  A wide variety of molecules have recently been detected in the Horsehead
nebula photodissociation region (PDR) suggesting that: (i) gas-phase and grain
chemistries should both contribute to the formation of organic molecules, and
(ii) far-ultraviolet (FUV) photodesorption may explain the release into the gas
phase of grain surface species. In order to tackle these specific problems and
more generally in order to better constrain the chemical structure of these
types of environments we present a study of the Horsehead nebula gas-grain
chemistry. To do so we used the 1D astrochemical gas-grain code Nautilus with
an appropriate physical structure computed with the Meudon PDR Code and
compared our modeled outcomes with published observations and with previously
modeled results when available. The use of a large set of chemical reactions
coupled with the time-dependent code Nautilus allows us to reproduce most of
the observations well, including those of the first detections in a PDR of the
organic molecules HCOOH, CH2CO, CH3CHO and CH3CCH, which are mostly associated
with hot cores. We also provide some abundance predictions for other molecules
of interest. Understanding the chemistry behind the detection of these organic
molecules is crucial to better constrain the environments these molecules can
probe.
