The Galactic Cosmic Ray Ionization Rate SPECIAL FEATURE

The galactic cosmic ray ionization rate SPECIAL FEATURE A. Dalgarno† Harvard–Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138 Edited by William Klemperer, Harvard University, Cambridge, MA, and approved June 7, 2006 (received for review March 15, 2006) The chemistry that occurs in the interstellar medium in response to Dense Clouds cosmic ray ionization is summarized, and a review of the ionization Typical values for the ionization rate in dense gas lie in the range rates that have been derived from measurements of molecular from1to5ϫ 10Ϫ17 sϪ1 (7–14), depending on the details of the abundances is presented. The successful detection of large abun- ؉ measurements, the physical model, and the chemistry. There dances of H3 in diffuse clouds and the recognition that dissociative may be real variations in the ionizing flux. In some cases, values ؉ Ϫ Ϫ recombination of H3 is fast has led to an upward revision of the of ␨ as high as 10 15 s 1 have been derived and the enhanced rate derived ionization rates. In dense clouds the molecular abundances tentatively attributed to x-rays from a central source (15). A are sensitive to the depletion of carbon monoxide, atomic oxygen, range of values with an average of (2.8 Ϯ 1.6) ϫ 10Ϫ17 sϪ1 was nitrogen, water, and metals and the presence of large molecules derived by van der Tak and van Dishoeck (16) from measure- and grains. Measurements of the relative abundances of deuter- ments of HCOϩ in several dense molecular clouds in the ated species provide information about the ion removal mecha- ϩ direction of massive young stars, but from measurements of H3 nisms, but uncertainties remain. The models, both of dense and (17) rates in excess of 10Ϫ16 sϪ1 were obtained. A later analysis diffuse clouds, that are used to interpret the observations may be of the cloud in the direction of the massive star-forming region seriously inadequate. Nevertheless, it appears that the ionization AFGL 2591 yielded a rate of 5.6 ϫ 10Ϫ17 sϪ1, good to a factor rates differ in dense and diffuse clouds and in the intercloud of three (18). There was a discrepancy between the rates derived medium. ϩ ϩ from HCO and H3 that could easily be removed by a modifi- cation of the assumed distribution of material in front of the interstellar chemistry source. Because of the relative simplicity of the chemistry, the most ASTRONOMY he fractional ionization is a fundamental parameter, distin- direct determination of ␨ would seem to be from studies of the ϩ Tguishing the different physical regimes that occur in the abundance of H3 (17, 19, 20). In a cloud of H2 molecules, cosmic ϩ interstellar medium. Because of the presence of electrons and rays ionize H2 and produce H ions that immediately react with 2 ϩ positive ions the gas dynamics is modified by the coupling to the H2 in a fast ion-molecule process to yield H ions (4, 5, 21). The ϩ 3 magnetic field which controls the transfer of angular momentum H3 ions are removed by proton transfer reactions with the and the dissipation of turbulence. The fractional ionization neutral constituents X, where X is any of interstellar CO, O, N2, determines the strength of the coupling and is a crucial aspect of H2O, and HD and D. If k(X) is the rate coefficient for the the evolution of the interstellar gas and the formation of stars reaction and planets. Energetic cosmic rays penetrate planetary atmo- ϩ ϩ 3 ϩ ϩ spheres and create low-altitude ionospheric regions and drive a H3 X HX H2, [1] low-altitude chemistry. ϩ Local regions of ionization are created in the interstellar then in equilibrium the number density of H3 is given by (22) medium by hot stars and prestellar objects radiating at UV and x-ray wavelengths and are found also in warm regions of the gas ͑ ϩ͒ ϭ ␨ ͑ ͒ ͸ ͑ ͒ ͑ ͒ n H3 n H2 Ͳ k X n X . [2] subjected to energetic shock waves generated by stellar outflows X and supernovae. Cosmic rays are a global source of ionization distributed through the Galaxy. Measurements have been made The rate coefficient for the reaction with CO of the cosmic ray flux at high energies, but at energies below 100 ϩ ϩ 3 ϩ ϩ MeV the cosmic rays are excluded from the heliosphere by the H3 CO HCO H2 [3] solar wind and the