The chemistry of interstellar molecules containing the halogen elements

Although they are only minor constituents of the interstellar medium, halogen-containing molecules are of special interest because of their unique thermochemistry. Here, we present a theoretical study of the chemistry of interstellar molecules containing the halogen elements chlorine and fluorine. We have modeled both diffuse and dense molecular clouds, making use of updated estimates for the rates of several key chemical processes. We present predictions for the abundances of the three halogen molecules that have been detected to date in the interstellar medium: HF, CF+ and HCl. As in our previous study of fluorine-bearing interstellar molecules, we predict HF to be the dominant gas-phase reservoir of fluorine within both diffuse and dense molecular clouds; we expect the Herschel Space Observatory to detect widespread absorption in the HF J=1-0 transition. Our updated model now overpredicts the CF+ abundance by a factor ~10 relative to observations of the Orion Bar; this discrepancy has widened because we now adopt a laboratory measurement of the CF+ dissociative recombination rate that is smaller than the estimate we adopted previously. This disagreement suggests that the reaction of C+ with HF proceeds more slowly than the capture rate assumed in our model; a laboratory measurement of this reaction rate would be very desirable. Our model predicts diffuse cloud HCl abundances that are similar to those predicted previously and detected tentatively toward zeta Oph. Two additional species are potentially detectable from photodissociation regions: the H2Cl+ and HCl+ molecular ions. Ortho-H2Cl+ has its lowest-lying transition in the millimeter spectral region observable from the ground, and the lowest rotational transition of HCl+ is observable with Herschel's HIFI instrument.Comment: 35 pages, including 14 figures. Accepted for publication in Ap

The chemistry of fluorine-bearing molecules in diffuse and dense interstellar gas clouds

We present a theoretical investigation of the chemistry of fluorine-bearing molecules in diffuse and dense interstellar gas clouds. The chemistry of interstellar fluorine is qualitatively different from that of any other element, because - unlike the neutral atoms of any other element found in diffuse or dense molecular clouds - atomic fluorine undergoes an exothermic reaction with molecular hydrogen. Over a wide range of conditions attained within interstellar gas clouds, the product of that reaction - hydrogen fluoride - is predicted to be the dominant gas-phase reservoir of interstellar fluorine nuclei. Our model predicts HF column densities ~ 1.E+13 cm-2 in dark clouds and column densities as large as 1.E-11 cm-2 in diffuse interstellar gas clouds with total visual extinctions as small as 0.1 mag. Such diffuse clouds will be detectable by means of absorption line spectroscopy of the J = 1 - 0 transition at 243.2 micron using the Stratospheric Observatory for Infrared Astronomy (SOFIA) and the Herschel Space Observatory (HSO). The CF+ ion is predicted to be the second most abundant fluorine-bearing molecule, with typical column densities a factor ~ 100 below those of HF; with its lowest two rotational transitions in the millimeter-wave spectral region, CF+ may be detectable from ground-based observatories. HF absorption in quasar spectra is a potential probe of molecular gas at high redshift, providing a possible bridge between the UV/optical observations capable of probing H2 in low column density systems and the radio/millimeter-wavelength observations that probe intervening molecular clouds of high extinction and large molecular fraction; at redshifts beyond ~ 0.3, HF is potentially detectable from ground-based submillimeter observatories in several atmospheric transmission windows.Comment: 34 pages, including 11 figures (10 color), accepted for publication in Ap

The Effelsberg-Bonn HI Survey (EBHIS)

The Effelsberg-Bonn HI survey (EBHIS) comprises an all-sky survey north of Dec = -5 degrees of the Milky Way and the local volume out to a red-shift of z ~ 0.07. Using state of the art Field Programmable Gate Array (FPGA) spectrometers it is feasible to cover the 100 MHz bandwidth with 16.384 spectral channels. High speed storage of HI spectra allows us to minimize the degradation by Radio Frequency Interference (RFI) signals. Regular EBHIS survey observations started during the winter season 2008/2009 after extensive system evaluation and verification tests. Until today, we surveyed about 8000 square degrees, focusing during the first all-sky coverage of the Sloan-Digital Sky Survey (SDSS) area and the northern extension of the Magellanic stream. The first whole sky coverage will be finished in 2011. Already this first coverage will reach the same sensitivity level as the Parkes Milky Way (GASS) and extragalactic surveys (HIPASS). EBHIS data will be calibrated, stray-radiation corrected and freely accessible for the scientific community via a web-interface. In this paper we demonstrate the scientific data quality and explore the expected harvest of this new all-sky survey.Comment: 19 pages, 11 figures, accepted for publication by Astronomical Note

The Chemistry of Interstellar OH+, H2O+, and H3O+: Inferring the Cosmic Ray Ionization Rates from Observations of Molecular Ions

