  We have compared molecular line emission to dust continuum emission and
modeled molecular lines using Monte Carlo simulations in order to study the
depletion of molecules and the ionization fraction in three preprotostellar
cores, L1512, L1544, and L1689B. L1512 is much less dense than L1544 and
L1689B, which have similar density structures. L1689B has a different
environment from those of L1512 and L1544. We used density and temperature
profiles, calculated by modeling dust continuum emission in the submillimeter,
for modeling molecular line profiles. In addition, we have used molecular line
profiles and maps observed in several different molecules toward the three
cores. We find a considerable diversity in chemical state among the three
cores. The molecules include those sensitive to different timescales of
chemical evolution such as CCS, the isotopes of CO and HCO+, DCO+, and N2H+.
The CO molecule is significantly depleted in L1512 and L1544, but not in
L1689B. CCS may be in the second enhancement of its abundance in L1512 and
L1544 because of the significant depletion of CO molecules. N2H+ might already
start to be depleted in L1512, but it traces very well the distribution of dust
emission in L1544. On the other hand, L1689B may be so young that N2H+ has not
reached its maximum yet. The ionization fraction has been calculated using
H13CO+ and DCO+. This study suggests that chemical evolution depends on the
absolute timescale during which a core stays in a given environment as well as
its density structure.
