2005MNRAS.359..663D

We describe results from a survey for J = 3-2 ¹²CO emission from visible stars classified as having an infrared excess. The line is clearly detected in 21 objects, and significant molecular gas (≥10^–3 Jupiter masses) is found to be common in targets with infrared excesses ≥0.01 (≥56 per cent of objects), but rare for those with smaller excesses (∼10 per cent of objects).

A simple geometrical argument based on the infrared excess implies that disc opening angles are typically ≥12° for objects with detected CO; within this angle, the disc is optically thick to stellar radiation and shields the CO from photodissociation. Two or three CO discs have an unusually low infrared excess (≤0.01), implying the shielding disc is physically very thin (≤1°).

Around 50 per cent of the detected line profiles are double-peaked, while many of the rest have significantly broadened lines, attributed to discs in Keplerian rotation. Simple model fits to the line profiles indicate outer radii in the range 30-300 au, larger than found through fitting continuum SEDs, but similar to the sizes of debris discs around main-sequence stars. As many as five have outer radii smaller than the Solar System (50 au), with a further four showing evidence of gas in the disc at radii smaller than 20 au. The outer disc radius is independent of the stellar spectral type (from K through to B9), but there is evidence of a correlation between radius and total dust mass. Also the mean disc size appears to decrease with time: discs around stars of age 3-7 Myr have a mean radius ∼210 au, whereas discs of age 7-20 Myr are a factor of three smaller. This shows that a significant mass of gas (at least 2 M_⊕) exists beyond the region of planet formation for up to ∼7 Myr, and may remain for a further ∼10Myr within this region.

The only bona fide debris disc with detected CO is HD9672; this shows a double-peaked CO profile and is the most compact gas disc observed, with a modelled outer radius of 17 au. In the case of HD141569, detailed modelling of the line profile indicates gas may lie in two rings, with radii of 90 and 250 au, similar to the dust structure seen in scattered light and the mid-infrared. In both AB Aur and HD163296 we also find that the sizes of the molecular disc and the dust scattering disc are similar; this suggests that the molecular gas and small dust grains are closely co-located.