DENSITIES OF CLOUDS DETECTED IN MOLECULAR ABSORPTION

First detections are presented of the rotational transition HCN J=3-2 in absorption towards background continuum sources. The lines of sight studied were towards Sgr B2 and W49 A, and approximately eleven foreg round clouds were detected, at velocities associated with spiral arms and the large-scale bar. In contrast to lower frequency absorption lin es previously observed, HCN J=3-2 is a good tracer of dense molecular gas, and LVG radiative transfer modelling has been used to estimate cl oud densities. The mean n(H-2) of four clouds seen towards Sgr B2 is 8 x 10(3) cm(-3) (at a kinetic temperature of 20 K), with an upper limi t of a few 10(4) cm(-3). However, modelling of CS J=2-1 and J=3-2 abso rption lines indicates n(H-2) less than or similar to 600 cm(-3) for t hree of these clouds, suggesting that a range of densities is present. The cloud-averaged n(H-2) is found to be close to 200 cm(-3), in whic h case cloud masses are a few 100 M(circle dot) . The n(H-2) range fou nd in the absorption clouds covers a regime from the average in GMCs t o somewhat less than in star-forming cores, which suggests that these clouds represent an intermediate stage of fragmentation.