  Prestellar cores exhibit a strong chemical differentiation, which is mainly
caused by the freeze-out of molecules onto the grain surfaces. Understanding
this chemical structure is important, because molecular lines are often used as
probes to constrain the core physical properties. Here we present new
observations and analysis of the C18O (1-0) and H13CO+ (1-0) line emission in
the L1498 and L1517B prestellar cores, located in the Taurus-Auriga molecular
complex. We model these observations with a detailed chemistry network coupled
to a radiative transfer code. Our model successfully reproduces the observed
C18O (1-0) emission for a chemical age of a few 10^5 years. On the other hand,
the observed H13CO+ (1-0) is reproduced only if cosmic-ray desorption by
secondary photons is included, and if the grains have grown to a bigger size
than average ISM grains in the core interior. This grain growth is consistent
with the infrared scattered light ("coreshine") detected in these two objects,
and is found to increase the CO abundance in the core interior by about a
factor four. According to our model, CO is depleted by about 2-3 orders of
magnitude in the core center.
