2011ApJ...731...23S

We present results of a study on the Circinus cloud based on ¹³CO (J = 1 - 0) data as well as visual to near-infrared (JHK_S) 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 2.5x10⁴ M_☉ for the assumed distance 700 pc. Two massive clumps in the cloud, called Circinus-W and Circinus-E, have a mass of ∼5x10³ M_☉. 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)/A_V ratio in the Circinus cloud to be 1.25x10¹⁵/cm2/mag 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 M_☉ at the zero-age main sequence.