SiO J=1-0 emission toward massive SFRs

The determined median SiO column density, N(SiO), and abundance, X(SiO), relative to N(H_2_) are 8.12x10^12^cm^-2^ and 1.28x10^-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 (<~10-15km/s). For sources with dust temperatures (T_dust_)<~20K, 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 Tdust. 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 with H_2_O and Class I CH_3_OH masers, we find a significant correlation between L_SiO_/L_bol_ and L_CH_3_OH_/L_bol_ ratios, whereas no similar correlation is seen for the H_2_O maser emission. This suggests a similar origin for the SiO and Class I CH_3_OH emission in these sources. We demonstrate that in cold regions SiO J=1-0 may be a better tracer of shocks than higher J transition of SiO.

Lower Tdust (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_<~20K), which do not contain identified outflow sources, may be related to other mechanisms producing low-velocity shocks (5-15km/s) for example, arising from cloud-cloud collisions, shocks triggered by expanding HII regions, global infall, or converging flows.