Chemical variations across the TMC-1 boundary: molecular tracers from
  translucent phase to dense phase
  We investigated the chemical evolutions of gas phase and grain surface
species across the Taurus molecular cloud-1 (TMC-1) filament from translucent
phase to dense phase. By comparing observations with modeling results from an
up-to-date chemical network, we examined the conversion processes for the
carbon-, oxygen-, nitrogen- and sulfur-bearing species, i.e.from their initial
atomic form to their main molecular reservoir form both in the gas phase and on
the grain surface. The conversion processes were found to depend on the species
and A$_V$. The effect of initial carbon to oxygen elemental abundances ratio
(C/O) by varying O on the chemistry was explored, and an initial carbon
elemental abundance of 2.5 $\times$ 10$^{-4}$ and a C/O ratio of 0.5 could best
reproduce the abundances of most observed molecules at TMC-1 CP, where more
than 90 molecules have been identified. Based on the TMC-1 condition, we
predicted a varied grain ice composition during the evolutions of molecular
clouds, with H$_2$O ice as the dominant ice composition at A$_V$ $>$ 4 mag,
CO$_2$ ice as the dominant ice composition at A$_V$ $<$ 4 mag, while CO ice
severely decreased at A$_V$ around 4--5 mag.
