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

L. Deharveng, A. Zavagno+1 more|ArXiv.org|Dec 22, 2004

Astrophysics and Star Formation StudiesPhysics and Astronomy47 references128 citations

TL;DR

This paper identifies 17 Galactic H II regions as prime candidates for triggered massive-star formation via the 'collect and collapse' mechanism, where expanding ionized gas compresses and fragments a surrounding molecular shell to form new stars. The key contribution is a targeted selection of regions showing high-luminosity near-IR clusters and mid-IR point sources at their peripheries—strong indicators of second-generation star formation—validated by multi-wavelength follow-up observations in subsequent studies.

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.

Motivation & Objective

• To identify and select candidate H II regions where the 'collect and collapse' process may be triggering massive star formation at their peripheries.
• To address the long-standing lack of convincing observational evidence for the 'collect and collapse' mechanism in Galactic H II regions.
• To provide a prioritized list of regions for detailed follow-up observations using near-IR and millimetre wavelengths.
• To distinguish between primary and secondary ionizing sources in peripheral star-forming regions to confirm the triggering mechanism.
• To investigate whether the process leads to the formation of massive stars or clusters in dense, compressed shells of neutral gas.

Proposed method

• Selected candidate regions based on the presence of high-luminosity near-IR clusters and mid-IR point sources at the peripheries of H II regions.
• Used mid-IR and near-IR data from IRAS, MSX, and 2MASS to identify protostellar and young stellar objects at H II region borders.
• Applied criteria for identifying dense molecular shells and cold dust emission rings via millimetre continuum and line observations (e.g., IRAM, SEST, MAMBO).
• Analyzed velocity shifts in ionized gas (e.g., champagne flows) to assess whether peripheral clusters are the primary ionizing sources.
• Mapped molecular gas and dust emission to detect the presence of a compressed, shocked layer and potential fragmentation patterns.
• Compared observed morphologies (e.g., ring-like dust structures) with theoretical predictions of the 'collect and collapse' model.

Experimental results

Research questions

• RQ1 Are there observable signatures of the 'collect and collapse' process in Galactic H II regions, such as dense, compressed shells and peripheral star clusters?
• RQ2 Do the observed high-luminosity near-IR clusters and mid-IR point sources at H II region peripheries indicate second-generation massive star formation?
• RQ3 Is the molecular shell surrounding the H II region dense and continuous enough to support gravitational fragmentation, as required by the model?
• RQ4 Can the observed morphology—especially ring-like dust structures—be explained by the 'collect and collapse' mechanism, or do they suggest alternative processes?
• RQ5 Are the peripheral clusters in these regions ionized by their own stars, or are they triggered by external ionization from central H II regions?

Key findings

• Seventeen candidate regions were identified based on the presence of high-luminosity near-IR clusters and mid-IR point sources at the peripheries of H II regions.
• Regions such as Sh 104, RCW 40, RCW 79, and RCW 120 are surrounded by nearly complete rings of cold dust emission, indicating a dense, compressed shell.
• In RCW 79, multiple star formation stages were observed: a compact H II region, an H2O maser, and dust condensations without associated IR sources, suggesting ongoing fragmentation.
• Sh 104 showed clear evidence of a dense molecular shell and fragmentation, confirming the 'collect and collapse' process in a prior study (Deharveng et al. 2003b).
• For Sh 217 and Sh 219, the surrounding material is primarily atomic hydrogen at low density, making the collect and collapse scenario less certain.
• In some cases, such as Sh 219, velocity shifts suggest a 'champagne flow', but the central exciting star is distinct from the peripheral cluster, supporting external triggering.