2023A&A...679A.123K

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 (T_dust) ≲ 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, L_SiO is highly correlated with dust clump mass, and L_SiO/L_bol also has a strong negative correlation with T_dust. This shows that colder and younger clumps have high L_SiO/L_bol 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 L_SiO/L_bol ratios lack wing features. Comparing SiO emission with H₂O and Class I CH₃OH masers, we find a significant correlation between L_SiO/L_bol 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 T_dust (and so probably less globally evolved) sources appear to have higher L_SiO relative to their L_bol. The SiO 1-0 emission toward infrared dark sources (T_dust ≲ 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.