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
June 21, 2011
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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
June 21, 2011
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3. Ion-Molecule Reactions
N2H+ N2 CR H2 CO H2
H2+
H3+
HCO+ O CR O H2 H2 H2 H H+ O+
OH+
H2O+
H3O+
Low proton affinity of H2 makes H3+ especially willing to transfer its charge
June 21, 2011
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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
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5. H3+ Chemistry Formation Destruction 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)
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6. Steady State Equation
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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
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8. Validity of Approximation
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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)
June 21, 2011
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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: μm
R(1,0): μm
R(1,1)l: μm
Q(1,1): μm
Q(1,0): μm
Energy level diagram for the ground vibrational state of H3+
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11. Survey Status
Dame et al. 2001
Observations targeting H3+ in diffuse clouds have been made in 50 sight lines
H3+ is detected in 21 of those
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12. Example Spectra
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13. Inferred Ionization Rates
mean ionization rate: ζ2=3.3±0.410-16 s-1
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14. ζ2 versus Galactic Longitude
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15. ζ2 versus Total Column Density
Dense cloud results from Kulesa 2002 and van der Tak & van Dishoeck 2000
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16. Particle Range Range for a 1 MeV proton is ~31020 cm-2
Diffuse cloud column densities are about 1021 ≤ NH ≤ 1022 cm-2
Padovani et al. 2009
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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
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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
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19. SNR versus Diffuse ISM Ionization rates near IC 443
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
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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
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21. Acknowledgments Brian Fields Geoff Blake Miwa Goto Tomonori Usuda
June 21, 2011
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