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

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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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Steady State Equation

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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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Validity of Approximation

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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

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Example Spectra

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Inferred Ionization Rates

mean ionization rate: ζ2=3.3±0.410-16 s-1

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ζ2 versus Galactic Longitude

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ζ2 versus Total Column Density

Dense cloud results from Kulesa 2002 and van der Tak & van Dishoeck 2000

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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

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Acknowledgments

• Brian Fields• Geoff Blake• Miwa Goto• Tomonori Usuda