The Distribution of Cosmic-Ray Ionization Rates in Diffuse Molecular Clouds as Probed by H 3 +

1. The Distribution of Cosmic-Ray Ionization Rates in Diffuse Molecular Clouds as Probed by H3+ Nick Indriolo Johns Hopkins University Chemistry, Astronomy, & Physics of H3+

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

3. Outline • Introduction to cosmic rays and interstellar H3+ • Calculating the cosmic-ray ionization rate • Observations of H3+ and example spectra • Line-of-sight properties and ζ2 • The distribution of ionization rates • Ionization rate and location • Complementary tracers and future work

4. Cosmic Rays • Discovered in 1912 by Victor Hess during balloon borne experiments that showed increased radiation at higher altitudes • Later dubbed cosmic rays (Millikan 1926) • Now known to be highly energetic charged particles (p, e-, e+, α,nuclei)

5. 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 + γ4.44 MeV • Inelastic Collisions p + H  p' + H + 0 γ + γ

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. Ion-Molecule Reactions N2H+ N2 CR H2 CO H3+ HCO+ H2 H2+ O H2 H2 CR O H2 OH+ H2O+ H3O+ H H+ O+ e- H e- e- H3+ H+ hν hν H2O OH O

9. ζ2 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. Necessary Parameters • ke measured in lab (adopt McCall et al. 2004) • xe approximated by x(C+)≈1.510-4Cardelli et al. 1996; Sofia et al. 2004 • nH estimated from rotational excitation analysis of C2 (Sonnentrucker et al. 2007) or thermal pressure analysis of fine structure lines of C I (Jenkins et al. 1983; Jenkins & Tripp 2001)

13. Necessary Parameters (cont.) • N(H2) • UV observations of H2 lines (Savage et al. 1977; Rachford et al. 2002, 2009) • N(CH)/N(H2)≈3.510-8 (Sheffer et al. 2008) • NH≈E(B-V)5.81021 cm-2 mag-1 (Bohlin et al. 1978; Rachford et al. 2002) • All that remains is N(H3+)

14. 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+

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

16. 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

17. Example Spectra CRIRES at VLT

18. ζ2 vs. Position

19. ζ2 vs. Total Column Density

20. 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

21. Distribution of Ionization Rates

22. Where do we stand?

23. Regional Distributions

24. Ophiuchus-Scorpius region E N HD 147889 oSco 3 pc ρOph D χOph Image Credit: Rogelio Bernal Andreo

25. Per OB2 region N 40 Per o Per N(H2)=4.8×1020 cm-2 E BD +31 643 10.5 pc 4.6 pc ζ Per X Per PSR J0357+3205 ξ Per N(H2)=4.1×1020 cm-2 Image Credit: Rogelio Bernal Andreo

26. Why the Differences? • Particle range determined by energy • Cosmic rays must get from acceleration site to observed clouds • Ionization rate controlled by proximity of cloud to nearest acceleration site • If true, ζ2 should be large near known source, e.g. IC 443, a supernova remnant

27. IC 443 Survey ALS 8828 HD 254755 HD 43582 HD 254577 Image credit: Gerhard Bachmayer

28. IC 443 Results • SNRs accelerate cosmic rays • Proximity to IC 443 boosts ζ2 • Ionization rates vary over a few pc

29. Conclusions • ζ2 is NOTa constant!!!!! • !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! • The cosmic-ray ionization rate is controlled by the distance between a cloud and acceleration site • Different regions of the sky show different distributions of ζ2 • ζ2 can vary on size scales of a few pc

30. H3+ work in progress/development • Observe H3+ near W 28 & Vela SNRs • High S/N survey of compact regions (Sco-Oph & Per OB2) • Southern hemisphere survey (CRIRES) • ESO Archive survey

31. Ion-Molecule Reactions N2H+ N2 CR H2 CO H3+ HCO+ H2 H2+ O H2 H2 CR O H2 OH+ H2O+ H3O+ H H+ O+ e- H e- e- H3+ H+ hν hν H2O OH O

32. ζ from OH+ and H2O+ • Oxygen chemistry closely linked to ionization rate of H (ζH) • ε is fraction of H+ that forms OH+ • Needs to be determined

33. W 51

34. Preliminary results • from H3+: ζ2=(12.5±9.3)×10-16 s-1 • from OH+: ζHε=(0.52±0.28)×10-16 s-1 • taking 2.3ζH=1.5ζ2: ζ2ε=(0.79±0.43)×10-16 s-1 • ε=0.06 ± 0.03 • >90% of the time, cosmic-ray ionization of H does not result in OH+ • Grain neutralization on PAH-

35. γ-ray Signatures W28, Fermi-LAT; Abdo et al. 2010 IC 443, VERITAS; Acciari et al. 2009 W51C, Fermi-LAT; Carmona et al. 2011

36. Diffuse γ-ray Signatures

37. Thanks!