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

¹ Purple Mountain Observatory & Key Laboratory of Radio Astronomy, Chinese Academy of Sciences, 2 West Beijing Road, 210008 Nanjing, PR China
e-mail: ygong@pmo.ac.cn
² University of Chinese Academy of Sciences, No. 19A Yuquan Road, 100049 Beijing, PR China

Received: 9 September 2015
Accepted: 29 January 2016

Abstract

Context. Molecular clouds trace the spiral arms of the Milky Way and all its star forming regions. Large-scale mapping of molecular clouds will provide an approach to understand the processes that govern star formation and molecular cloud evolution.

Aims. As a part of the Milky Way Imaging Scroll Painting (MWISP) survey, the aim is to study the physical properties of molecular clouds and their associated star formation toward the Galactic plane within 216.25° ≤ l ≤ 218.75° and −0.75° ≤ b ≤ 1.25°, which covers the molecular cloud complex S287.

Methods. Using the 3 × 3 Superconducting Spectroscopic Array Receiver (SSAR) at the PMO-13.7m telescope, 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 also make use of archival data to study star formation within the molecular clouds.

Results. We reveal three molecular clouds, the 15 km s^-1 cloud, the 27 km s^-1 cloud, and the 50 km s^-1 cloud, in the surveyed region. The 50 km s^-1 cloud is resolved with an angular resolution of ~1′ for the first time. Investigating their morphology and velocity structures, we find that the 27 km s^-1 cloud is likely affected by feedback from the stellar association Mon OB3 and the 50 km s^-1 cloud is characterized by three large expanding molecular shells. The surveyed region is mapped in C¹⁸O (1–0) for the first time. We discover seven C¹⁸O clumps that are likely to form massive stars, and 15 dust clumps based on the Bolocam Galactic Plane Survey (BGPS) archive data. Using infrared color-color diagrams, we find 56 Class I and 107 Class II young stellar object (YSO) candidates toward a slightly larger region of 5.0 square degrees. Based on the distribution of YSO candidates, an overdensity is found around the HII region S287 and the intersection of two shells; this is probably indicative of triggering. The star formation efficiency (SFE) and rate (SFR) of the 27 km s^-1 cloud are discussed. Comparing the observed values of the filament S287-main with fragmentation models, we suggest that turbulence controls the large-scale fragmentation in the filament, while gravitational fragmentation plays an important role in the formation of YSOs on small scales. We find that star-forming gas tends to have a higher excitation temperature, a higher ¹³CO (1–0) opacity, and a higher column density than non-star-forming gas, which is consistent with the point that star formation occurs 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^-1 cloud.