STAR FORMATION AND DISTRIBUTIONS OF GAS AND DUST IN THE CIRCINUS CLOUD

We present results of a study on the Circinus cloud based on 13CO (J = 1 - 0) data as well as visual to near-infrared (JHKS) extinction maps, to investigate the distributions of gas and dust around the cloud. The global 13CO distribution of the Circinus cloud is revealed for the first time, and the total molecular mass of the cloud is estimated to be 2.5 x 104 Msun for the assumed distance 700 pc. Two massive clumps in the cloud, called Circinus-W and Circinus-E, have a mass of ∼5 x 103 Msun. These clumps are associated with a number of young stellar objects (YSOs) searched for in the literature, indicating that they are the most active star-forming sites in Circinus. All of the extinction maps show good agreement with the 13CO distribution. We derived the average N(13CO)/AV ratio in the Circinus cloud to be 1.25 x 1015 cm-2 mag-1 by comparing the extinction maps with the 13CO data. The extinction maps also allowed us to probe into the reddening law over the Circinus cloud. We found that there is a clear change in dust properties in the densest regions of Circinus-W and Circinus-E, possibly due to grain growth in the dense cloud interior. Among the YSOs found in the literature, we attempted to infer the ages and masses of the Hα emission-line stars forming in the two clumps, and found that they are likely to be younger than 1 Myr, having a relatively small mass of ∼<2 Msun at the zero-age main sequence.