CS emission near MIR-bubbles
We survey 44 young stellar objects located near the edges of mid-IR-identified bubbles in CS (1-0) using the Green Bank Telescope. We detect emission in 18 sources, indicating young protostars that are good candidates for being triggered by the expansion of the bubble. We calculate CS column densities and abundances. Three sources show evidence of infall through non-Gaussian line-shapes. Two of these sources are associated with dark clouds and are promising candidates for further exploration of potential triggered star formation. We obtained on-the-fly maps in CS (1-0) of three sources, showing evidence of significant interactions between the sources and the surrounding environment.
Prior to post-main-sequence evolution, ionizing radiation is one of the most important mechanisms by which massive stars influence their surrounding environments. This ionizing radiation may potentially trigger subsequent star-formation. The influence of ionizing radiation is observed in the form of bubble-shaped emission in the 8 \(\mu\)m band of the Spitzer-GLIMPSE survey of the Galactic Plane (Benjamin et al., 2003). Churchwell et al. (2006); Churchwell et al. (2007) observed bubble-shaped 8 \(\mu\)m emission to be common throughout the Galactic plane. Watson et al. (2008); Watson et al. (2009) found 24 \(\mu\)m and 20 cm emission centered within the 8 \(\mu\)m emission and interpreted the bubbles seen in the GLIMPSE data as caused by hot stars ionizing their surroundings, creating 20 cm free-free emission, and at larger distances exciting PAHs, creating 8 \(\mu\)m emission. Deharveng et al. (2010) also interpreted the bubbles as classical HII regions.
Watson et al. (2010) used 2MASS and GLIMPSE photometry and Spectral Energy Distribution (SED)-fitting to analyze the 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. These YSOs are candidates for being 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 (Elmegreen et al., 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 (Lefloch et al., 1994), for example, describes clumps already present in the ambient material whose contraction is aided by the external radiation of the hot star.
Bubbles with an overabundance of YSOs along the bubble-interstellar medium (ISM) boundary are a potentially excellent set of sources to study the mechanisms of triggered star-formation. The method of identifying YSOs through photometry, however, is limited. Robitaille et al. (2006) showed that YSO age is degenerate with the observer’s inclination angle. 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 IR. Thus, we require other diagnostics of the YSOs along the bubble edge to determine the youngest, and most likely to have been triggered, YSOs. Additionally, a line-diagnostic allows us to rule out any line-of-sight coincidence associations.
For the current project we selected a subset of the bubbles identified above to identify those YSOs associated with infall, outflows or hot cores by observing the CS (1-0) transition near 49 GHz with the Green Bank Telescope (GBT11The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.). CS is a probe of young star-formation. It has been detected in outflows from protostars, infall, disks and in hot cores (Dutrey et al., 1997; Bronfman et al., 1996; Morata et al., 2012). The chemistry is, naturally, complex, and it appears that CS can play several roles (Beuther et al., 2002), such as tracing outflows (Wolf-Chase et al., 1998) or hot cores (Chandler et al., 1997). Our aim here 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 (\(\S\) 2) and numerical results (\(\S\) 3), we analyze the Herschel-HiGAL emission toward all our sources to determine, along with our CS detections, the CS abundances (\(\S\) 4.1). We also analyze three sources with evidence of rapid infall (\(\S\) 4.2) and three mapped regions (\(\S\) 4.3). We end with a summary of the conclusions.