ON THE MASS AND THE GAS-TO-DUST RATIO OF THE RHO-OPHIUCHI CLOUD CORE

We have mapped the rho Oph main cloud in the J=2-1 transition of CS wi th the Nagoya 4m millimetre-wave telescope. The overall morphology of the cloud is similar to that mapped in (CO)-O-18, but the high density regions rho Oph A and B are more evident in CS. Depending on the adop ted model for the CS excitation, these observations lead to estimated cloud masses 540 M. (LTE) or 2500 M. (NLTE) for a value of the CS abun dance equal to 10(-9). We find, in particular, that the assumption of LTE is inadequate and that the mass of the rho Oph main cloud may prev iously have been underestimated by at least a factor of three. This do es also imply that the formerly determined high star formation efficie ncy of the cloud would have to be revised downwards accordingly. From IRAS observations we determine the mass of the dust emitting at 60 mu m to be at least 5.5 M.. Average mass ratios of the gas-to-dust are th us found to be 100 or 450. Detailed comparison of the CS and IRAS imag es reveals, however, that average values are rarely, if ever, found on local scales on the order of 0.15 pc.