Formation of Dense Gas and Stars near the End of the Galactic Bar

<jats:title>Abstract</jats:title> <jats:p>We carried out H$ ^{13}$ CO$ ^{+}$ ($ J$$ =$ 1–0) observations of a molecular cloud containing a massive star-forming region, G23.44$-$ 0.18, using the Nobeyama 45-m radio telescope. We identified three clumps, named Clumps A, B, and C, on the periphery of the H II region in the cloud. The most massive clump, Clump A, has a radius of 0.74 $ \pm$ 0.20 pc and a mass of 1100$ ^{+800}_{-\hskip.35mm 640}\ M\odot$ . Both Clumps B and C, however, have much smaller size and mass than Clump A. These three clumps seem not to be virialized. We also found four sub-clumps in Clump A. These sub-clumps may be affected by the strong outflow penetrating Clump A. The star-formation efficiency (SFE) of the entire cloud is $ \sim$ 0.4%, which is typical for galactic star-forming clouds. The SFE of Clump A is 25$ ^{+14}_{-\hskip.35mm 11}$ %. This value is higher than other embedded cluster-forming clumps, which implies that stars, including massive ones, are formed efficiently and actively in Clump A. The dense gas fraction estimated from H$ ^{13}$ CO$ ^{+}$ ($ J$$ =$ 1–0) and $ ^{13}$ CO ($ J$$ =$ 1–0) of the cloud are lower than those of other star-forming regions. The results mean that this star-forming region is young, and suggest that the formation of dense gas and stars in the cloud has just begun. Comparing the dynamical age of the H II region with the fragmentation timescale for the collect and collapse process, molecular gas is accumulating through expansion of the H II region in Clumps B and C. Since there are massive young stellar objects (YSOs) in Clump A, we suggest that this clump has already become denser than the other regions in the cloud due to converging flow, or some external factors, such as an old bubble and cloud–cloud interaction.</jats:p>