Molecules in the transition disk orbiting T Chamaeleontis

Aims: We seek to establish the presence and properties of gas in the circumstellar disk orbiting T Cha, a nearby (d ~ 110 pc), relatively evolved (age ~5-7 Myr) yet actively accreting 1.5 M_⊙ T Tauri star.
Methods: We used the Atacama Pathfinder Experiment (APEX) 12 m radiotelescope to search for submillimeter molecular emission from the T Cha disk, and we reanalyzed archival XMM-Newton imaging spectroscopy of T Cha to ascertain the intervening absorption due to disk gas along the line of sight to the star (N_H).
Results: We detected submillimeter rotational transitions of ¹²CO, ¹³CO, HCN, CN, and HCO⁺ from the T Cha disk. The ¹²CO line (and possibly the ¹³CO line) appears to display a double-peaked line profile indicative of Keplerian rotation; hence, these molecular line observations constitute the first direct demonstration of the presence of cold molecular gas orbiting T Cha. Analysis of the CO emission line data indicates that the disk around T Cha has a mass (M_disk,H2 = 80 M_⊕) similar to, but more compact (R_disk,CO ~ 80 AU) than other nearby, evolved molecular disks (e.g., V4046 Sgr, TW Hya, MP Mus) in which cold molecular gas has been previously detected. The HCO⁺/¹³CO and HCN/¹³CO line ratios measured for T Cha appear similar to those of other evolved circumstellar disks (i.e., TW Hya and V4046 Sgr). The CN/¹³CO ratio appears somewhat weaker, but due to the low signal-to-noise ratio of our detection, this discrepancy is not strongly significant. Analysis of the XMM-Newton X-ray spectroscopic data shows that the atomic absorption N_H toward T Cha is one to two orders of magnitude larger than toward the other nearby T Tauri with evolved disks, which are seen at much lower inclination angles. Furthermore, the ratio between atomic absorption and optical extinction N_H/A_V toward T Cha is higher than the typical value observed for the interstellar medium and young stellar objects in the Orion nebula cluster. This may suggest that the fraction of metals in the disk gas is higher than in the interstellar medium. However, an X-ray absorption model appropriate for the physical and chemical conditions of a circumstellar disk is required to address this issue.
Conclusions: Our results confirm that pre-main-sequence stars older than ~5 Myr retain cold molecular disks when accreting, and that those relatively evolved disks display similar physical and chemical properties.

Based on submillimeter and X-ray observations. Submillimeter observations have been collected at the European Organisation for Astronomical Research in the Southern Hemisphere, Chile, with the Atacama Pathfinder Experiment APEX (Prog. ID 088.C-0441 and E-089.C-0518A). X-ray archival observations used in this paper have been obtained with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA member states and NASA.