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

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 TF03

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 TF03

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 June 21, 2011 TF03

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) June 21, 2011 TF03

Steady State Equation June 21, 2011 TF03

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 June 21, 2011 TF03

Validity of Approximation June 21, 2011 TF03

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 TF03

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+ June 21, 2011 TF03

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 June 21, 2011 TF03

Example Spectra June 21, 2011 TF03

Inferred Ionization Rates mean ionization rate: ζ2=3.3±0.410-16 s-1 June 21, 2011 TF03

ζ2 versus Galactic Longitude June 21, 2011 TF03

ζ2 versus Total Column Density Dense cloud results from Kulesa 2002 and van der Tak & van Dishoeck 2000 June 21, 2011 TF03

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 June 21, 2011 TF03

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 June 21, 2011 TF03

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 June 21, 2011 TF03

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 June 21, 2011 TF03

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 June 21, 2011 TF03

Acknowledgments Brian Fields Geoff Blake Miwa Goto Tomonori Usuda June 21, 2011 TF03