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.Comment: 43 pages, 11 figures, 5 tables accepted to Ap

Carbon Recombination Lines toward the Riegel-Crutcher Cloud and other Cold HI Regions in the inner Galaxy

We report here, for the first time, the association of low frequency CRRL with \HI\ self-absorbing clouds in the inner Galaxy and that the CRRLs from the innermost $\sim 10^{\circ}$ of the Galaxy arise in the Riegel-Crutcher (R-C) cloud. The R-C cloud is amongst the most well known of \HI\ self-absorbing (HISA) regions located at a distance of about 125 pc in the Galactic centre direction. Taking the R-C cloud as an example, we demonstrate that the physical properties of the HISA can be constrained by combining multi-frequency CRRL and \HI\ observations. The derived physical properties of the HISA cloud are used to determine the cooling and heating rates. The dominant cooling process is emission of the \CII\ 158 \mum line whereas dominant heating process in the cloud interior is photoelectric emission. Constraints on the FUV flux (G0 $\sim$ 4 to 7) falling on the R-C cloud are obtained by assuming thermal balance between the dominant heating and cooling processes. The H$_2$ formation rate per unit volume in the cloud interior is $\sim$ 10$^{-10}$ -- 10$^{-12}$ s$^{-1}$ \cmthree, which far exceeds the H$_2$ dissociation rate per unit volume. We conclude that the self-absorbing cold \HI\ gas in the R-C cloud may be in the process of converting to the molecular form. The cold \HI\ gas observed as HISA features are ubiquitous in the inner Galaxy and form an important part of the ISM. Our analysis shows that combining CRRL and \HI\ data can give important insight into the nature of these cold gas. We also estimate the integration times required to image the CRRL forming region with the upcoming SKA pathfinders. Imaging with the MWA telescope is feasible with reasonable observing times.Comment: 11 pages, 4 figures, 5 tables, accepted by MNRA

A compact steep spectrum radio source in NGC1977

A compact steep spectrum radio source (J0535-0452) is located in the sky coincident with a bright optical rim in the HII region NGC1977. J0535-0452 is observed to be $\leq 100$ mas in angular size at 8.44 GHz. The spectrum for the radio source is steep and straight with a spectral index of -1.3 between 330 and 8440 MHz. No 2 \mu m IR counter part for the source is detected. These characteristics indicate that the source may be either a rare high redshift radio galaxy or a millisecond pulsar (MSP). Here we investigate whether the steep spectrum source is a millisecond pulsar.The optical rim is believed to be the interface between the HII region and the adjacent molecular cloud. If the compact source is a millisecond pulsar, it would have eluded detection in previous pulsar surveys because of the extreme scattering due to the HII region--molecular cloud interface. The limits obtained on the angular broadening along with the distance to the scattering screen are used to estimate the pulse broadening. The pulse broadening is shown to be less than a few msec at frequencies \gtsim 5 GHz. We therefore searched for pulsed emission from J0535-0452 at 14.8 and 4.8 GHz with the Green Bank Telescope (GBT). No pulsed emission is detected to 55 and 30 \mu Jy level at 4.8 and 14.8 GHz. Based on the parameter space explored by our pulsar search algorithm, we conclude that, if J0535-0452 is a pulsar, then it could only be a binary MSP of orbital period \ltsim 5 hrs.Comment: Accepted for publication in A&A (3pages, 1 fig

Hydrogen 2p--2s transition: signals from the epochs of recombination and reionization

We propose a method to study the epoch of reionization based on the possible observation of 2p--2s fine structure lines from the neutral hydrogen outside the cosmological H {\sc ii} regions enveloping QSOs and other ionizing sources in the reionization era. We show that for parameters typical of luminous sources observed at $z \simeq 6.3$ the strength of this signal, which is proportional to the H {\sc i} fraction, has a brightness temperature $\simeq 20 \mu K$ for a fully neutral medium. The fine structure line from this redshift is observable at $\nu \simeq 1 \rm GHz$ and we discuss prospects for the detection with several operational and future radio telescopes. We also compute the characteristics of this signal from the epoch of recombination: the peak brightness is expected to be $\simeq 100 \mu K$; this signal appears in the frequency range 5-10 MHz. The signal from the recombination era is nearly impossible to detect owing to the extreme brightness of the Galactic emission at these frequencies.Comment: 17 pages, 1 figure, to appear in Ap