A search for molecular gas components in prototypal Vega-excess systems

Abstract

We report the results of a deep search for submillimetre lines of 12CO around the nearby main-sequence stars Vega (α Lyr), ε Eridanus, Fomalhaut (α PsA), β Leo and β Pictoris. Upper limits on CO masses about 10 to 100 times lower than those previously reported have been obtained. If ‘normal’ interstellar medium CO abundances are assumed, the data suggest that the gas:dust mass ratio is reduced by a factor of ≳ 103 below that found in molecular clouds. These apparently low values can be explained by photodissociation of CO in the stellar or, in the case of ε Eri, the interstellar UV radiation field. The predicted CO column densities are similar to our limits if the CO is not replenished. Although the β Pic disc has the highest dust mass, the large grain size results in a relatively low UV extinction to the central star, and so CO photodissociation can readily occur. It has been suggested by others that the CO could be replenished by the occasional infall of comets, and we place an upper limit on the rate of cometary accretion for ε Eri.