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 (2001A&A...373..641K). The [CII] 157.7µm line was searched toward Vega and β 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 β Pictoris. For Vega the [CII] observations indicate an upper limit of 0.2M_⊕ 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.