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

Abstract

Aims. To search for evidence of triggered star formation within four bright-rimmed clouds, SFO 58, SFO 68, SFO 75 and SFO 76. Methods. We present the results of radio continuum and molecular line observations conducted using the Mopra millimetre-wave telescope and Australia Telescope Compact Array. We use the J = 1−0 transitions of 12 CO, 13 CO and C 18 O to trace the distribution of molecular material and to study its kinematics. The radio continuum data is used to trace the distribution of the ionised gas and to derive its parameters. Combining these observations with archival data allows us to build up a comprehensive picture of the current state of star formation within these clouds. Results. These observations reveal the presence of a dense core (nH2 > 10 4 cm −3 ) 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 (>25 cm −3 ) 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). Using a simple pressure-based argument, photoionisation is shown to have a profound effect on the stability of these cores, leaving SFO 58 and SFO 68 on the edge of gravitational stability, and is also likely to have rendered SFO 75 and SFO 76 unstable to gravitational collapse. 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, H2O 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. Conclusions. 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.