A tale of two cores: Triggered massive star formation in the bright-rimmed cloud SFO 75

Context.Bright-rimmed clouds (BRCs) are isolated molecular clouds located on the edges of evolved HII regions. Star formation within the BRCs may have been triggered through the propagation of photoionisation-induced shocks driven by the expansion of the HII region.

Aims.The main focus of this paper is to investigate the current level of star formation within one of these clouds and evaluate to what extent, if any, star formation may have been triggered.

Methods.We present a detailed multi-wavelength study of the BRC SFO 75, including 1.3 cm and 1.2 mm continuum, and 13CO and ammonia spectral line observations. To build up a comprehensive picture of the local environment we complement our observations with archival data from the 2MASS, GLIMPSE and IRAS surveys.

Results.The 13CO and 1.2 mm emission reveals the presence of a dense core located behind the bright rim of the cloud which is approximately coincident with that of the IRAS point source. From an analysis of the IRAS and 1.2 mm fluxes we derive a dust temperature of ~30 K, a luminosity of $L_{\rm {bol}}$ = 1.6$\times$104 $L_\odot$ and estimate the core mass to be ~570 $M_\odot$. The higher resolution ammonia observations resolve the 1.2 mm core into two distinct cores, one directly behind the cloud's rim (Core A) and the second located slightly farther back (Core B). These have masses of 8-15 $M_\odot$ and 3.5-7 $M_\odot$ for Core A and Core B respectively, which are significantly larger than their virial masses. Comparing the morphology of Core A with that of the photon-dominated region and ionised boundary layer leaves little doubt that it is being strongly affected by the ionisation front. 2MASS and GLIMPSE archive data which reveal a small cluster of three deeply embedded ($A_{\rm {v}}\sim$ 20 mag) high- and intermediate-mass young stellar objects towards Core A leads us to conclude that the star formation found towards this core has been triggered. In stark contrast, Core B appears to have a much simpler, almost spherical, morphology. No stars are found towards Core B. We find evidence supporting the presence of shocked gas within the surface layers of the cloud which appears to extend to midway between the two ammonia cores.

Conclusions.The scenario that emerges from our analysis is one where the two ammonia cores pre-date the arrival of the ionisation front. Since its arrival the over-pressure of the ionised gas at the surface of the cloud has driven shocks into the surface layers of the cloud. The propagation of these shocks through Core A have triggered the formation of a small cluster of massive stars, however, the shock front has not yet propagated deeply enough into the cloud to have affected the evolution of Core B.