Helium Ionization in the Diffuse Ionized Gas surrounding Ultra-compact HII regions
Abstract
We observed radio recombination lines (RRLs) from regions surrounding three Ultra-compact HII (UCHII) regions at frequencies near 5 GHz. The observations were made with the Green Bank Telescope (GBT). From existing observations we know that helium in the diffuse ionized gas (DIR), located far from the ionizing source, is not fully ionized. The objectives of our observations are to determine (a) the distance from the ionizing stars where helium is under ionized for a variety of physical conditions and (b) whether the helium ionization depends on the age of the ionizing star. With these objectives, we observed RRLs towards 16 positions in the envelops of UCHII regions G10.15-0.34, G23.46-0.20 and G29.96-0.02. Helium lines were detected toward 10 of the observed positions and hydrogen RRLs were detected toward all the observed positions. The observed ratio of ionized helium to ionized hydrogen (He^+/H^+) at the positions where helium lines are detected range between 0.03 and 0.09. At positions where helium lines are not detected the upper limit on the ratio is ~ 0.05. We discuss the dependence of He^+/H^+ ratio on the distance from and age of the ionizing star clusters in the observed sources.
- Publication:
-
American Astronomical Society Meeting Abstracts #227
- Pub Date:
- January 2016
- Bibcode:
- 2016AAS...22740903A