Observations of H{sup +}{sub 3} in the Galactic diffuse interstellar medium have led to various surprising results, including the conclusion that the cosmic-ray ionization rate ({zeta}{sub 2}) is about one order of magnitude larger than previously thought. The present survey expands the sample of diffuse cloud sight lines with H{sup +}{sub 3} observations to 50, with detections in 21 of those. Ionization rates inferred from these observations are in the range (1.7 {+-} 1.3) Multiplication-Sign 10{sup -16} s{sup -1} < {zeta}{sub 2} < (10.6 {+-} 8.2) Multiplication-Sign 10{sup -16} s{sup -1} with a mean value of {zeta}{sub 2} = (3.5{sup +5.3}{sub -3.0}) Multiplication-Sign 10{sup -16} s{sup -1}. Upper limits (3{sigma}) derived from non-detections of H{sup +}{sub 3} are as low as {zeta}{sub 2} < 0.4 Multiplication-Sign 10{sup -16} s{sup -1}. These low upper limits, in combination with the wide range of inferred cosmic-ray ionization rates, indicate variations in {zeta}{sub 2} between different diffuse cloud sight lines. A study of {zeta}{sub 2} versus N{sub H} (total hydrogen column density) shows that the two parameters are not correlated for diffuse molecular cloud sight lines, but that the ionization rate decreases when N{sub H} increases to values typical of dense molecular clouds. Bothmore » the difference in ionization rates between diffuse and dense clouds and the variation of {zeta}{sub 2} among diffuse cloud sight lines are likely the result of particle propagation effects. The lower ionization rate in dense clouds is due to the inability of low-energy (few MeV) protons to penetrate such regions, while the ionization rate in diffuse clouds is controlled by the proximity of the observed cloud to a site of particle acceleration.« less

https://iopscience.iop.org/article/10.1088/0004-637X/745/1/91/pdf

Investigating the cosmic-ray ionization rate in the galactic diffuse interstellar medium through observations of h+ 3

Cosmic-ray astrophysics has advanced rapidly in recent years, and its impact on other astronomical disciplines has broadened. Many new experiments measuring these particles, both directly in the atmosphere or space and indirectly via γ rays and synchrotron radiation, have widened the range and quality of the information available on their origin, propagation, and interactions. The impact of low-energy cosmic rays on interstellar chemistry is a fast-developing topic, including the propagation of these particles into the clouds in which the chemistry occurs. Cosmic rays, via their γ-ray production, also provide a powerful way to probe the gas content of the interstellar medium. Substantial advances have been made in the observations and modelling of the interplay between cosmic rays and the interstellar medium. Focusing on energies up to 1 TeV, these interrelating aspects are covered at various levels of detail, giving a guide to the state of the subject.

http://publications.lib.chalmers.se/records/fulltext/222464/local_222464.pdf

The Nine Lives of Cosmic Rays in Galaxies

To date, 204 individual molecular species, comprised of 16 different elements, have been detected in the interstellar and circumstellar medium by astronomical observations. These molecules range in size from two atoms to seventy, and have been detected across the electromagnetic spectrum from cm-wavelengths to the ultraviolet. This census presents a summary of the first detection of each molecular species, including the observational facility, wavelength range, transitions, and enabling laboratory spectroscopic work, as well as listing tentative and disputed detections. Tables of molecules detected in interstellar ices, external galaxies, protoplanetary disks, and exoplanetary atmospheres are provided. A number of visual representations of this aggregate data are presented and briefly discussed in context.

