Cluster formation in the W40 and Serpens South complex triggered by the expanding HII region

TL;DR

This study maps the W40 and Serpens South regions, revealing their physical connection and demonstrating that expanding shells from the HII region likely triggered the formation of the young cluster in Serpens South.

Contribution

It provides a three-dimensional model of the complex and links shell expansion to star cluster formation, a novel insight into triggered star formation mechanisms.

Findings

C18O emission shows the physical connection between W40 and Serpens South.

Two expanding shells are identified in position-velocity diagrams.

Cluster formation is likely triggered by shell expansion.

Abstract

We present results of the mapping observations covering a large area of 1 square degree around W40 and Serpens South carried out in the 12CO (J=1-0), 13CO (J=1-0), C18O (J=1-0), CCS (J_{N}=8_{7}-7_{6}), and N_2H+ (J=1-0) emission lines with the 45 m Nobeyama Radio Telescope. W40 is a blistered Hii region, and Serpens South is an infrared dark cloud accompanied by a young cluster. The relationship between these two regions which are separated by ~20' on the sky has not been clear so far. We found that the C18O emission is distributed smoothly throughout the W40 and Serpens South regions, and it seems that the two regions are physically connected. We divided the C18O emission into four groups in terms of the spatial distributions around the HII region which we call 5, 6, 7, and 8 km s^{-1} components according to their typical LSR velocities, and propose a three-dimensional model of the W40 and Serpens South complex. We found two elliptical structures in position-velocity diagrams, which can be explained as a part of two expanding shells. One of the shells is the small inner shell just around the HII region, and the other is the large outer shell corresponding to the boundary of the HII region. Dense gas associated with the young cluster of Serpens South is likely to be located at the surface of the outer shell, indicating that the natal clump of the young cluster is interacting with the outer shell being compressed by the expansion of the shell. We suggest that the expansion of the shell induced the formation of the young cluster.