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ATLASGAL-selected massive clumps in the inner Galaxy IV. Millimeter hydrogen recombination lines from associated HII regions

Kim, W.-J., Wyrowski, F., Urquhart, J.S., Menten, K.M., Csengeri, T. (2017) ATLASGAL-selected massive clumps in the inner Galaxy IV. Millimeter hydrogen recombination lines from associated HII regions. Astronomy and Astrophysics, 602 . A37. ISSN 0004-6361. E-ISSN 1432-0746. (doi:10.1051/0004-6361/201629764) (KAR id:61363)

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Aims: Observations of millimeter wavelength radio recombination lines (mm-RRLs) are used to search for H ii 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 H ii regions and to provide velocity information of ionized gas.

Methods: We carried out targeted mm-RRL observations (39 ≤ principal quantum number (n) ≤ 65 and Δn = 1, 2, 3, and 4, named Hnα, Hnβ, Hnγ, and Hnδ) using the IRAM 30 m and Mopra 22 m 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 H ii regions. The nature of the other 44 detections is unclear, but 8 detections are thought to be potentially new H ii regions while the mm-RRL emission from the others may be due to contamination from nearby evolved H ii 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 H ii regions. We find that the systemic velocity of the associated dense molecular gas, traced by H13CO+(1−0), is consistent with the mm-RRL velocities and confirms them as embedded H ii regions. We also find that the linewidth of the H13CO+(1−0) emission is significantly wider than those without mm-RRL detection, indicating a physical connection between the embedded H ii 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 H ii regions with ne < 105 cm-3 and H ii region diameter >0.03 pc.

Conclusions: We detected mm-RRLs toward 178 clumps and identified eight new H ii region candidates. The broad mm-RRL from nine clumps may indicate that they arise in very young hyper-compact H ii 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.

Item Type: Article
DOI/Identification number: 10.1051/0004-6361/201629764
Uncontrolled keywords: surveys ? stars: massive ? stars: formation ? Hii region: ISM
Divisions: Faculties > Sciences > School of Physical Sciences > Centre for Astrophysics and Planetary Sciences
Depositing User: James Urquhart
Date Deposited: 18 Apr 2017 10:50 UTC
Last Modified: 11 Jul 2019 15:09 UTC
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