New detections of (sub)millimeter hydrogen radio recombination lines towards high-mass star-forming clumps

Abstract

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. We made observations of submm-RRLs with low principal quantum numbers ($n$ $\le$ 30) using the APEX 12 m telescope, toward 104 HII regions associated with massive dust clumps from ATLASGAL. The observations covered the H25$\alpha$, H28$\alpha$, and H35$\beta$ transitions. Toward a small subsample the H26$\alpha$, H27$\alpha$, H29$\alpha$, and H30$\alpha$ lines were observed to avoid contamination by molecular lines at adjacent frequencies. We have detected submm-RRLs (signal-to-noise $\ge$ 3 $\sigma$) 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 H35$\beta$/H28$\alpha$ 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, $L_{\mathrm{bol}}/M_{\mathrm{clump}}$, 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 $Q$(H28$\alpha$) measurements provide estimates of the Lyman continuum photon flux consistent with those determined from 5 GHz radio continuum emission.

20 pages, 13 figures, 8 tables. Accepted for publication in A&A

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Cited by in corpus (12)

• ATLASGAL -- Evolutionary trends in high-mass star formation
• A population of hypercompact HII regions identified from young HII regions
• SEDIGISM-ATLASGAL: Dense Gas Fraction and Star Formation Efficiency Across the Galactic Disk
• Fragmentation and filaments at the onset of star and cluster formation: SABOCA 350 $\mu$m view of ATLASGAL selected massive clumps
• ATOMS: ALMA Three-millimeter Observations of Massive Star-forming regions -XIV. Properties of resolved UC Hii regions
• The accretion burst of the massive young stellar object G323.46 -0.08
• Infall and Outflow Towards High-mass Starless Clump Candidates
• ATLASGAL-selected massive clumps in the inner Galaxy: VIII. Chemistry of photodissociation regions
• Surveying the Giant HII Regions of the Milky Way with SOFIA: III. W49A
• Turbulent Pressure Heats Gas and Suppresses Star Formation in Galactic Bar Molecular Clouds
• The SOFIA Massive (SOMA) Star Formation Q-band Follow-up. II. Hydrogen Recombination Lines Toward High-Mass Protostars
• Simultaneous 22 GHz Water and 44 GHz Methanol Masers Survey of Ultracompact HII Regions