Triggered massive-star formation on the borders of Galactic HII regions. I. A search for `collect and collapse' candidates

Abstract

Young massive stars or clusters are often observed at the peripheries of HII regions. What triggers star formation at such locations? Among the scenarios that have been proposed, the `collect and collapse' process is particularly attractive because it permits the formation of massive objects via the fragmentation of the dense shocked layer of neutral gas surrounding the expanding ionized zone. However, until our recent article on Sh 104, it had not been convincingly demonstrated that this process actually takes place. In the present paper we present our selection of seventeen candidate regions for this process; all show high-luminosity near-IR clusters and/or mid-IR point sources at their peripheries. The reality of a `collect and collapse' origin of these presumably second-generation stars and clusters will be discussed in forthcoming papers, using new near-IR and millimetre observations.