  Spectral lines of ammonia, NH$_3$, are useful probes of the physical
conditions in dense molecular cloud cores. In addition to advantages in
spectroscopy, ammonia has also been suggested to be resistant to freezing onto
grain surfaces, which should make it a superior tool for studying the interior
parts of cold, dense cores. Here we present high-resolution NH$_3$ observations
with the Very Large Array (VLA) and Green Bank Telescope (GBT) towards a
prestellar core. These observations show an outer region with a fractional
NH$_3$ abundance of X(NH$_3$) = (1.975$\pm$0.005)$\times 10^{-8}$ ($\pm 10\%$
systematic), but it also reveals that after all, the X(NH$_3$) starts to
decrease above a H$_2$ column density of $\approx 2.6 \times 10^{22}$
cm$^{-2}$. We derive a density model for the core and find that the break-point
in the fractional abundance occurs at the density n(H$_2$) $\sim 2\times10^5$
cm$^{-3}$, and beyond this point the fractional abundance decreases with
increasing density, following the power law $n^{-1.1}$. This power-law behavior
is well reproduced by chemical models where adsorption onto grains dominates
the removal of ammonia and related species from the gas at high densities. We
suggest that the break-point density changes from core to core depending on the
temperature and the grain properties, but that the depletion power law is
anyway likely to be close to $n^{-1}$ owing to the dominance of accretion in
the central parts of starless cores.
