Chemistry of Protosolar-like Nebulae: Cold Gas component of the DM Tau and GG Tau Disks

Report by: A.Dutrey, S.Guilloteau, M.Guélin
Institut de Radio Astronomie Millimétrique, 300 Rue de la Piscine, F-38406 Saint Martin d'Hères, France
Observing the chemical and molecular composition of the protosolar nebula has, for a long time, been a haunting, albeit unreachable dream. Yet, in some way this old dream is becoming true: lately, astronomers have uncovered protoplanetary disks around a number of low-mass pre-main sequence stars (e.g. T Tauri stars), and, thanks to recent gains in sensitivity at mm wavelengths, we are about to learn the composition of these disks.

The observation of these disks is not easy, due to their small sizes and to confusion with fore-/background gas. It requires to select the nearest possible objects, when they are well detached from their parent cloud. The T Tauri stars DM Tau and GG Tau are in that sense unique since (1) they are located only 140 pc away, in a region devoid of CO emission at the edge of the L 1551 core (2) they are both surrounded by dusty disks, with relatively strong CO and thermal dust emission; the disks have radii of several arcsec, hence are well resolved by mm interferometry; they show the clear signature of Keplerian rotation (Guilloteau & Dutrey 1994, A&A, 291, L23; Dutrey et al.  1994, A&A 286, 149; Guilloteau et al. , 1996 in preparation).

DM Tau is one of the oldest T Tauri stars of the Taurus region ( yr), whereas GG Tau is a young binary system (separation 0.26'' in the plane of sky), which has cleared up a large central hole by tidal interaction (inner radius AU and outer radius AU). Comparing the molecular content of two objects originating from the same molecular cloud, but at different evolution paths, may allow to understand better the physico-chemical processes leading to planetary formation.

We have investigated with the IRAM 30-m telescope the molecular content of the DM Tau and GG Tau disks and were able, for the first time, to detect a large variety of molecular species in such objects. These include CO, CS, CN, HCN, HNC, , and a first organic molecule, (ortho and para). So far, both disks look surprisingly similar. The gas column densities seem quite large, as indicated by our probable detection of and in DM Tau. Assuming the CS, HCN and abundances are similar to those in TMC1, the disk masses derived are between 20 and 100 times lower than those determined from the dust thermal emission, in agreement with our previous results derived from CO and isotopes (Guilloteau & Dutrey ibid).

 
Figure: Molecular spectra observed towards DM Tau. Double-peaked profiles, characteristic of a tilted Keplerian disk, are detected for , CS, and . Like in the case of GG Tau, they have the same width and the same systemic velocity than the profiles.