  We have studied the influence of far ultraviolet (UV) radiation from a
massive young stellar object (YSO) on the chemistry of its own envelope by
extending the models of Doty et al. (2002) to include a central source of UV
radiation. The models are applied to the massive star-forming region AFGL 2591
for different inner UV field strengths. Depth-dependent abundance profiles for
several molecules are presented and discussed. We predict enhanced column
densities for more than 30 species, especially radicals and ions. Comparison
between observations and models is improved with a moderate UV field incident
on the inner envelope, corresponding to an enhancement factor G0~10-100 at 200
AU from the star with an optical depth tau~15-17. Subtle differences are found
compared with traditional models of Photon Dominated Regions (PDRs) because of
the higher temperatures and higher gas-phase H2O abundance caused by
evaporation of ices in the inner region. In particular, the CN/HCN ratio is not
a sensitive tracer of the inner UV field, in contrast with the situation for
normal PDRs: for low UV fields, the extra CN reacts with H2 in the inner dense
and warm region and produces more HCN. It is found that the CH+ abundance is
strongly enhanced and grows steadily with increasing UV field. High-J lines of
molecules like CN and HCN are most sensitive to the inner dense region where UV
radiation plays a role. Thus, even though the total column density affected by
UV photons is small, comparison of high-J and low-J lines can selectively trace
and distinguish the inner UV field from the outer one. In addition, future
Herschel-HIFI observations of hydrides can sensitively probe the inner UV
field.