2018 Census of Interstellar, Circumstellar, Extragalactic, Protoplanetary Disk, and Exoplanetary Molecules

In diffuse interstellar clouds the chemistry that leads to the formation of the oxygen-bearing ions OH+, H2O+, and H3O+ begins with the ionization of atomic hydrogen by cosmic rays, and continues through subsequent hydrogen abstraction reactions involving H2 Given these reaction pathways, the observed abundances of these molecules are useful in constraining both the total cosmic-ray ionization rate of atomic hydrogen (ζH) and molecular hydrogen fraction (f_H_2) We present observations targeting transitions of OH+, H2O+, and H3O+ made with the Herschel Space Observatory along 20 Galactic sight lines toward bright submillimeter continuum sources Both OH+ and H2O+ are detected in absorption in multiple velocity components along every sight line, but H3O+ is only detected along 7 sight lines From the molecular abundances we compute f_H_2 in multiple distinct components along each line of sight, and find a Gaussian distribution with mean and standard deviation 0042 ± 0018 This confirms previous findings that OH+ and H2O+ primarily reside in gas with low H2 fractions We also infer ζH throughout our sample, and find a lognormal distribution with mean log (ζH) = –1575 (ζH = 178 × 10–16 s–1) and standard deviation 029 for gas within the Galactic disk, but outside of the Galactic center This is in good agreement with the mean and distribution of cosmic-ray ionization rates previously inferred from H_3^+ observations Ionization rates in the Galactic center tend to be 10-100 times larger than found in the Galactic disk, also in accord with prior studies

/pdf/herschel-survey-of-galactic-oh-h2o-and-h3o-probing-the-2jody6e017.pdf

Herschel Survey of Galactic OH+, H2O+, and H3O+: Probing the Molecular Hydrogen Fraction and Cosmic-Ray Ionization Rate

Water is observed throughout the universe, from diffuse interstellar clouds to protoplanetary disks around young stars, and from comets in our own solar system and exoplanetary atmospheres to galaxies at high redshifts This review summarizes the spectroscopy and excitation of water in interstellar space as well as the basic chemical processes that form and destroy water under interstellar conditions Three major routes to water formation are identified: low temperature ion-molecule chemistry, high-temperature neutral-neutral chemistry and gas-ice chemistry The rate coefficients of several important processes entering the networks are discussed in detail; several of them have been determined only in the last decade through laboratory experiments and theoretical calculations Astronomical examples of each of the different chemical routes are presented using data from powerful new telescopes, in particular the Herschel Space Observatory Basic chemical physics studies remain critically important to analyze astronomical data

Interstellar water chemistry: from laboratory to observations

We report the detection of absorption lines by the reactive ions OH+, H2O+ and H3O+ along the line of sight to the submillimeter continuum source G10.6$-$0.4 (W31C). We used the Herschel HIFI instrument in dual beam switch mode to observe the ground state rotational transitions of OH+ at 971 GHz, H2O+ at 1115 and 607 GHz, and H3O+ at 984 GHz. The resultant spectra show deep absorption over a broad velocity range that originates in the interstellar matter along the line of sight to G10.6$-$0.4 as well as in the molecular gas directly associated with that source. The OH+ spectrum reaches saturation over most velocities corresponding to the foreground gas, while the opacity of the H2O+ lines remains lower than 1 in the same velocity range, and the H3O+ line shows only weak absorption. For LSR velocities between 7 and 50 kms$^{-1}$ we estimate total column densities of $N$(OH+) $> 2.5 \times 10^{14}$ cm$^{-2}$, $N$(H2O+) $\sim 6 \times 10^{13}$ cm$^{-2}$ and $N$(H3O+) $\sim 4.0 \times 10^{13}$ cm$^{-2}$. These detections confirm the role of O$^+$ and OH$^+$ in initiating the oxygen chemistry in diffuse molecular gas and strengthen our understanding of the gas phase production of water. The high ratio of the OH+ by the H2O+ column density implies that these species predominantly trace low-density gas with a small fraction of hydrogen in molecular form.

/pdf/interstellar-oh-h2o-and-h3o-along-the-sight-line-to-g10-6-0-27xmnzo0qx.pdf

Interstellar OH+, H2O+ and H3O+ along the sight-line to G10.6-0.4

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Interstellar OH+, H2O+ and H3O+ along the sight-line to G10.6-0.4