total ionization rate cannot be directly Ϫ Ϫ ϩ determined. is 1.7 ϫ 10 9 cm3⅐s 1 (23). The HCO ion then undergoes Spitzer and Tomasko (1) used the available data to obtain a dissociative recombination ϫ Ϫ18 Ϫ1 ␨ probable lower limit of 6.7 10 s for the ionization rate ϩ ϩ 3 ϩ of hydrogen atoms and from a general consideration of energies HCO e H CO. [4] released in supernovae estimated a probable upper limit of 1.2 ϫ ϩ ϩ HCO is also removed by H O, producing H O . The reactions 10Ϫ15 sϪ1. More recently, Webber (2) used data from the 2 3 with atomic oxygen, Voyager and Pioneer spacecraft acquired at distances from the Sun out to 60 AU to determine a probable minimum rate of ϩ ϩ 3 ϩ ϩ H3 O OH H2 [5] (3–4) ϫ 10Ϫ17 sϪ1 for H atoms. Webber drew attention to the Ϫ16 Ϫ1 3 ϩ ϩ possibility of enhanced rates exceeding 10 s near massive H2O H, [6] stars with strong stellar winds. Ϫ Ϫ Cosmic rays heat the interstellar gas and drive an interstellar have a total rate coefficient of 8.0 ϫ 10 10 cm3⅐s 1 (24). The chemistry. Following the attribution by Klemperer (3) of an process initiates an ion-molecule sequence that terminates in interstellar emission line at 89,188 MHz to the molecular ion HCOϩ, the influence of cosmic rays on the chemistry of dense clouds was discussed by Herbst and Klemperer (4) and Watson Conflict of interest statement: No conflicts declared. (5) and on the chemistry of diffuse clouds by Black and Dalgarno This paper was submitted directly (Track II) to the PNAS office. (6). It is applications of chemical models to interpret measure- Abbreviation: PAH, polycyclic aromatic hydrocarbon. ments of interstellar molecular abundances that has yielded the †E-mail: [email protected]. values of ␨ in use today. © 2006 by The National Academy of Sciences of the USA www.pnas.org͞cgi͞doi͞10.1073͞pnas.0602117103 PNAS ͉ August 15, 2006 ͉ vol. 103 ͉ no. 33 ͉ 12269–12273 Downloaded by guest on October 2, 2021 ϩ ϩ Ϫ H3O (4), which in turn leads to OH (22). The observation of PAH ϩ PAH 3 PAH products, [16] ϩ H3O would be a valuable diagnostic of oxygen depletion (25). ϩ The reaction with N2 is similar to that for CO. It produces N2H , formed by attachment processes which can be lost by dissociative recombination, e ϩ PAH 3 PAHϪ [17] ϩ ϩ 3 ϩ N2H e H N2 ϩ (34, 35). The equilibrium abundance of H3 may be written 3 N ϩ NH, [7] ͑ ϩ͒ ϭ ␨ ͑ ͒ ͸ ͑ ͒ ͑ ͒ ϩ ͸ ͑ ͒ ͑ ͒ and by reaction with CO, n H3 n H2 Ͳͭ n X k X n M k M X M ϩ ϩ 3 ϩ ϩ N2H CO HCO N2. [8] ϩ ϩ ␣ ͑ ͒ ϩ ͑ ͒ ϩ ͑ Ϫ͒ Eq. 2 shows that the abundance of H is a constant, increasing n e k0n PAH k1n PAH ͮ , [18] 3 with the ionizing flux and with the amount of depletion of the heavy neutral gas components. For ␨ ϭ 5 ϫ 10Ϫ17 sϪ1 and no ϩ ϭ ϫ Ϫ5 Ϫ3 where k0 is the rate coefficient of depletion, n(H3 ) 5 10 cm . With increasing depletion dissociative recombination ϩ ϩ 3 ϩ ϩ ϩ H3 PAH H2 (H PAH) [19] ϩ ϩ 3 ϩ H3 e H H2 [9] and k1 is the rate coefficient of 3 ϩ ϩ H H H [10] ϩ ϩ Ϫ 3 ϩ ϩ H3 PAH H2 H PAH. [20] becomes significant. The rate of 9 and 10 was once thought to be negligible. It is now known to be rapid (26), and most of the early Model chemistries of dense clouds appear not to include reac- estimates of ␨ in diffuse clouds are too small. If ␣ is the rate tions involving PAHs, presumably on the assumption that any ϩ PAH molecules that are present are absorbed onto grains and do coefficient of 9 and 10 combined, the abundance of H3 is given by not remain in the gas phase. Their influence could be substantial (36). Recombination onto grain surfaces may be still more significant (27, 32). We add n(g)k(g) to the denominator of Eq. ͑ ϩ͒ ϭ ␨ ͑ ͒ ͸ ͑ ͒ ͑ ͒ ϩ ␣ ͑ ͒ n H3 n H2 Ͳͭ n X k X n e ͮ , [11] 18. Inclusion of any metals or PAHs or grains will increase the X inferred ionization rate. The possible effects of depletion, metals, large molecules, and where n(e) is the electron number density. ϩ grains can be assessed by a consideration of the deuteration of Positive ions X may also be removed by charge transfer with molecular ions by fractionation processes. Observations show metal atoms M, that the ratio of the abundances of molecular ions such as DCOϩ ϩ ϩ ϩ to HCO and neutral molecules such as H CO and HDCO is X ϩ M 3 X ϩ M .

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