CS Depletion in Prestellar Cores
  The CS molecule is known to be absorbed onto dust in the cold and dense
conditions, causing it to get significantly depleted in the central region of
cores. This study is aimed to investigate the depletion of the CS molecule
using the optically thin C$^{34}$S molecular line observations. We mapped five
prestellar cores, L1544, L1552, L1689B, L694-2, and L1197 using two molecular
lines, C$^{34}$S $(J=2-1)$ and N$_2$H$^+$ $(J=1-0)$ with the NRO 45-m
telescope, doubling the number of cores where the CS depletion was probed using
C$^{34}$S. In most of our targets, the distribution of C$^{34}$S emission shows
features that suggest that the CS molecule is generally depleted in the center
of the prestellar cores. The radial profile of the CS abundance with respect to
H$_2$ directly measured from the CS emission and the Herschel dust emission
indicates that the CS molecule is depleted by a factor of $\sim$3 toward the
central regions of the cores with respect to their outer regions. The degree of
the depletion is found to be even more enhanced by an order of magnitude when
the contaminating effect introduced by the presence of CS molecules in the
surrounding envelope that lie along the line-of-sight is removed. Except for
L1197 which is classified as relatively the least evolved core in our targets
based on its observed physical parameters, we found that the remaining four
prestellar cores are suffering from significant CS depletion at their central
region regardless of the relative difference in their evolutionary status.
