2015A&A...578A..31R

We attempt to determine the molecular composition of disks around young low-mass stars in the ρ Oph region and to compare our results with a similar study performed in the Taurus-Auriga region. We used the IRAM 30m telescope to perform a sensitive search for CN N=2-1 in 29 T Tauri stars located in the ρ Oph and upper Scorpius regions. ¹³CO J=2-1 is observed simultaneously to provide an indication of the level of confusion with the surrounding molecular cloud. The bandpass also contains two transitions of ortho-H₂CO, one of SO, and the C¹⁷O J=2-1 line, which provides complementary information on the nature of the emission. Contamination by molecular cloud in ¹³CO and even C¹⁷O is ubiquitous. The CN detection rate appears to be lower than for the Taurus region, with only four sources being detected (three are attributable to disks). H₂CO emission is found more frequently, but appears in general to be due to the surrounding cloud. The weaker emission than in Taurus may suggest that the average disk size in the ρ Oph region is smaller than in the Taurus cloud. Chemical modeling shows that the somewhat higher expected disk temperatures in ρ Oph play a direct role in decreasing the CN abundance. Warmer dust temperatures contribute to convert CN into less volatile forms. In such a young region, CN is no longer a simple, sensitive tracer of disks, and observations with other tracers and at high enough resolution with ALMA are required to probe the gas disk population.