DETECTION OF A CIRCUMSTELLAR GAS AROUND DM-TAURI - A PROTOPLANETARY DISK AROUND A SINGLE STAR

Sensitive molecular line observations carried out with the Nobeyama 45 m telescope have resulted in the detection of the (CO)-C-12 (d = 1-0) and (CO)-C-13 (J = 1-0) emission centered on the young classical T Ta uri star, DM Tau. The derived peak antenna temperatures are 0.3 K in ( CO)-C-12 and 0.1 K in (CO)-C-13. NO (CO)-O-18 emission was detected at an upper limit (3 a) of 45 mK. The emission feature has a line width of 1.7 km s(-1) and is centered at upsilon(LSR) = 5.9 km s(-1), which coincides well with the reported radial velocity for DM Tau (5.9 +/- 2 .1 km s(-1)). The (CO)-C-13 profile shows a symmetric double peak, sug gesting a Keplerian rotating disk. These observations suggest that the molecular gas is associated with and is most probably gravitationally bound to DM Tau. The derived radius of the gaseous disk is about 1000 AU using an optically thick disk model and a Keplerian rotating disk model. The gaseous mass is between 7 x 10(-4) and 1 x 10(-3) M. from t he (CO)-C-13 intensity and the upper limit of (CO)-O-18 intensity. Our results suggest that molecular gas is depleted from the standard gas- to-dust mass ratio or that mass distribution in the disk is steeper th an the standard law, r(-1.5), although our observations cannot detect emission from an optically thick gaseous component inward of r < 100 A U, if the emission comes from a Keplerian rotating disk.