Molecular clouds and star formation toward the Galactic plane within 216.25° ≤l≤ 218.75° and −0.75° ≤b≤ 1.25°

Abstract

As a part of the Milky Way Imaging Scroll Painting (MWISP) survey, we performed a simultaneous 12CO(1-0), 13CO(1-0), C18O(1-0) mapping toward molecular clouds in a region encompassing 3.75 square degrees. We reveal three molecular clouds, the 15 km/s cloud, the 27 km/s cloud, and the 50 km/s cloud, in the surveyed region. The 50 km/s cloud is resolved with an angular resolution of around 1' for the first time. Investigating their morphology and velocity structures, we find that the 27 km/s cloud is likely affected by feedback from the stellar association Mon OB3 and the 50 km/s cloud is characterised by three large expanding molecular shells. The region is mapped in C18O (1-0) for the first time. We discover seven C18O clumps, which are likely to form massive stars, and 15 dust clumps based on the BGPS archival data. Using infrared color-color diagrams, we find 56 Class I and 107 Class II young stellar object (YSO) candidates. According to the distribution of YSO candidates, an overdensity is found around the HII region S287 and the intersection of two shells, indicative of triggering. The star formation efficiency and rate of the 27~km/s cloud are discussed. Comparing the observed values of the filament S287-main with the models of fragmentation, we suggest that turbulence controls the large scale of fragmentation in the filament while gravitational fragmentation plays an important role in the formation of YSOs on the small scale. We find that star-forming gas tends to have higher excitation temperature, higher 13CO opacity, and higher column density than non-star-forming gas, which is consistent with the point that star formation happens in denser gas and star-forming gas is heated by YSOs. Using the 1.1 mm dust emission to trace dense gas, we obtain a dense gas fraction of 2.7%-10.4% for the 27 km/s cloud.