2020MNRAS.499.6018Z

In order to search for shocks in the very early stage of star formation, we performed single-point surveys of SiO J = 1-0, 2-1, and 3-2 lines and the H₂CO 2₁₂-1₁₁ line towards a sample of 100 high-mass starless clump candidates (SCCs) by using the Korean VLBI Network (KVN) 21-m radio telescopes. The detection rates of the SiO J = 1-0, 2-1, 3-2 lines, and the H₂CO line are 31.0, 31.0, 19.5, and 93.0 per cent, respectively. Shocks seem to be common in this stage of massive star formation. The widths of the observed SiO lines [full width at zero power (FWZP)] range from 3.4 to 55.1 km s^–1. A significant fraction (∼29 per cent) of the detected SiO spectra have broad line widths (FWZP > 20 km s^–1), which are very likely associated with fast shocks driven by protostellar outflows. This result suggests that about one third of the SiO-detected SCCs are not really starless but protostellar. On the other hand, about 40 per cent of the detected SiO spectra show narrow line widths (FWZP < 10 km s^–1) probably associated with low-velocity shocks which are not necessarily protostellar in origin. The estimated SiO column densities are mostly 0.31-4.32 x 10¹² cm^–2. Comparing the SiO column densities derived from SiO J = 1-0 and 2-1 lines, we suggest that the SiO molecules in the SCCs may be in the non-LTE condition. The SiO abundances to H₂ are usually 0.20-10.92 x 10^–10.