Searching for shocks in high-mass starless clump candidates
  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 H$_2$CO
$2_{12}-1_{11}$ line toward 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 H$_2$CO line are
$31.0\%$, $31.0\%$, $19.5\%$ and $93.0\%$, 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 ($\sim29\%$) 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$\%$ 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 $0.31-4.32\times10^{12}~cm^{-2}$. 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 H$_2$ are usually
$0.20-10.92\times10^{-10}$.
