We have studied the influence of far ultraviolet ( UV ) radiation ( 6 < h \nu < 13.6 eV ) 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 G _ { 0 } \approx 10 – 100 at 200 AU from the star with an optical depth \tau \approx 15 – 17 .
The chemical networks of various species are explored .
Subtle differences are found compared with traditional models of Photon Dominated Regions ( PDRs ) because of the higher temperatures and higher gas-phase H _ { 2 } O abundance caused by evaporation of ices in the inner region .
In particular , the \@element [ ] [ ] [ ] [ ] { \mathrm { CN } } / \@element [ ] [ ] [ ] [ ] { \mathrm { 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 H _ { 2 } in the inner dense and warm region and produces more HCN .
It is found that the \@element [ ] [ ] [ ] [ ] { \mathrm { CH ^ { + } } } abundance is strongly enhanced and grows steadily with increasing UV field .
In addition , the ratio \@element [ ] [ ] [ ] [ ] { \mathrm { CH ^ { + } } } / \@element [ ] [ ] [ ] [ ] { \mathrm { CH } } is increased by a factor of 10 ^ { 3 } – 10 ^ { 5 } depending on the inner UV flux .
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 .