  Aims: We aim to simulate radial profiles of molecular abundances and the gas
temperature in cold and heavily shielded starless cores by combining chemical
and radiative transfer models. Methods: A determination of the dust temperature
in a modified Bonnor-Ebert sphere is used to calculate initial radial molecular
abundance profiles. The abundances of selected cooling molecules corresponding
to two different core ages are then extracted to determine the gas temperature
at two time steps. The calculation is repeated in an iterative process yielding
molecular abundances consistent with the gas temperature. Line emission
profiles for selected substances are calculated using simulated abundance
profiles. Results: The gas temperature is a function of time; the gas heats up
as the core gets older because the cooling molecules are depleted onto grain
surfaces. The contributions of the various cooling molecules to the total
cooling power change with time. Radial chemical abundance profiles are
non-trivial: different species present varying degrees of depletion and in some
cases inward-increasing abundances profiles, even at t > 10^5 years. Line
emission simulations indicate that cores of different ages can present
significantly different line emission profiles, depending on the tracer species
considered. Conclusions: Chemical abundances and the associated line cooling
power change as a function of time. Most chemical species are depleted onto
grain surfaces at densities exceeding ~10^5 cm^-3. Notable exceptions are NH_3
and N2H^+; the latter is largely undepleted even at n_H~10^6 cm-3. On the other
hand, chemical abundances are not significantly developed in regions of low gas
density even at t~10^5 years, revealed by inward-increasing abundance
gradients. The gas temperature can be significantly different from the dust
temperature; this may have implications on core stability.
