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June 21, 2011 TF03 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 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
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CRH2+
H2
H2 H3+
CO HCO
+
O
OH+
N2
N2H+
H2 H2O+
H2 H3O+
CRH H
+
OO+
H2
Ion-Molecule Reactions
• Low proton affinity of H2 makes H3+
especially willing to transfer its charge
June 21, 2011 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
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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)
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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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Necessary Parameters
• ke measured
• xe approximated by x(C+)≈1.510-4
• 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)
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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
+
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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
June 21, 2011 TF03 15
ζ2 versus Total Column Density
Dense cloud results from Kulesa 2002 and van der Tak & van Dishoeck 2000
June 21, 2011 TF03 16
Particle Range
Padovani et al. 2009
Range for a 1 MeV proton is ~31020 cm-2
Range for a 10 MeV proton is ~21022 cm-2
Diffuse cloud column densities are about 1021 ≤ NH ≤ 1022 cm-2
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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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Reproducing High Inferred ζ2
Using both components: ζ2=3.710-16 s-1
Using only base component: ζ2=0.1410-16 s-1
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SNR versus Diffuse ISM
• Ionization rates near IC 443– ζ2~20±1010-16 s-1
• Ionization rates in the diffuse ISM– mean: ζ2=3.3±0.410-16 s-1
– max: ζ2=10.6±6.810-16 s-1
– min: ζ2<0.410-16 s-1
• Consistent with theory that ionization rates are higher near acceleration sites
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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