A survey of SiO J = 1−0 emission toward massive star-forming regions
Authors: Kim W.-J. , Urquhart J. S. , Veena V. S. , Fuller G. A. , Schilke P. , Kim K.-T. 
Published: October 2023

Aims. The application of silicon monoxide (SiO) as a shock tracer arises from its propensity to occur in the gas phase as a result of shock-induced phenomena, including outflow activity and interactions between molecular clouds and expanding HII regions or supernova remnants. For this work, we searched for indications of shocks toward 366 massive star-forming regions by observing the ground rotational transition of SiO (v = 0, J = 1-0) at 43 GHz with the Korean VLBI Network (KVN) 21 m telescopes to extend our understanding on the origins of SiO in star-forming regions.
Methods. We analyzed the thermal SiO 1-0 emission and compared the properties of SiO emission with the physical parameters of associated massive dense clumps as well as 22 GHz H2O and Class I 44 GHz CH3OH maser emission.
Results. We detected SiO emission toward 104 regions that consist of 57 IRDCs, 21 HMPOs, and 26 UCHIIs. Out of 104 sources, 71 and 80 sources have 22 GHz H2O and 44 GHz Class I CH3OH maser counterparts, respectively. The determined median SiO column density, N(SiO), and abundance, X(SiO), relative to N(H2) are 8.12 × 1012 cm−2 and 1.28 × 10−10, respectively. These values are similar to those obtained toward other star-forming regions and also consistent with predicted values from shock models with low-velocity shocks (≲ 10–15 km s−1). For sources with dust temperatures of (Tdust) ≲ 20 K, we find that N(SiO) and X(SiO) derived with the J = 1–0 transition are a factor ~3 larger than those from the previous studies obtained with SiO 2–1. While the X(SiO) does not exhibit any strong correlation with the evolutionary stages of their host clumps, LSiO is highly correlated with dust clump mass, and LSiO/Lbol also has a strong negative correlation with Tdust. This shows that colder and younger clumps have high LSiO/Lbol suggestive of an evolutionary trend. This trend is not due to excess emission at higher velocities, such as SiO wing features, as the colder sources with high LSiO/Lbol ratios lack wing features. Comparing SiO emission with H2O and Class I CH3OH masers, we find a significant correlation between LSiO/Lbol and ${{{L_{{\rm{C}}{{\rm{H}}_3}{\rm{OH}}}}} \mathord{\left/ {\vphantom {{{L_{{\rm{C}}{{\rm{H}}_3}{\rm{OH}}}}} {{L_{{\rm{bol}}}}}}} \right. \kern-\nulldelimiterspace} {{L_{{\rm{bol}}}}}}$ ratios, whereas no similar correlation is seen for the H2O maser emission. This suggests a similar origin for the SiO and Class I CH3OH emission in these sources.
Conclusions. We demonstrate that in cold regions SiO J = 1–0 may be a better tracer of shocks than a higher J transition of SiO. Lower Tdust (and so probably less globally evolved) sources appear to have higher LSiO relative to their Lbol. The SiO 1–0 emission toward infrared dark sources (Tdust ≲ 20 K), which do not contain identified outflow sources, may be related to other mechanisms producing low-velocity shocks (5–15 km s−1) for example, arising from cloud-cloud collisions, shocks triggered by expanding HII regions, global infall, or converging flows.