Nick Indriolo Johns Hopkins University (University of Illinois at Urbana-Champaign)

1. Investigating the Cosmic-Ray Ionization Rate in the Galactic Interstellar Medium through Observations of H3+ Nick Indriolo Johns Hopkins University (University of Illinois at Urbana-Champaign)

2. In Collaboration with: • Ben McCall (University of Illinois) • Brian Fields (University of Illinois) • Takeshi Oka (University of Chicago) • Tom Geballe (Gemini Observatory) • Miwa Goto (MPIA) • Tomonori Usuda (Subaru Telescope) • Geoff Blake (Caltech)

3. Introduction • CH+ first discovered in ISM 70 years ago (Dunham 1937; Douglas & Herzberg 1941) • Now, more than 150 molecules have been identified in the ISM (H2 to C70) • Gas phase chemistry (ion-molecule) proposed in forming smaller molecules (Watson 1973; Herbst & Klemperer 1973) • Requires a source of ionization

4. Introduction (cont.) • Photoionization efficient only for species with ionization potentials below 13.6 eV • H: 13.6 eV; H2: 15.4 eV; O: 13.62 eV • N: 14.5 eV; He: 24.6 eV; C: 11.3 eV • Carbon expected to be fully ionized in diffuse clouds via photons • Other species are ionized by cosmic rays

5. Particle Energy Distribution • Power law in energy (φ~E-2.7) spanning 12 decades in E, and 30 decades in flux • Spectral shape is consistent for all species Ave et al. 2008 Swordy 2001

6.  +  Particle Interactions • Ionization • p + H2  H2+ + e- + p’ • Spallation and Fusion • [p, ] + [12C, 14N, 16O]  [6Li,7Li,9Be,10B,11B] • Nuclear Excitation • [p, ] + 12C  12C*  12C + 4.44 MeV • Inelastic Collisions • p + H  p’ + H + 0

7. Ionization by Cosmic Rays • 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

8. 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 extra proton

9. 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

10. 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)

11. Steady State Equation

12. 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

13. Validity of Approximation

14. 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)

15. 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+

16. Instruments & Telescopes IRCS: Subaru CGS4: UKIRT NIRSPEC: Keck II Phoenix: Gemini South CRIRES: VLT UT1

17. 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

18. Example Spectra

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

20. ζ2 versus Galactic Longitude

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

22. 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

23. 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

24. 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

25. The Ionization Rate Near a Supernova Remnant Indriolo et al. 2010 Image credit: Gerhard Bachmayer

26. Acciari et al. 2009; TeV gamma-ray map

27. HD 43703 ALS 8828 HD 254755 HD 43582 HD 254577 HD 43907

28. Results

29. 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

30. 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

31. Where to go from here? • Study ionization rate near more SNRs • 4 sight lines toward W28 (observed) • 9 sight lines toward Vela (in queue) • Combine H3+ analysis with that from OH+, H2O+, and H3O+ (Herschel) • Add depth-dependent cosmic-ray ionization rate to chemical models