HERSCHELSURVEY OF GALACTIC OH⁺, H₂O⁺, AND H₃O⁺: PROBING THE MOLECULAR HYDROGEN FRACTION AND COSMIC-RAY IONIZATION RATE

Abstract

In diffuse interstellar clouds the chemistry that leads to the formation of the oxygen-bearing ions OH + , H 2 O + , and H 3 O + begins with the ionization of atomic hydrogen by cosmic rays, and continues through subsequent hydrogen abstraction reactions involving H 2. Given these reaction pathways, the observed abundances of these molecules are useful in constraining both the total cosmic-ray ionization rate of atomic hydrogen (ζ H) and molecular hydrogen fraction (f H 2). We present observations targeting transitions of OH + , H 2 O + , and H 3 O + made with the Herschel Space Observatory along 20 Galactic sight lines toward bright submillimeter continuum sources. Both OH + and H 2 O + are detected in absorption in multiple velocity components along every sight line, but H 3 O + 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 0.042 ± 0.018. This confirms previous findings that OH + and H 2 O + primarily reside in gas with low H 2 fractions. We also infer ζ H throughout our sample, and find a lognormal distribution with mean log(ζ H) = −15.75 (ζ H = 1.78 × 10 −16 s −1) and standard deviation 0.29 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.

Detection of extragalactic argonium, ArH+, toward PKS 1830−211

“…The observations were reproduced by models in which argonium is present only in the almost purely atomic diffuse ISM, with peak abundance at molecular fractions in the 10 −4 to 10 −3 range. Basically, high values of ζ favor its formation, consistent with analyses of OH + and H 2 O + , which trace slightly higher, but still low H 2 fractions (see, e.g., Hollenbach et al 2012;Indriolo et al 2015), whereas molecular fractions 10 −3 favor its destruction by the reaction with H 2 to produce Ar and H + 3 . Although ArH + abhors molecular clouds, it does rely upon a small amount of H 2 for its existence.…”

“…The large column densities of OH + with respect to H 2 O + were explained by both molecules residing preferentially in the largely atomic, diffuse interstellar medium (ISM), with the largest fractional abundances at a molecular fraction of Appendix A is available in electronic form at http://www.aanda.org around 0.04, and their unexpectedly large column densities require cosmic ray ionization rates ζ considerably larger than in the dense ISM (Hollenbach et al 2012;Indriolo et al 2015).…”

Water in star-forming regions withHerschel(WISH)

“…The difference may result from the inaccuracy of the transition frequency of the H 2 O + line; it would amount to a correction of +1.0 MHz, to be added to the H 2 O + frequencies listed in Table 2. A correction was discussed by Neufeld et al (2010) and Indriolo et al (2015), who reported a value of +5 MHz.…”

“…The detection of SH + toward W3 IRS5 was reported in . Foreground clouds of some high-mass objects in this selection at velocities offset from the YSO were analyzed by Indriolo et al (2015).…”

“…Indeed, in diffuse clouds OH + /H 2 O + ratios of 1-15 have been observed, suggesting densities <100 cm −3 with a molecular fraction of only a few percent (Gerin et al 2010b;Indriolo et al 2015). Such low densities are unlikely to be present in protostellar systems on the scales covered by the Herschel beam.…”