Clustered and Triggered Star Formation in W5: Observations with Spitzer

TL;DR

This study uses Spitzer observations to analyze star formation in W5, revealing multiple generations of stars, clustering properties, and potential triggered star formation mechanisms within the region.

Contribution

First detailed infrared survey of W5 with Spitzer, identifying star clusters, analyzing their properties, and suggesting triggered star formation as a key process.

Findings

40-70% of IR excess sources are in clusters with >10 members

Higher Class II to Class I ratio in evacuated cavities suggests older stellar populations

Evidence supports multiple star formation episodes, possibly triggered by earlier generations

Abstract

We present images and initial results from our extensive Spitzer Space Telescope imaging survey of the W5 H II region with the Infrared Array Camera (IRAC) and Multiband Imaging Photometer for Spitzer (MIPS). We detect dense clusters of stars, centered on the O stars: HD 18326, BD +60 586, HD 17505 and HD 17520. At 24 microns substantial extended emission is visible, presumably from heated dust grains that survive in the strongly ionizing environment of the H {\sc ii} region. With photometry of more than 18000 point sources, we analyze the clustering properties of objects classified as young stars by their IR spectral energy distributions (a total of 2064 sources) across the region using a minimal-spanning-tree algorithm. We find ~40--70% of infrared excess sources belong to clusters with >10 members. We find that within the evacuated cavities of the H II regions that make up W5, the ratio of Class II to Class I sources is ~7 times higher than for objects coincident with molecular gas as traced by 12CO emission and near-IR extinction maps. We attribute this contrast to an age difference between the two locations, and postulate that at least two distinct generations of star formation are visible across W5. Our preliminary analysis shows that triggering is a plausible mechanism to explain the multiple generations of star formation in W5, and merits further investigation.