Aims. Observations of millimeter wavelength radio recombination lines (mm-RRLs) are used to search for Hii regions in an unbiased way that is complementary to many of the more traditional methods previously used (e.g., radio continuum, far-infrared colors, maser emission). The mm-RRLs can be used to derive physical properties of Hii regions and to provide velocity information of ionized gas. Methods. We carried out targeted mm-RRL observations (39 ≤ principal quantum number (n) ≤ 65 and ∆n = 1, 2, 3, and 4, named Hnα, Hnβ, Hnγ, and Hnδ) using the IRAM 30m and Mopra 22m telescopes. In total, we observed 976 compact dust clumps selected from a catalog of ∼10,000 sources identified by the APEX Telescope Large Area Survey of the Galaxy (ATLASGAL). The sample was selected to ensure a representative mix of star-forming and quiescent clumps such that a variety of different evolutionary stages is represented. Approximately half of the clumps are mid-infrared quiet while the other half are mid-infrared bright. Results. We detected Hnα mm-RRL emission toward 178 clumps; Hnβ, Hnγ, and Hnδ were also detected toward 65, 23, and 22 clumps, respectively. This is the largest sample of mm-RRLs detections published to date. Comparing the positions of these clumps with radio continuum surveys we identified compact radio counterparts for 134 clumps, confirming their association with known Hii regions. The nature of the other 44 detections is unclear, but 8 detections are thought to be potentially new Hii regions while the mm-RRL emission from the others may be due to contamination from nearby evolved Hii regions. Broad linewidths are seen toward nine clumps (linewidth > 40 km s −1 ) revealing significant turbulent motions within the ionized gas; in the past, such wide linewidths were found toward very compact and dense Hii regions. We find that the systemic velocity of the associated dense molecular gas, traced by H 13 CO + (1−0), is consistent with the mm-RRL velocities and confirms them as embedded Hii regions. We also find that the linewidth of the H 13 CO + (1−0) emission is significantly wider than those without mm-RRL detection, indicating a physical connection between the embedded Hii region and their natal environments. We also find a correlation between the integrated fluxes of the mm-RRLs and the 6 cm continuum flux densities of their radio counterparts (the correlation coefficient, ρ, is 0.70). By calculating the electron densities we find that the mm-RRL emission is associated with Hii regions with n e <10 5 cm −3 and Hii region diameter > 0.03 pc. Conclusions. We detected mm-RRLs toward 178 clumps and identified eight new Hii region candidates. The broad mm-RRL from nine clumps may indicate that they arise in very young hyper-compact Hii regions. The mm-RRLs trace the radio continuum sources detected by high-resolution observations and their line parameters show associations with the embedded radio sources and their parental molecular clumps.

Aims. Previous radio recombination line (RRL) observations of dust clumps identified in the APEX Telescope Large Area Survey of the Galaxy (ATLASGAL) have led to the detection of a large number of RRLs in the 3mm range. Here, we aim to study their excitation with shorter wavelength (sub)millimeter radio recombination line (submm-RRL) observations. Methods. We made observations of submm-RRLs with low principal quantum numbers (n ≤ 30) using the APEX 12 m telescope, toward 104 Hii regions associated with massive dust clumps from ATLASGAL. The observations covered the H25α, H28α, and H35β transitions. Toward a small subsample the H26α, H27α, H29α, and H30α lines were observed to avoid contamination by molecular lines at adjacent frequencies.Results. We have detected submm-RRLs (signal-to-noise ≥ 3 σ) from compact Hii regions embedded within 93 clumps. The submm-RRLs are approximately a factor of two brighter than the mm-RRLs and consistent with optically thin emission in local thermodynamic equilibrium (LTE). The average ratio (0.31) of the measured H35β/H28α fluxes is close to the LTE value of 0.28. No indication of RRL maser emission has been found. The Lyman photon flux, bolometric, and submm-RRL luminosities toward the submm-RRL detected sources present significant correlations. The trends of dust temperature and the ratio of bolometric luminosity to clump mass, L bol /M clump , indicate that the Hii regions are related to the most massive and luminous clumps. By estimating the production rate of ionizing photons, Q, from the submm-RRL flux, we find that the Q(H28α) measurements provide estimates of the Lyman continuum photon flux consistent with those determined from 5 GHz radio continuum emission. Six RRL sources show line profiles that are a combination of a narrow and a broad Gaussian feature. The broad features are likely associated with high-velocity ionized flows. Conclusions. We have detected submm-RRLs toward 93 ATLASGAL clumps. Six RRL sources have high-velocity RRL components likely driven by high-velocity ionized flows. Their observed properties are consistent with thermal emission that correlates well with the Lyman continuum flux of the Hii regions. The sample of Hii regions with mm/submm-RRL detections probes, in our Galaxy, luminous clumps (L bol > 10 4 L ) with high L bol /M clump . We also provide suitable candidates for further studies of the morphology and kinematics of embedded, compact Hii regions with the Atacama Large Millimeter/submillimeter Array (ALMA).

Aims. Hydrocarbons are ubiquitous in the interstellar medium, but their formation is still not well understood, depending on the physical environment they are found in. Messier 8 (M8) is host to one of the brightest Hii regions and photodissociation regions (PDRs) in our galaxy. With the observed C 2 H and c-C 3 H 2 data toward M8, we aim at obtaining their densities and abundances and to shed some light on their formation mechanism. Methods. Using the Atacama Pathfinder Experiment (APEX) 12 m, and the Institut de Radioastronomie Millimétrique (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 × 1.3 pc around Her 36 of various transitions of C 2 H and C 3 H 2 . We used both Local Thermodynamic Equilibrium (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 polycyclic aromatic hydrocarbons (PAHs) in the formation of small hydrocarbons and to investigate their association with the Hii region, the PDR and the molecular cloud, we compared archival Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE) 8 µm and the Spectral and Photometric Imaging Receiver (SPIRE) 250 µm continuum images with the C 2 H emission maps.Results. We observed a total of three rotational transitions of C 2 H with their hyperfine structure components and four rotational transitions of C 3 H 2 with ortho and para symmetries toward the Hii region and the PDR of M8. Fragmentation of PAHs seems less likely to contribute to the formation of small hydrocarbons as the 8 µm emission does not follow the distribution of C 2 H emission, which is more associated with the molecular cloud toward the north-west of Her 36. From the quantitative analysis, we obtained abundances of ∼ 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(H 2 ) ∼ 5 × 10 4 -5 × 10 6 cm −3 . Conclusions. 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.

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 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 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.