Observational study of sites of triggered star formation: CO and mid-infrared observations

(Abridged) Bright-rimmed clouds (BRCs) are isolated molecular clouds located on the edges of evolved HII regions where star formation is thought may have been triggered. In this paper we investigate the current level of star formation within a sample of BRCs and evaluate to what extent star formation may have been induced. We present the results of a programme of position-switched CO observations towards 45 southern BRCs. The 12CO, 13CO and C18O (J=1-0) were simultaneously observed using the 22m Mopra telescope. We complement these observations with archival mid-IR submm and radio data. Analysis of the CO, mid-IR and radio data result in the clouds being divided into three distinct groups. We refer to these groups as spontaneous, triggered, and zapped clouds, respectively. Comparing the physical parameters of spontaneous and triggered samples we find striking differences in luminosity, surface temperature and column density with all three quantities significantly enhanced for the clouds considered to have been triggered. Furthermore, we find strong evidence for star formation within the triggered sample by way of methanol and H_2O masers, embedded mid-IR point sources and CO wings, however, we find evidence of ongoing star formation within only two of the spontaneous sample. We have used CO, mid-IR and radio data to identify 24 of the 45 southern BRCs that are undergoing a strong interaction with their HII region. We can therefore exclude ~50% from future studies. 14 of the 24 interacting BRCs are found to be associated with embedded mid-IR point sources and we find strong evidence of that these clouds are forming stars. The absence of mid-infrared sources towards the remaining ten clouds leads us to conclude that these represent an earlier evolutionary stage of star formation.

💡 Research Summary

This paper investigates whether bright‑rimmed clouds (BRCs) on the peripheries of evolved H II regions are sites of triggered star formation. A sample of 45 southern BRCs was observed with the 22 m Mopra telescope, obtaining simultaneous J = 1‑0 spectra of 12CO, 13CO, and C18O. The CO data provide estimates of molecular mass, excitation temperature, optical depth, and kinematics for each cloud. In addition, archival mid‑infrared (Spitzer‑IRAC, MSX), sub‑millimetre (ATLASGAL, LABOCA), and radio continuum (NVSS) data were compiled to identify embedded point sources, PAH emission, dense cores, and ionized gas interaction.

The authors classify the BRCs into three categories based on the combined multi‑wavelength diagnostics: (1) Spontaneous clouds, which show low surface temperatures (≲20 K), modest column densities (10²¹–10²² cm⁻²), narrow CO lines, and lack maser emission or embedded IR sources; these are interpreted as undergoing star formation driven primarily by internal gravitational collapse. (2) Triggered clouds, which exhibit higher surface temperatures (30–40 K), elevated column densities (>10²² cm⁻²), broadened CO line wings, and frequent detections of 6.7 GHz methanol and 22 GHz water masers, as well as embedded mid‑IR point sources. These signatures point to radiation‑driven implosion (RDI) caused by the external ionization front. (3) Zapped clouds, where the ionizing radiation appears to have eroded or evaporated the molecular material; CO emission is weak or absent, and no star‑formation tracers are found.

Statistical comparison reveals that the triggered subset is significantly more luminous (average luminosity ≈ three times that of the spontaneous group), hotter, and denser. Fourteen of the 24 BRCs identified as interacting with their H II region host embedded mid‑IR sources, confirming active star formation. The remaining ten interacting clouds lack such sources but display CO wings and maser activity, suggesting they are in an earlier evolutionary stage prior to the emergence of detectable IR point sources. Only two of the spontaneous clouds show any star‑formation indicators, underscoring the contrast between the two populations.

The study concludes that roughly half of the surveyed BRCs (24/45) are undergoing strong interaction with their surrounding H II region and should be prioritized for detailed follow‑up, while the other half can be excluded from triggered‑star‑formation studies. The authors emphasize that the combination of CO line diagnostics with mid‑IR, sub‑mm, and radio data provides a robust framework for distinguishing between spontaneous and externally triggered star formation. They acknowledge limitations such as distance uncertainties for many southern BRCs and the need for higher‑density tracers (e.g., NH₃, N₂H⁺) to probe core interiors. Future work with ALMA and JWST is recommended to resolve the internal structure of the triggered clouds, to trace the earliest phases of star formation, and to refine the physical parameters governing radiation‑driven implosion.