We have surveyed 44 YSOs located near the edges of MIR-identified bubbles in CS(1-0) using the GBT. Our conclusions are:
We have detected CS toward 18 sources.
Using Herschel/HiGal survey data, we calculated CS abundances for these sources to be \(\sim\)10\(^{-7}\) and range between 0.16-9.36 \(\times\)10\(^{-7}\).
Three sources show non-Gaussian line-profiles with strong emission on the blue-shifted side. We interpret this profile as caused by gas infall onto a protostar.
Two of the infall candidates (N62-1 and N90-2) are embedded in infrared dark clouds along the edge of their expanding bubbles. The combination of photometry-based YSO identification, CS-based infall, location inside an IRDC and on the edge of an expanding bubble is strongly suggestive of triggered star formation.
Using a two-component model, we estimate that one infall candidate, N117-3, has an average infall speed of 0.31 km/s and a mass infall rate of 2.9 \(\times\) 10\(^{-5}\) M\(_\odot\)/yr. These numerical results are consistent with intermediate to massive star-formation.
Our interpretation of infall in N62-1, N90-2, and N117-3 assumes that the observed CS emission is optically thick. However, our interpretation of the asymmetric, non-Gaussian line profile in N65-2 is that there are two line-of-sight clouds contributing to the emission. It is possible that a similar mechanism could produce the profiles seen in N62-1, N90-2, and N117-3. Further observations of an optically thin line, for example \(^{34}\)CS, are needed to distinguish between the two possible interpretations.
The three infall candidates are promising sources for further study to better determine the mechanisms involved in triggered star-formation. The two candidates embedded in IRDCs are especially promising and are being mapped in a follow-up study (Devine et al., in prep.).