| Abstract
| - 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 +, C 2H, c-C 3H 2, CN, H 13CN, HC 15N, and HN 13C as PDR tracers, and H 13CO + and C 18O as dense gas tracers. By using estimated optical depths of C 2H and H 13CN 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: H II regions, infrared bright non-H II regions, and infrared dark non-H II regions. Results. We detected C 18O, H 13CO +, C 2H, c-C 3H 2, CN, and HN 13C toward most of the observed dust clumps (detection rate >94%), and H 13CN is also detected with a detection rate of 75%. On the other hand, HCO and HC 15N show detection rates of 32 and 39%, respectively, toward the clumps, which are mostly associated with H II region sources: detection rates of HCO and HC 15N toward the H II regions are 66 and 79%. We find that the abundances of HCO, CN, C 2H, and c-C 3H 2 decrease as the H 2 column density increases, indicating high visual extinction, while those of high-density tracers (i.e., H 13CO + and HC 15N) are constant. In addition, N(HCO)/ N(H 13CO +) ratios significantly decrease as H 2 column density increases, and, in particular, 82 clumps have X(HCO) ≳ 10 −10 and N(HCO)/ N(H 13CO +) ≳ 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-C 3H 2)/ N(C 2H) ratios found for H II regions that have high HCO abundances (≳10 −10) are associated with more evolved clumps with high Lbol/ Mclump. 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 H II regions. In addition, some fraction of the measured abundances of the small hydrocarbons of the H II sources may be the result of the photodissociation of PAH molecules.
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