Searching for shocks in high-mass starless clump candidates

Abstract

ABSTRACT 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 $93.0{{\ \rm per\ cent}}$, 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 ($\sim 29{{\ \rm per\ cent}}$) 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 ${{\ \rm per\ cent}}$ 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 × 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 × 10−10.