Triggered Star Formation

TL;DR

This paper reviews the mechanisms and observational evidence of triggered star formation in shells and pillars around HII regions, discussing criteria, timescales, and the role of triggering compared to other star formation processes.

Contribution

It provides a comprehensive review of triggered star formation phenomena, criteria, and timescales, highlighting its role among various star formation mechanisms.

Findings

Dense molecular shells often show triggered star formation.

Star formation rate in galaxies is mainly influenced by molecular mass.

Triggered star formation is one of multiple processes leading to star formation.

Abstract

Triggered star formation in bright rims and shells is reviewed. Shells are commonly observed in the Milky Way and other galaxies, but most diffuse shells seen in HI or the infrared do not have obvious triggered star formation. Dense molecular shells and pillars around HII regions often do have such triggering, although sometimes it is difficult to see what is triggered and what stars formed in the gas before the pressure disturbances. Pillar regions without clear age gradients could have their stars scattered by the gravity of the heads. Criteria and timescales for triggering are reviewed. The insensitivity of the average star formation rate in a galaxy to anything but the molecular mass suggests that triggering is one of many processes that lead to gravitational collapse and star formation.