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We attempt to determine the molecular composition of disks around younglow-mass stars in the $\rho$ Oph region and to compare our results with asimilar study performed in the Taurus-Auriga region. We used the IRAM 30 mtelescope to perform a sensitive search for CN N=2-1 in 29 T Tauri starslocated in the $\rho$ Oph and upper Scorpius regions. $^{13}$CO J=2-1 isobserved simultaneously to provide an indication of the level of confusion withthe surrounding molecular cloud. The bandpass also contains two transitions ofortho-H$_2$CO, one of SO, and the C$^{17}$O J=2-1 line, which providescomplementary information on the nature of the emission. Contamination bymolecular cloud in $^{13}$CO and even C$^{17}$O is ubiquitous. The CN detectionrate appears to be lower than for the Taurus region, with only four sourcesbeing detected (three are attributable to disks). H$_2$CO emission is foundmore frequently, but appears in general to be due to the surrounding cloud. Theweaker emission than in Taurus may suggest that the average disk size in the$\rho$ Oph region is smaller than in the Taurus cloud. Chemical modeling showsthat the somewhat higher expected disk temperatures in $\rho$ Oph play a directrole in decreasing the CN abundance. Warmer dust temperatures contribute toconvert CN into less volatile forms. In such a young region, CN is no longer asimple, sensitive tracer of disks, and observations with other tracers and athigh enough resolution with ALMA are required to probe the gas disk population.

Sensitive survey for 13CO, CN, H2CO, and SO in the disks of T Tauri and Herbig Ae stars II: Stars in $\rho$ Oph and upper Scorpius