Molecular clouds toward three Spitzer bubbles S116, S117, and S118: Evidence for a cloud–cloud collision which formed the three H ii regions and a 10 pc scale molecular cavity

• Published in: Publications of the Astronomical Society of Japan 2018-05, Vol.70 (SP2)
• Main Authors: Fukui, Yasuo, Ohama, Akio, Kohno, Mikito, Torii, Kazufumi, Fujita, Shinji, Hattori, Yusuke, Nishimura, Atsushi, Yamamoto, Hiroaki, Tachihara, Kengo
• Format: Article
• Language: English
• ISSN: 0004-6264; 2053-051X
• DOI: 10.1093/pasj/psy005

Abstract We carried out a molecular-line study toward the three Spitzer bubbles S116, S117, and S118, which show active formation of high-mass stars. We found molecular gas consisting of two components with a velocity difference of ∼5 km s−1. One of them, the small cloud, has a typical velocity of −63 km s−1 and the other, the large cloud, has one of −58 km s−1. The large cloud has a nearly circular intensity depression, the size of which is similar to that of the small cloud. We present an interpretation that its cavity was created by a collision between the two clouds and that this collision compressed the gas into a dense layer elongating along the western rim of the small cloud. In this scenario, the O stars including those in the three Spitzer bubbles were formed in the interface layer compressed by the collision. Assuming that the relative motion of the clouds has a tilt of 45° to the line of sight, we estimate that the collision continued for the last 1 Myr at a relative velocity of ∼10 km s−1. In the S116-S117-S118 system the H ii regions are located outside of the cavity. This morphology is ascribed to the density-bound distribution of the large cloud which caused the H ii regions to expand more easily toward the outer part of the large cloud than towards the inside of the cavity. The present case proves that a cloud–cloud collision creates a cavity without the action of O-star feedback, and suggests that the collision-compressed layer is highly filamentary.