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Probing Cosmic-Ray Acceleration and Propagation with H 3 + Observations

Probing Cosmic-Ray Acceleration and Propagation with H 3 + Observations. Nick Indriolo, Brian D. Fields, & Benjamin J. McCall University of Illinois at Urbana-Champaign. Collaborators. Takeshi Oka – University of Chicago Tom Geballe – Gemini Observatory Tomonori Usuda – Subaru Telescope

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Probing Cosmic-Ray Acceleration and Propagation with H 3 + Observations

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  1. 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

  2. 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

  3. 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

  4. 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, 2175 - Spitzer, L., Jr., & Tomasko, M. G. 1968, ApJ, 152, 971 6 – Indriolo, N., Fields, B. D., & McCall, B. J. 2009, ApJ, 694, 257

  5. 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

  6.  +  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

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

  8. Pionic Gamma-Rays & Supernova Remnants

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

  10. 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

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

  12. 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

  13. H3+ Chemistry • Formation • 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

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

  15. 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

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

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

  18. 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

  19. 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

  20. HD 43703 ALS 8828 HD 254755 HD 43582 HD 254577 HD 43907 Target Sight Lines

  21. Results Indriolo et al. 2010, ApJ, in press

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

  23. Either ζ2 is large, or xenH is small Results

  24. 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

  25. 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 Spitzer & Tomasko 1968, ApJ, 152, 971 Torres et al. 2008, MNRAS, 387, L59

  26. 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)

  27. 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

  28. 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

  29. 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

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