2016AJ....152..117Y

We investigated the physical properties of molecular clouds and star formation (SF) processes around infrared bubbles, which are essentially expanding H ii regions. We performed observations of 13 galactic infrared bubble fields containing 18 bubbles. We observed five molecular lines-¹²CO (J=1→0), ¹³CO (J=1→0), C¹⁸O (J=1→0), HCN (J=1→0), and HCO⁺ (J=1→0)-and several publicly available surveys were used for comparison: Galactic Legacy Infrared Mid-Plane Survey Extraordinaire, Multiband Imaging Photometer for Spitzer Galactic Plane Survey, APEX Telescope Large Area Survey of the Galaxy, Bolocam Galactic Plane Survey, Very Large Array (VLA) Galactic Plane Survey, Multi-Array Galactic Plane Imaging Survey, and NRAO VLA Sky Survey. We find that these bubbles are generally connected with molecular clouds, most of which are giant. Several bubble regions display velocity gradients and broad-shifted profiles, which could be due to the expansion of bubbles. The masses of molecular clouds within bubbles range from 100 to 19,000 M_☉, and their dynamic ages are about 0.3-3.7 Myr, which takes into account the internal turbulence pressure of surrounding molecular clouds. Clumps are found in the vicinity of all 18 bubbles, and molecular clouds near four of these bubbles with larger angular sizes show shell-like morphologies, indicating that either collect-and-collapse or radiation-driven implosion processes may have occurred. Due to the contamination of adjacent molecular clouds, only six bubble regions are appropriate to search for outflows, and we find that four have outflow activities. Three bubbles display ultra-compact H ii regions at their borders, and one is probably responsible for its outflow. In total, only six bubbles show SF activities in the vicinity, and we suggest that SF processes might have been triggered.