We model the production of OH+, H2O+, and H3O+ in interstellar clouds, using a steady state photodissociation region code that treats the freeze-out of gas species, grain surface chemistry, and desorption of ices from grains. The code includes PAHs, which have important effects on the chemistry. All three ions generally have two peaks in abundance as a function of depth into the cloud, one at A_V<~1 and one at A_V~3-8, the exact values depending on the ratio of incident ultraviolet flux to gas density. For relatively low values of the incident far ultraviolet flux on the cloud ({\chi}<~ 1000; {\chi}= 1= local interstellar value), the columns of OH+ and H2O+ scale roughly as the cosmic ray primary ionization rate {\zeta}(crp) divided by the hydrogen nucleus density n. The H3O+ column is dominated by the second peak, and we show that if PAHs are present, N(H3O+) ~ 4x10^{13} cm^{-2} independent of {\zeta}(crp) or n. If there are no PAHs or very small grains at the second peak, N(H3O+) can attain such columns only if low ionization potential metals are heavily depleted. We also model diffuse and translucent clouds in the interstellar medium, and show how observations of N(OH+)/N(H) and N(OH+)/N(H2O+) can be used to estimate {\zeta}(crp)/n, {\chi}/n and A_V in them. We compare our models to Herschel observations of these two ions, and estimate {\zeta}(crp) ~ 4-6 x 10^-16 (n/100 cm^-3) s^-1 and \chi/n = 0.03 cm^3 for diffuse foreground clouds towards W49N

Infrared emission from ultracompact H II regions

Models of circumstellar dust shells around ultracompact (UC) H II regions were constructed that accurately fit the observed IR flux distributions. The models assume spherically symmetric dust shells illuminated by stars whose bolometric luminosity is inferred from the integrated FIR flux densities. Assuming ionization by a single zero age main sequence (ZAMS) star, the relations of Panagia were used to infer the stellar radius and effective temperature for a given luminosity. The grain mixture in the dust shell consists of bare graphite and silicate grains with the optical properties of Draine and Lee and the size distribution of Mathis et al. The computer code of Wolfire et al was used to solve the radiative transfer equations through a spherical dust shell. The model provides monochromatic luminosities, dust temperatures, and opacities through the shell. Aside from the stellar and dust properties, the only other input parameters to the model are the distance to the shell, the form of its density distribution, and its outer radius. Predictions of the model are compared with observations of a typical UC H II region and the run of dust temperature with radius and the optical depth with frequency are discussed

The temperature of the WNM in the Milky Way

We report high spectral resolution Australia Telescope Compact Array HI 21 cm observations resulting in the detection of the warm neutral medium (WNM) of the Galaxy in absorption against two extragalactic radio sources, PKS 1814-637 and PKS 0407-658. The two lines of sight were selected on the basis of the simplicity of their absorption profiles and the strength of the background sources; the high velocity resolution of the spectra then enabled us to estimate the kinetic temperatures of the absorbing gas by fitting multiple Gaussians to the absorption profiles. Four separate WNM components were detected toward the two sources, with peak optical depths $\tau_{max} = (1.0 \pm 0.08) \times 10^{-2}$, $(1.4 \pm 0.2) \times 10^{-3}$, $(2.2 \pm 0.5) \times 10^{-3}$ and $(3.4 \pm 0.5) \times 10^{-3}$ and kinetic temperatures $T_{k} = 3127 \pm 300$ K, $3694 \pm 1595$ K, $3500 \pm 1354$ K and $2165 \pm 608$ K respectively. All four components were thus found to have temperatures in the thermally unstable range $500 < T_{k} < 5000$ K; this suggests that thermal equilibrium has not been reached throughout the WNM.Comment: 5 pages, 6 figures. Accepted for publication in MNRAS (Letters). Minor typos removed to match version in pres

Physical properties of a very diffuse HI structure at high Galactic latitude

The main goal of this analysis is to present a new method to estimate the physical properties of diffuse cloud of atomic hydrogen observed at high Galactic latitude. This method, based on a comparison of the observations with fractional Brownian motion simulations, uses the statistical properties of the integrated emission, centroid velocity and line width to constrain the physical properties of the 3D density and velocity fields, as well as the average temperature of HI. We applied this method to interpret 21 cm observations obtained with the Green Bank Telescope of a very diffuse HI cloud at high Galactic latitude located in Firback North 1. We first show that the observations cannot be reproduced solely by highly-turbulent CNM type gas and that there is a significant contribution of thermal broadening to the line width observed. To reproduce the profiles one needs to invoke two components with different average temperature and filling factor. We established that, in this very diffuse part of the ISM, 2/3 of the column density is made of WNM and 1/3 of thermally unstable gas (T ~2600 K). The WNM gas is mildly supersonic (~1) and the unstable phase is definitely sub-sonic (~0.3). The density contrast (i.e., the standard deviation relative to the mean of density distribution) of both components is close to 0.8. The filling factor of the WNM is 10 times higher that of the unstable gas, which has a density structure closer to what would be expected for CNM gas. This field contains a signature of CNM type gas at a very low level (N_H ~ 3 x 10^19) which could have been formed by a convergent flow of WNM gas.Comment: 13 pages, 12 figures, accepted for publication in A&