Dense gas discs around T Tauri stars

The circumstellar discs of six T Tauri stars, forming an age sequence from approximately 1 to 10 Myr, have been observed in the molecular line HCO+ J = 4-3. The dense gas traced by this line lies typically at less than half the radius characteristic of more easily excited CO lines, and the most compact disc solutions have outer radii of only around 100 au. Within the uncertainties set by the disc chemistry, the lowest-mass solutions are found to possess at least 2 to 15 M-Jupiter of gas, in all but one of the six systems. The dense gas present within similar to100 au could provide fuel for the zone at tens of au where giants planets may form, and this mass reservoir is sufficient to form such planets. Only the oldest star, LkCa 15 at 8-12 Myr, has a large central cavity lacking in dense material that may reflect gas removal at late times.