Observational study of hydrocarbons in the bright photodissociation
  region of Messier 8
  Hydrocarbons are ubiquitous in the interstellar medium, but their formation
is still not well understood, depending on the physical environment they are
found in. M8 is host to one of the brightest HII regions and PDRs in our
galaxy. Using the APEX, and the IRAM 30 m telescopes, we performed a line
survey toward Herschel 36 (Her 36), which is the main ionizing stellar system
in M8, and an imaging survey within 1.3 $\times$ 1.3 pc around Her 36 of
various transitions of C$_{2}$H and c-C$_{3}$H$_{2}$. We used both LTE and
non-LTE methods to determine the physical conditions of the emitting gas along
with the column densities and abundances of the observed species, which we
compared with (updated) gas phase photochemical PDR models. In order to examine
the role of PAHs in the formation of small hydrocarbons and to investigate
their association with M8, we compared archival GLIMPSE 8 $\mu$m and the SPIRE
250 $\mu$m continuum images with the C$_{2}$H emission maps. We observed a
total of three rotational transitions of C$_{2}$H with their hyperfine
structure components and four rotational transitions of c-C$_{3}$H$_{2}$ with
ortho and para symmetries toward M8. Fragmentation of PAHs seems less likely to
contribute to the formation of small hydrocarbons as the 8 $\mu$m emission does
not follow the distribution of C$_{2}$H emission, which is more associated with
the molecular cloud. From the quantitative analysis, we obtained abundances of
$\sim$ 10$^{-8}$ and 10$^{-9}$ for C$_{2}$H and c-C$_{3}$H$_{2}$ respectively,
and volume densities of the hydrocarbon emitting gas in the range $n(\rm H_2)$
$\sim$ 5 $\times$ 10$^{4}$--5 $\times$ 10$^{6}$ cm$^{-3}$. The observed column
densities of C$_{2}$H and c-C$_3$H$_{2}$ are reproduced reasonably well by our
PDR models. This supports the idea that in high-UV flux PDRs, gas phase
chemistry is sufficient to explain hydrocarbon abundances.
