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Helium Ionization in the Diffuse Ionized Gas Surrounding UCH ii Regions
- Source :
- The Astrophysical Journal. 838:144
- Publication Year :
- 2017
- Publisher :
- American Astronomical Society, 2017.
-
Abstract
- We present measurements of the singly ionized helium to hydrogen ratio ($n_{He^+}/n_{H^+}$) 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 $n_{He^+}/n_{H^+}$ 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 $n_{He^+}/n_{H^+}$ 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 $n_{He^+}/n_{H^+}$ 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 $n_{He^+}/n_{H^+}$ 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.<br />43 pages, 11 figures, 5 tables accepted to ApJ
- Subjects :
- Physics
FOS: Physical sciences
chemistry.chemical_element
Astronomy and Astrophysics
Astrophysics::Cosmology and Extragalactic Astrophysics
Plasma
Astrophysics - Astrophysics of Galaxies
01 natural sciences
chemistry
Space and Planetary Science
Astrophysics of Galaxies (astro-ph.GA)
Ionization
0103 physical sciences
Physics::Atomic and Molecular Clusters
Physics::Atomic Physics
Atomic physics
010306 general physics
010303 astronomy & astrophysics
Astrophysics::Galaxy Astrophysics
Helium
Subjects
Details
- ISSN :
- 15384357
- Volume :
- 838
- Database :
- OpenAIRE
- Journal :
- The Astrophysical Journal
- Accession number :
- edsair.doi.dedup.....61fb8d1c4caa6c0fc7505935b19a8ffc