CO emission from discs around isolated HAeBe and Vega-excess stars
  We describe results from a survey for J=3-2 12CO emission from visible stars with an infrared excess. The line is clearly detected in 21 objects, with molecular gas (>10^-3 Jupiter masses) common in targets with infrared excesses >0.01 (>56% of objects). Such high excesses indicate the presence of a disc of opening angle >12 degrees; within this, the optically thick disc prevents CO photodissociation. Two or three stars with associated CO have an excess <0.01, implying a disc opening angle <1 degree. Most line profiles are double-peaked or relatively broad. Model fits, assuming a Keplerian disc, indicate outer radii, R_out, of ~20-300 au. As many as 5 discs have outer radii smaller than the Solar System (50 au), and a further 4 have gas at radii <20 au. R_out is independent of the stellar spectral type (from K through to B9), but is correlated with total dust mass. R_out appears to decrease with time: discs around stars of age 3-7 Myr have a mean radius of ~210 au, whereas discs of age 7-20 Myr are a factor of 3 smaller. The only bona fide debris disc with detected CO is HD9672; this has a double peaked line profile and is the most compact gas disc observed, with a modelled radius 17 au). A fit to HD141569 suggests the gas lies in two rings of radii 90 and 250 au, similar to the scattered light structure. In both AB Aur and HD163296 the sizes of the molecular and dust scattering discs are also similar, suggesting that the gas and small dust grains are co-located. 
W. R. F. Dent, J. S. Greaves, I. M. Coulson