Volume 602, June 2017
|Number of page(s)||19|
|Section||Interstellar and circumstellar matter|
|Published online||02 June 2017|
ATLASGAL-selected massive clumps in the inner Galaxy
IV. Millimeter hydrogen recombination lines from associated H II regions ⋆
1 Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
2 School of Physical Sciences, University of Kent, Ingram Building, Canterbury, Kent CT2 7NH, UK
Received: 21 September 2016
Accepted: 13 January 2017
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.
Key words: surveys / stars: massive / stars: formation / H II regions / ISM: general
Full Tables 3 and 6, spectra (Fig. 2 plus their fitted line parameters) and mid-infrared three-color composite images (Fig. 7) are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (188.8.131.52) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/602/A37
© ESO, 2017
Current usage metrics show cumulative count of Article Views (full-text article views including HTML views, PDF and ePub downloads, according to the available data) and Abstracts Views on Vision4Press platform.
Data correspond to usage on the plateform after 2015. The current usage metrics is available 48-96 hours after online publication and is updated daily on week days.
Initial download of the metrics may take a while.