Helium Ionization in the Diffuse Ionized Gas surrounding UCHII regions
  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.
