CO observations of the molecular gas in the galactic HII region Sh2-48; Evidence for cloud-cloud collision as a trigger of high-mass star formation

Kazufumi Torii, Yusuke Hattori, Mitsuhiro Matsuo, Shinji Fujita, Atsushi Nishimura, Mikito Kohno, Mika Kuriki, Yuya Tsuda, Tetsuhiro Minamidani, Tomofumi Umemoto, Nario Kuno, Satoshi Yoshiike, Akio Ohama, Kengo Tachihara, Yasuo Fukui, Kazuhiro Shima, Asao Habe, Thomas J. Haworth

Published 2017-06-22Version 1

Understanding the mechanism of high-mass star formation is one of the top-priority issues in contemporary astrophysics. Sh2-48 is a galactic HII region located at 3.8kpc from the Sun. It harbors an O9.5-type star at the center of the HII region which is extended for ~10 arcsin. As a part of the FOREST Unbiased Galactic plane Imaging survey with the Nobeyama 45-m telescope (FUGIN) project, we obtained the CO J=1-0 dataset for a large area of Sh2-48 at a spatial resolution of 21 arcsec, which corresponds to ~0.4pc at 3.8kpc. The CO data revealed that the molecular gas having a total molecular mass of 8.5x10^4 M_sun is associated with Sh2-48, which shows a characteristic line-symmetric velocity gradient over ~4km/s. Such a velocity gradient cannot be formed by a spherical expansion the HII region. In this paper, we discuss a cloud-cloud collision scenario to interpret the observed signatures including the velocity gradient. By comparing between the observations and simulations, we found that this line-symmetric velocity gradient is an expected outcome of a collision between a cylindrical cloud and a large spherical cloud, and we concluded that the high-mass star formation in Sh2-48 was triggered by the collision. Our results reinforce the importance of cloud-cloud collision for high-mass star formation in the Milky Way.