Prior to post-main-sequence evolution, ionizing radiation is one of the most important mechanisms by which massive stars influence their surrounding environments. For example, ionizing radiation potentially triggers subsequent star-formation. The influences of massive stars are observed in the form of bubble-shaped emission in the 8 \(\mu\)m band of the Spitzer-GLIMPSE survey of the Galactic Plane \cite{Benjamin_2003}. \citet{Churchwell_2006,Churchwell_2007} observed bubble-shaped 8 \(\mu\)m emission to be common throughout the Galactic plane. \citet{Watson_2008, Watson_2009} found 24 \(\mu\)m and 20 cm emission centered within the 8 \(\mu\)m emission. They interpreted the bubbles seen in the GLIMPSE data as caused by hot stars which ionize their surroundings, creating 20 cm free-free emission, and at larger distances excite PAHs, creating 8 \(\mu\)m emission. \citet{Deharveng_2010} also interpreted the bubbles as classical HII regions.
\citet{Watson_2010} used 2MASS and GLIMPSE photometery and SED-fitting to analyze the young stellar object (YSO) population around 46 bubbles and found about a quarter showed an overabundance of YSOs near the boundary between the ionized interior and molecular exterior. Bubbles with an overabundance of YSOs along the bubble-ISM boundary are a potentially excellent set of sources to study the mechanisms of triggered star-formation. Star formation along the bubble rims may be triggered by the expanding ionization and shock fronts created by the hot star. Star formation triggered by previous generations of stars is known to occur but the specific physical mechanism is still undetermined. The collect-and-collapse model \cite{Elmegreen_1977} describes ambient material swept up by the shock fronts which eventually becomes gravitationally unstable, resulting in collapse. Other mechanisms, however, have been proposed. Radiatively-driven implosion \cite{1994A&A...289..559L}, for example, describes clumps already present in the ambient material whose contraction is aided by the external radiation from the hot star.
The method of identifying YSOs through SED-fitting used in \citet{Watson_2010}, however, is limited. Robitaille & Whitney (2006) showed that YSO age is degenerate with the observer’s inclination angle. Briefly, an early-stage YSO and a late-stage YSO seen edge on, so the accretion or debris disk is observed as thick and blocking the inner regions, can appear similar, even in the infrared. Thus, we require other diagnostics of the YSOs along the bubble edge to determine the youngest, and most likely to have been triggered, YSOs.
We selected a subset of the YSOs identified as triggered star formation candidates by \citet{Watson_2010} for follow-up observations in the CS (1-0) transition near 49 GHz with the Green Bank Telescope (GBT1). We sought evidence of infall, outflows or hot cores associated with these YSOs. CS is a probe of young star-formation. It has been detected in outflows from protostars, infall, disks and in hot cores \cite{1997A&A...317L..55D,1996A&AS..115...81B,Morata_2012}. The chemistry is, naturally, complex, and it appears that CS can play several roles \cite{Beuther_2002}, such as tracing outflows \cite{Wolf_Chase_1998} or hot cores \cite{1997MNRAS.287..445C}. Our aim is to use CS as a broad identifier of young star-formation and use any non-Gaussian line-shapes to infer molecular gas behavior.
After describing the CS survey and CS mapping observations (sec 2) and numerical results (sec 3), we analyze the Herschel-HiGAL emission toward all our sources to determine, along with our CS detections, the CS abundances (sec 4.1). We also analyze three sources for evidence of rapid infall (sec 4.2) and three mapped regions (sec 4.3). We end with a summary of the conclusions (sec 5).
The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.↩