CS emission near MIR-bubbles
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 (Benjamin 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. 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. Deharveng et al. (2010) also interpreted the bubbles as classical HII regions.
Watson et al. (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 (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 (Lefloch 1994), 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 Watson et al. (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 Watson et al. (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 (Dutrey 1997, Bronfman 1996, Morata 2012). The chemistry is, naturally, complex, and it appears that CS can play several roles (Beuther 2002), such as tracing outflows (Wolf-Chase 1998) or hot cores (Chandler 1997). 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.↩
Candidate YSO locations were identified using the SED fitter tool developed by Robitaille et al. (2006, 2007). Briefly, this method uses the 2MASS and GLIMPSE point source catalogues to identify sources that are not well-fit by main-sequence SEDs and are well-fit by YSO SEDs. Watson et al. (2010) fit all point sources within 1’ of the bubble edges using this method. From this set of point sources, four sources were selected near each bubble based on association with either diffuse, bright 8 \(\mu\)m emission or IR dark clouds. Forty point sources in total were selected. The names, Galactic longitude and Galactic latitude are reported in Table 2. Each point source was observed for CS using the Green Bank Telescope (GBT) for two 5 minute integrations. The spectrometer was set-up in frequency switching mode to maximize on-source observing time. The setup parameters and calibration sources are listed in Table 1.
|Channel width||1.5 kHz|
|Rest frequency||48.99095 GHz|
|Frequency switching shift||8 MHz|
Data were calibrated and analyzed using GBTIDL. Typical system temperatures were between 105 K and 120 K. Typical rms noise in the resulting calibrated spectra was 0.20 K. Non-detections and detections are listed in Tables 2 and 3, respectively. We estimate uncertainty due to flux calibration of 20%.
In addition to single pointings, we mapped three regions (N56, N65 2 and N77 1) that displayed strong CS emission. The map sizes were 1’x1’ (N56 and N77-1) and 2’x2’ (N65-2), both using a Nyquist-sampling step-size of 6.12"
We also used observations of all of the sources in this study from Hi-Gal (Molinari 2010), a Herschel Space Telescope imaging survey of the Galactic plane conducted at wavelengths between 60 \(\mu\)m and 600 \(\mu\)m. Data were downloaded from the Spitzer Science Center website. Fully calibrated Level 2 data products were used.