Line emission from circumstellar disks around A stars^*

¹ Leiden Observatory, PO Box 9513, 2300 RA Leiden, The Netherlands
² Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany

Corresponding author: I. Kamp, kamp@strw.leidenuniv.nl

Received: 11 September 2002
Accepted: 29 October 2002

Abstract

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 (2001). The [] 157.7 μm 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 [] observations indicate an upper limit of 0.2  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 –2 line.