Radio continuum and molecular line observations of four bright-rimmed clouds

Abstract

We present the results of radio continuum and molecular line observations conducted using the Mopra millimetre-wave telescope and Australia Telescope Compact Array. These observations reveal the presence of a dense core embedded within each cloud, and the presence of a layer of hot ionised gas coincided with their bright-rims. The ionised gas has electron densities significantly higher than the critical density above which an ionised boundary layer can form and be maintained, strongly supporting the hypothesis that these clouds are being photoionised by the nearby OB star(s). From an evaluation of the pressure balance between the ionised and molecular gas, SFO 58 and SFO 68 are identified as being in a post-pressure balance state, while SFO 75 and SFO 76 are more likely to be in a pre-pressure balance state. We find secondary evidence for the presence of ongoing star formation within SFO 58 and SFO 68, such as molecular outflows, OH, H$_2$O and methanol masers, and identify a potential embedded UC HII region, but find no evidence for any ongoing star formation within SFO 75 and SFO 76. Our results are consistent with the star formation within SFO 58 and SFO 68 having been triggered by the radiatively driven implosion of these clouds.