The chemistry and spatial distribution of small hydrocarbons in
  UV-irradiated molecular clouds: the Orion Bar PDR
  We study the spatial distribution and chemistry of small hydrocarbons in the
Orion Bar PDR. We used the IRAM-30m telescope to carry out a millimetre line
survey towards the Orion Bar edge, complemented with ~2'x2' maps of the C2H and
c-C3H2 emission. We analyse the excitation of the detected hydrocarbons and
constrain the physical conditions of the emitting regions with non-LTE
radiative transfer models. We compare the inferred column densities with
updated gas-phase photochemical models including 13CCH and C13CH isotopomer
fractionation. ~40% of the lines in the survey arise from hydrocarbons (C2H,
C4H, c-C3H2, c-C3H, C13CH, 13CCH, l-C3H and l-H2C3). We detect new lines from
l-C3H+ and improve its rotational spectroscopic constants. Anions or deuterated
hydrocarbons are not detected: [C2D]/[C2H]<0.2%, [C2H-]/[C2H]<0.007% and
[C4H-]/[C4H]<0.05%. Our gas-phase models can reasonably match the observed
column densities of most hydrocarbons (within factors <3). Since the observed
spatial distribution of the C2H and c-C3H2 emission is similar but does not
follow the PAH emission, we conclude that, in high UV-flux PDRs,
photodestruction of PAHs is not a necessary requirement to explain the observed
abundances of the smallest hydrocarbons. Instead, gas-phase endothermic
reactions (or with barriers) between C+, radicals and H2 enhance the formation
of simple hydrocarbons. Observations and models suggest that the [C2H]/[c-C3H2]
ratio (~32 at the PDR edge) decreases with the UV field attenuation. The
observed low cyclic-to-linear C3H column density ratio (<3) is consistent with
a high electron abundance (Xe) PDR environment. In fact, the poorly constrained
Xe gradient influences much of the hydrocarbon chemistry in the more
UV-shielded gas. We propose that reactions of C2H isotopologues with 13C+ and H
atoms can explain the observed [C13CH]/[13CCH]=1.4(0.1) fractionation level.
