Probing Cosmic-Ray Acceleration and Propagation with H3+ Observations Nick Indriolo, Brian D. Fields, & Benjamin J. McCall University of Illinois at Urbana-Champaign Image credit: Gerhard Bachmayer

Collaborators Takeshi Oka – University of Chicago Tom Geballe – Gemini Observatory Tomonori Usuda – Subaru Telescope Miwa Goto – Max Planck Institute for Astronomy Geoff Blake – California Institute of Technology Ken Hinkle – NOAO

Cosmic Ray Basics Energetic charged particles and nuclei Thought to be primarily accelerated in supernova remnants Diffuse throughout the interstellar medium along magnetic field lines Generally assumed that the cosmic-ray spectrum is uniform in the Galaxy

Example Cosmic-Ray Spectra 1 - Nath, B. B., & Biermann, P. L. 1994, MNRAS, 267, 447 2 - Hayakawa, S., Nishimura, S., & Takayanagi, T. 1961, PASJ, 13, 184 3 - Valle, G., Ferrini, F., Galli, D., & Shore, S. N. 2002, ApJ, 566, 252 4 - Kneller, J. P., Phillips, J. R., & Walker, T. P. 2003, ApJ, 589, 217 5 - Spitzer, L., Jr., & Tomasko, M. G. 1968, ApJ, 152, 971 6 – Indriolo, N., Fields, B. D., & McCall, B. J. 2009, ApJ, 694, 257

Interactions with the ISM Ionization and excitation of atoms and molecules CR + H  CR’ + p + e- CR + H2  CR’ + H2+ + e- Spallation of ambient nuclei and of heavier cosmic rays CR + [C,N,O]  CR’ + [Li,Be,B] + fragments

Interactions with the ISM Excitation of nuclear states, resulting in gamma-ray emission CR + 12C  CR’ + 12C*  12C + 4.44 CR + 16O  CR’ + 16O*  16O + 6.13 Production of mesons (+, -, 0) during inelastic collisions CR + H  CR’ + H + 0  + 

Cross Sections Bethe, H. 1933, Hdb. d Phys. (Berlin: J. Springer), 24, Pt. 1, 491 Read, S. M., & Viola, V. E. 1984, Atomic Data Nucl. Data, 31, 359 Meneguzzi, M. & Reeves, H. 1975, A&A, 40, 91

Pionic Gamma-Rays & Supernova Remnants

Pionic Gamma-Rays & Supernova Remnants VERITAS gamma-ray map of IC 443: Acciari et al. 2009, ApJ, 698, L133

Pionic Gamma-Rays & Supernova Remnants HESS gamma-ray map of W 28 Aharonian et al. 2008, A&A, 481, 401 Fermi-LAT gamma-ray map of W 28 Abdo et al. 2010, ApJ, 718, 348

Pionic Gamma-Rays & Supernova Remnants Supernova remnants accelerate hadronic cosmic rays Ekin > 280 MeV Abdo et al. 2010, ApJ, 718, 348

Tracing Lower-Energy Cosmic Rays Formation of molecular ion H3+ begins with ionization of H2 CR + H2  H2+ + e- + CR’ H2+ + H2  H3+ + H Cross section for ionization increases as cosmic-ray energy decreases, so H3+ should trace MeV particles

H3+ Chemistry Formation Destruction Steady state in diffuse clouds CR + H2  H2+ + e- + CR’ H2+ + H2  H3+ + H Destruction H3+ + CO  HCO+ + H2 (dense clouds) H3+ + e-  H2 + H or H + H + H (diffuse clouds) Steady state in diffuse clouds

Calculating the Ionization Rate Sheffer et al. 2008, ApJ, 687, 1075 N(H2) from N(CH) xe from C+; Cardelli et al. 1996, ApJ, 467, 334 nH from C2; Sonnentrucker et al. 2007, ApJS, 168, 58

Observations Transitions of the 2  0 band of H3+ are available in the infrared R(1,1)u: 3.66808 m; R(1,0) : 3.66852 m R(1,1)l : 3.71548 m; Q(1,1) : 3.92863 m Q(1,0) : 3.95300 m; R(3,3)l : 3.53367 m Weak absorption lines (typically 1-2%) require combination of a large telescope and high resolution spectrograph

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

Select H3+ Spectra Crabtree et al. 2010, ApJ, submitted

Current Survey Status Searched for H3+ in about 50 diffuse cloud sight lines Detected absorption in 20 of those Column densities range from a few times 1013 cm-2 to a few times 1014 cm-2 Inferred ionization rates of 2–810-16 s-1, with 3 upper limits as low as 710-17 s-1 Dame et al. 2001, ApJ, 547, 792

Implications Variations in the ionization rate suggest that the cosmic-ray spectrum may not be uniform at lower energies If true, the cosmic-ray flux should be much higher in close proximity to the site of particle acceleration Search for H3+ near the supernova remnant IC 443

Target Sight Lines HD 43703 ALS 8828 HD 254755 HD 43582 HD 254577

Results Indriolo et al. 2010, ApJ, in press

HD 43703 ALS 8828 HD 254755 HD 43582 HD 254577 HD 43907

Results Either ζ2 is large, or xenH is small N(H3+) ζ2 (1014 cm-2) ALS 8828 4.4 16±10 HD 254577 2.2 26±16 HD 254755 < 0.6 < 3.5 HD 43582 < 0.8 < 9.0 HD 43703 < 5.7 HD 43907 < 2.1 < 40 Either ζ2 is large, or xenH is small

Case 1: Low electron density By taking an average value from C+, have we overestimated the electron density? xe decreases from ~10-4 in diffuse clouds to ~10-8 in dense clouds C2 rotation-excitation and CN restricted chemical analyses indicate densities of 200-400 cm-3 (Hirschauer et al. 2009) Estimated values of x(CO) are ~10-6, much lower than 3×10-4 solar system abundance of carbon

Case 2: High Ionization Rate How can we explain the large difference between detections and upper limits? Cosmic-ray spectrum changes as particles propagate Perhaps ALS 8828 & HD 254577 sight lines probe clouds closer to SNR Torres et al. 2008, MNRAS, 387, L59 Spitzer & Tomasko 1968, ApJ, 152, 971

Propagation & Acceleration MHD effects May exclude lower-energy particles from entering denser regions Damping of Alfvén waves may limit time spent in denser regions Acceleration effects In models of diffusive shock acceleration, the highest energy particles escape upstream while the others are advected downstream (into the remnant)

Applications With sufficient spatial coverage (i.e. sight lines), it may be possible to track particle flux in supernova remnants This may be useful in constraining particle acceleration/escape efficiency in models Allow for better constraints on the interstellar cosmic-ray spectrum

Summary H3+ has been detected in 20 of ~50 diffuse cloud sight lines studied, and ionization rates range from 0.7–810-16 s-1 Ionization rates inferred near IC 443 are ~210-15 s-1, suggesting that the supernova remnant accelerates a large flux of low-energy cosmic rays Propagation effects and proximity to the acceleration site may cause non-uniformity in the cosmic-ray spectrum

Future Work Continue survey of H3+ in diffuse cloud sight lines Search for H3+ near more supernova remnants interacting with the ISM Where possible, perform necessary ancillary observations (H2, CH, CO, C, C+) to constrain sight line properties