Helium Ionization in the Diffuse Ionized Gas surrounding UCHII regions

Abstract

We present measurements of the singly ionized helium to hydrogen ratio ($n_{H{e}^{+}}/n_{H^{+}}$) toward diffuse gas surrounding three Ultra-Compact HII (UCHII ) regions: G10.15-0.34, G23.46-0.20 \& G29.96-0.02. We observe radio recombination lines (RRLs) of hydrogen and helium near 5 GHz using the GBT to measure the $n_{H{e}^{+}}/n_{H^{+}}$ ratio. The measurements are motivated by the low helium ionization observed in the warm ionized medium (WIM) and in the inner Galaxy diffuse ionized regions (DIR). Our data indicate that the helium is not uniformly ionized in the three observed sources. Helium lines are not detected toward a few observed positions in sources G10.15-0.34 \& G23.46-0.20 and the upper limits of the $n_{H{e}^{+}}/n_{H^{+}}$ ratio obtained are 0.03 and 0.05 respectively. The selected sources harbor stars of type O6 or hotter as indicated by helium line detection toward the bright radio continuum emission from the sources with mean $n_{H{e}^{+}}/n_{H^{+}}$ value 0.06$\pm$0.02. Our data thus show that helium in diffuse gas located a few pc away from the young massive stars embedded in the observed regions is not fully ionized.We investigate the origin of the non-uniform helium ionization and rule out the possibilities : (a) that the helium is doubly ionized in the observed regions and (b) that the low $n_{H{e}^{+}}/n_{H^{+}}$ values are due to additional hydrogen ionizing radiation produced by accreting low-mass stars (Smith 2014). We find that selective absorption of ionizing photons by dust can result in low helium ionization but needs further investigation to develop a self-consistent model for dust in HII regions.

43 pages, 11 figures, 5 tables accepted to ApJ

References in corpus (13)

• The WISE Catalog of Galactic HII Regions
• The Electron Temperature Gradient in the Galactic Disk
• The Parallax of W43: a Massive Star Forming Complex near the Galactic Bar
• Trigonometric Parallaxes of Massive Star Forming Regions: V. G23.01-0.41 and G23.44-0.18
• ATLASGAL - Kinematic distances and the dense gas mass distribution of the inner Galaxy
• Trigonometric parallaxes to star-forming regions within 4 kpc of the galactic center
• The HIPASS survey of the Galactic plane in Radio Recombination Lines
• The Distances of SNR W41 and overlapping HII regions
• Bipolar HII regions - Morphology and star formation in their vicinity - I - G319.88$+$00.79 and G010.32$-$00.15
• HII Region Ionization of the Interstellar Medium: A Case Study of NGC 7538
• Massive stars in the giant molecular cloud G23.3-0.3 and W41
• Evolutionary tracks of massive stars during formation
• Origin of the Lyman excess in early-type stars

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• Ionization Profiles of Galactic HII Regions
• ATOMS: ALMA Three-millimeter Observations of Massive Star-forming regions -XIV. Properties of resolved UC Hii regions
• A 4-6 GHz Radio Recombination Line Survey in the Milky Way
• Multiline observations of hydrogen, helium, and carbon radio-recombination lines toward Orion A: A detailed dynamical study and direct determination of physical conditions
• Extended Ionized Fe Objects in the UWIFE Survey