Density structure in giant molecular cloud

We present the results of a multitransition study of HC3N in cores of three giant molecular clouds in Orion, M17, and Cepheus A. In these regions we have mapped the J = 4 --> 3, J = 10 --> 9, J = 12 --> 11, and J = 16 --> 15 transitions of HC3N over a 4' x 12' area (360 positions) in Orion and a 4' x 5' area (120 positions) in M17 and Cepheus A. We have used the results of a previous study of the temperature structure of the dense gas in the same cloud cores, together with a non-LTE excitation model for HC3N, to derive the density of molecular hydrogen and the column density of HC3N. In all we have computed densities for 133 positions in Orion, 46 positions in M17, and 14 positions in Cepheus A. Despite the differences between the clouds, the range of densities in the three cores is found to be quite similar, with derived values of n(H2) between 3 x 10(5) and 5 x 10(6) cm(-3). The principal result of this study is that, in spite of the use of an optically thin tracer and the inclusion of an improved source temperature model, the density within each cloud core shows no evidence of large-scale variations. These observations are consistent with the results of previous efforts which utilized other tracers of the dense gas and assumed a constant temperature for the cloud. An examination of the size scale of the clouds implied by the observed density and the (CO)-O-18 column density demonstrates either that each cloud has a strikingly (greater than 10:1) flattened geometry, each with the short axis along the line of sight, or that the dense gas must be clumped and is filling only a small fraction of the volume (less than or similar to 5%). We have also examined the possibility of lower density material along the line of sight and find that the present data suggest a clump-to-interclump density ratio greater than 100.