Models of molecular clouds. I - Multitransition study of CS

Abstract

The dense cloud cores of M17, S140, and NGC 2024 have been mapped in four rotational transitions (J = 2-1, J = 3-2, J = 5-4, J = 6-5) of the CS molecule. These observations were made with three different telescopes to provide similar beam sizes; the angular resolution available was approx.1'. In addition, observations of the isotopic species C/sup 34/S were obtained in the J = 2-1 and J = 5-4 transitions. In each of the cloud cores the distribution of emission in all four CS transitions is similar. The similarity of the spatial distributions and the close agreement between line shapes of the transitions from different rotational levels suggest that the emission from all four transitions is produced in the same region of the cloud. The data for the four transitionns were fitted simultaneously to a model of the CS excitation to derive the density and column density of CS along each line of sight through the cloud core. Most positions in the three cloud cores could be fitted by this model. The mean density of the cloud cores were found to be 4--9 x 10/sup 5/ cm/sup -3/. Little density variation was found across the cloud cores,more » but the CS column density was found to vary by an order of magnitude, being largest at the center of the core and decreasing toward the edge. These results can be best explained if the CS emission in the cloud arises from numerous dense clumps, with little emission coming from the interclump medium. In this model, the filling factors of clumps are nearly unity at the centers of the cloud cores but decrease toward the edges of the cloud cores.« less