Nick Indriolo , 1 Ben McCall, 1 Tom Geballe, 2 & Takeshi Oka 3

1. Investigating the Cosmic-Ray Ionization Rate in the Galactic Interstellar Medium through Observations of H3+ Nick Indriolo,1 Ben McCall,1 Tom Geballe,2 & Takeshi Oka3 1University of Illinois at Urbana-Champaign; 2Gemini Observatory; 3University of Chicago TF03

2. Introduction • Gas phase chemistry (ion-molecule) proposed in forming smaller molecules (Watson 1973; Herbst & Klemperer 1973) • Requires a source of ionization • Cosmic rays ionize H, He, and H2 throughout diffuse molecular clouds, forming H+, He+, and H3+ • Initiates the fast ion-molecule reactions that drive chemistry in the ISM TF03

3. N2H+ N2 CR H2 CO H2 H3+ HCO+ H2+ O H2 CR H2 H2 O H H+ O+ OH+ H2O+ H3O+ Ion-Molecule Reactions • Low proton affinity of H2 makes H3+ especially willing to transfer its charge TF03

4. ζ Over the Past 50 Years Hayakawa et al. 1961; Spitzer & Tomasko 1968; O’Donnell & Watson 1974; Hartquist et al. 1978; van Dishoeck & Black 1986; Federman et al. 1996; Webber 1998; McCall et al. 2003; Indriolo et al. 2007; Gerin et al. 2010; Neufeld et al. 2010 TF03

5. H3+ Chemistry • Formation • CR + H2 H2+ + e- + CR’ • H2+ + H2  H3+ + H • Destruction • H3+ + e-  H + H + H (diffuse clouds) • H3+ + O  OH+ + H2 (diffuse & dense clouds) • H3+ + CO  HCO+ + H2 (dense clouds) • H3+ + N2  HN2+ + H2 (dense clouds) TF03

6. Steady State Equation TF03

7. More Complete Steady State • Proton transfer to O and CO also destroys H3+ • During formation process, H2+ can be destroyed prior to reaction with H2 • H2+ + H  H2 + H+ • H2+ + e-  H + H TF03

8. Validity of Approximation TF03

9. Necessary Parameters • ke measured • xe approximated by x(C+)≈1.510-4 • nH estimated from C2 analysis, C I analysis, or H & H2 (J=4) analysis • N(H2) from observations, estimated from E(B-V), or estimated from N(CH) TF03

10. Targeted Transitions • Transitions of the 2  0 band of H3+ are available in the infrared • Given average diffuse cloud temperatures (70 K) only the (J,K)=(1,0) & (1,1) levels are significantly populated • Observable transitions are: • R(1,1)u: 3.668083 μm • R(1,0): 3.668516 μm • R(1,1)l: 3.715479 μm • Q(1,1): 3.928625 μm • Q(1,0): 3.953000 μm Energy level diagram for the ground vibrational state of H3+ TF03

11. Survey Status • Observations targeting H3+ in diffuse clouds have been made in 50 sight lines • H3+ is detected in 21 of those Dame et al. 2001 TF03

12. Example Spectra TF03

13. Inferred Ionization Rates mean ionization rate: ζ2=3.3±0.410-16 s-1 TF03

14. ζ2 versus Galactic Longitude TF03

15. ζ2 versus Total Column Density Dense cloud results from Kulesa 2002 and van der Tak & van Dishoeck 2000 TF03

16. Particle Range Range for a 1 MeV proton is ~31020 cm-2 Range for a 10 MeV proton is ~21022 cm-2 Diffuse cloud column densities are about 1021 ≤ NH ≤ 1022 cm-2 Padovani et al. 2009 TF03

17. Implications • Likely that cosmic rays in the 2-10 MeV range operate throughout diffuse clouds • Only higher energy particles (E>10 MeV) contribute to ionization in dense clouds • Variations in ζ2 amongst diffuse clouds due to proximity to acceleration sites • Particle spectrum is not uniform in the Galactic ISM TF03

18. Reproducing High Inferred ζ2 Using both components: ζ2=3.710-16 s-1 Using only base component: ζ2=0.1410-16 s-1 TF03

19. SNR versus Diffuse ISM • Ionization rates near IC 443 • ζ2~20±1010-16 s-1 • Ionization rates in the diffuse ISM • mean: ζ2=3.3±0.410-16 s-1 • max: ζ2=10.6±6.810-16 s-1 • min: ζ2<0.410-16 s-1 • Consistent with theory that ionization rates are higher near acceleration sites TF03

20. Conclusions • Variations in ζ2 amongst diffuse clouds are due to differences in the cosmic-ray spectrum at MeV energies which result from particle propagation effects and proximity to acceleration sites • Supernova remnants accelerate MeV particles, but it is unclear if these can cause high ionization rates throughout the Galactic ISM TF03

21. Acknowledgments • Brian Fields • Geoff Blake • Miwa Goto • Tomonori Usuda TF03