ATLASGAL-selected massive clumps in the inner Galaxy. VIII. Chemistry of photodissociation regions

Aims: We study ten molecular transitions obtained from an unbiased 3 mm molecular line survey using the IRAM 30 m telescope toward 409 compact dust clumps identified by the APEX Telescope Large Area Survey of the Galaxy (ATLASGAL) to better understand the photodissociation regions (PDRs) associated with these clumps. The main goal of this study is to investigate whether the abundances of the selected molecules show any variations resulting from the PDR chemistry in different clump environments.
Methods: We selected HCO, HOC⁺, C₂H, c-C₃H₂, CN, H¹³CN, HC¹⁵N, and HN¹³C as PDR tracers, and H¹³CO⁺ and C¹⁸O as dense gas tracers. By using estimated optical depths of C₂H and H¹³CN and assuming optically thin emission for other molecular transitions, we derived the column densities of those molecules and their abundances. To assess the influence of the presence and strength of ultra-violet radiation, we compare abundances of three groups of the clumps: HII regions, infrared bright non-HII regions, and infrared dark non-HII regions.
Results: We detected C¹⁸O, H¹³CO⁺, C₂H, c-C₃H₂, CN, and HN¹³C toward most of the observed dust clumps (detection rate >94%), and H¹³CN is also detected with a detection rate of 75%. On the other hand, HCO and HC¹⁵N show detection rates of 32 and 39%, respectively, toward the clumps, which are mostly associated with HII region sources: detection rates of HCO and HC¹⁵N toward the HII regions are 66 and 79%. We find that the abundances of HCO, CN, C₂H, and c-C₃H₂ decrease as the H₂ column density increases, indicating high visual extinction, while those of high-density tracers (i.e., H¹³CO⁺ and HC¹⁵N) are constant. In addition, N(HCO)/N(H¹³CO⁺) ratios significantly decrease as H₂ column density increases, and, in particular, 82 clumps have X(HCO) ≳ 10^-10 and N(HCO)/N(H¹³CO⁺) ≳ 1, which are indications of far-ultraviolet (FUV) chemistry. This suggests the observed HCO abundances are likely associated with FUV radiation illuminating the PDRs. We also find that high N(c-C₃H₂)/N(C₂H) ratios found for HII regions that have high HCO abundances (≳10^-10) are associated with more evolved clumps with high L_bol/M_clump. This trend might be associated with grain-surface processes, which determine the initial abundances of these molecules, and time-dependent effects in the clumps corresponding to the envelopes around dense PDRs and HII regions. In addition, some fraction of the measured abundances of the small hydrocarbons of the HII sources may be the result of the photodissociation of PAH molecules.

Full Tables 2, 3, 4, 6 are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/644/A160