Star formation around RCW 120, the perfect bubble

TL;DR

This study uses multi-wavelength observations to analyze star formation processes around the RCW 120 bubble, revealing multiple triggering mechanisms and the presence of various young stellar objects in the swept-up shell.

Contribution

It provides detailed observational evidence of star formation triggered by the expansion of an HII region, highlighting the role of gravitational instabilities and identifying candidate Class 0 YSOs.

Findings

Dense neutral layer surrounds the HII region with over 2000 solar masses.

Multiple YSOs, including Class I and flat-spectrum sources, are found in the shell.

A potential Class 0 YSO candidate is identified near the ionization front.

Abstract

We take advantage of the very simple morphology of RCW 120 -- a perfect bubble -- to understand the mechanisms triggering star formation around an HII region and to establish what kind of stars are formed there. We present 870 microns observations of RCW 120, obtained with the APEX-LABOCA camera. These show the distribution of cold dust, and thus of neutral material. We use Spitzer-MIPS observations at 24 and 70 microns to detect the young stellar objects (YSOs) present in this region and to estimate their evolutionary stages. A layer of dense neutral material surrounds the HII region, having been swept up during the region's expansion. This layer has a mass greater than 2000 solar masses and is fragmented, with massive fragments elongated along the ionization front (IF). We measured the 24 microns flux of 138 sources. Of these, 39 are Class I or flat-spectrum YSOs observed in the direction of the collected layer. We show that several triggering mechanisms are acting simultaneously in the swept-up shell, where they form a second generation of stars. No massive YSOs are detected. However, a massive, compact 870 microns core lies adjacent to the IF. A 70 microns source with no 24 microns counterpart is detected at the same position. This source is a likely candidate for a Class 0 YSO. Also at 24 microns, we detect a chain of about ten regularly spaced Class I or flat spectrum sources, parallel to the IF, in the direction of the most massive fragment. We suggest that the formation of these YSOs is the result of Jeans gravitational instabilities in the collected layer. Finally, the 870 microns emission, the 24 microns emission, and the Halpha emission show the existence of an extended and partially ionized photodissociation region around RCW 120.