A SEARCH FOR MOLECULAR GAS COMPONENTS IN PROTOTYPAL VEGA-EXCESS SYSTEMS

We report the results of a deep search for submillimetre lines of (CO) -C-12 around the nearby main-sequence stars Vega (alpha Lyr), epsilon Eridanus, Fomalhaut (alpha PsA), beta Leo and beta Pictoris. Upper lim its on CO masses about 10 to 100 times lower than those previously rep orted have been obtained. If 'normal' interstellar medium CO abundance s are assumed, the data suggest that the gas:dust mass ratio is reduce d by a factor of greater than or similar to 10(3) below that found in molecular clouds. These apparently low values can be explained by phot odissociation of CO in the stellar or, in the case of epsilon Eri, the interstellar stellar UV radiation field. The predicted CO column dens ities are similar to our limits if the CO is not replenished. Although the beta Pic disc has the highest dust mass, the large grain size res ults in a relatively low UV extinction to the central star, and so CO photodissociation can readily occur. It has been suggested by others t hat the CO could be replenished by the occasional infall of comets, an d we place an upper limit on the rate of cometary accretion for epsilo n Eri.