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.