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The Implications of a High Cosmic-Ray Ionization Rate in Diffuse Interstellar Clouds

This study explores the effects of a high cosmic-ray ionization rate in diffuse interstellar clouds and its implications for various astrophysical processes. The results indicate the importance of low-energy cosmic rays and highlight the need for further investigation.

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The Implications of a High Cosmic-Ray Ionization Rate in Diffuse Interstellar Clouds

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  1. The Implications of a High Cosmic-Ray Ionization Rate in Diffuse Interstellar Clouds Nick Indriolo, Brian D. Fields, Benjamin J. McCall University of Illinois at Urbana-Champaign MWAM 08

  2. Image credit: NASA/CXC/UMass Amherst/M.D.Stage et al. Cosmic Ray Basics • Charged particles (e-, e+, p, α, etc.) with high energy (103-1019 eV) • Galactic cosmic rays are primarily accelerated in supernovae remnants

  3. Background • Cosmic rays have several impacts on the interstellar medium, all of which produce some observables • Ionization: molecules • CR + H2→ H2+ + e- + CR • H2+ + H2 → H3+ + H • Spallation: light element isotopes • [p, α] + [C, N, O] → [6Li, 7Li, 9Be, 10B, 11B] • Nuclear excitation: gamma rays • [p, α] + [C, O] → [C*, O*] → γ (4.4, 6.13 MeV)

  4. Motivations • Many astrochemical processes depend on ionization • Cosmic rays are the primary source of ionization in cold interstellar clouds • Low-energy cosmic rays (2-10 MeV) are the most efficient at ionization • The cosmic ray spectrum below ~1 GeV cannot be directly measured at Earth

  5. Example Cosmic Ray Spectra 1 - Herbst, E., & Cuppen, H. M. 2006, PNAS, 103, 12257 2 - Spitzer, L., Jr., & Tomasko, M. G. 1968, ApJ, 152, 971 3 - Kneller, J. P., Phillips, J. R., & Walker, T. P. 2003, ApJ, 589, 217 Shading – Mori, M. 1997, ApJ, 478, 225 4 - Valle, G., Ferrini, F., Galli, D., & Shore, S. N. 2002, ApJ, 566, 2525 - Hayakawa, S., Nishimura, S., & Takayanagi, T. 1961, PASJ, 13, 1846 - Nath, B. B., & Biermann, P. L. 1994, MNRAS, 267, 447Points – AMS Collaboration, et al. 2002, Phys. Rep., 366, 331

  6. Motivations • Recent results from H3+ give an ionization rate of ζ2=4×10-16 s-1 • Given a cosmic ray spectrum and cross section, the ionization rate can be calculated theoretically Indriolo, N., Geballe, T. R., Oka, T., & McCall, B. J. 2007, ApJ, 671, 1736

  7. Cosmic-Ray Ionization Rate (ζ2×10-17 s-1) Spectrum ζ2 (diffuse) ζ2 (dense) Propagated 1.4 4.3 Hayakawa et al. 165 96 Spitzer & Tomasko 0.7 0.7 Nath & Biermann 260 34 Kneller et al. 1.3 1.0 Valle et al. 3.6 2.7 Herbst & Cuppen 0.9 0.9 Observations 40a 3b Results from Various Spectra a – Indriolo, N., Geballe, T. R., Oka, T., & McCall, B. J. 2007, ApJ, 671, 1736 b – van der Tak, F. F. S., & van Dishoeck, E. F. 2000, A&AL, 358, L79

  8. p-4.3 p-2.0 p0.8 f=0.01 p-2.7 Add Flux at Low Energies

  9. Cosmic-Ray Ionization Rate (ζ2×10-17 s-1) Spectrum ζ2 (diffuse) ζ2 (dense) Broken Power Law 36 8.6 Carrot 37 2.6 Observations 40 3 High Flux Results • This is no surprise, as these spectra were tailored to reproduce the diffuse cloud ionization rate results

  10. Carrot Construction

  11. Light Element Results a – Anders, E. & Grevesse, N. 1989 Geochim. Cosmochim. Acta, 53, 197

  12. Diffuse Gamma-Ray Flux from the Central Radian (10-5 s-1 cm-2 rad-1) Energy INTEGRALa Propagated Power Law Carrot 4.44 MeV 10 0.9 8.3 3.0 6.13 MeV 10 0.4 5.9 2.4 a – Teegarden, B. J., & Watanabe, K. 2006, ApJ, 646, 965 Gamma-Ray Results

  13. Energy Constraints • There are approximately 3±2 supernovae per century, each releasing about 1051 erg of mechanical energy • The carrot spectrum requires 0.18×1051 erg per century, while the broken power law requires 0.17×1051 erg per century • Both are well within constraints

  14. Acceleration Mechanism • Carrot spectrum shape does not match acceleration by supernovae remnants • Voyager 1 observations at the heliopause show a steep slope at low energies • Possible that “astropauses” are accelerating cosmic rays throughout the Galaxy Fig. 2 - Stone, E. et al. 2005, Science, 309, 2017

  15. Conclusions • Carrot spectrum explains high ionization rate, and is broadly consistent with various observables • p-4.3 power law is inconsistent with acceleration from SNR • Perhaps weak shocks in the ISM are responsible for the vast majority of low-energy cosmic rays

  16. Brian Fields The McCall Group Acknowledgments

  17. 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

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