November 8, 2008MWAM 081 The Implications of a High Cosmic-Ray Ionization Rate in Diffuse...

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November 8, 2008MWAM 081 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 Slide 2 2 Cosmic Ray Basics Charged particles (e -, e +, p, , etc.) with high energy (10 3 -10 19 eV) Galactic cosmic rays are primarily accelerated in supernovae remnants Image credit: NASA/CXC/UMass Amherst/M.D.Stage et al. Slide 3 3 Background Cosmic rays have several impacts on the interstellar medium, all of which produce some observables Ionization: molecules CR + H 2 H 2 + + e - + CR H 2 + + H 2 H 3 + + H Spallation: light element isotopes [p, ] + [C, N, O] [ 6 Li, 7 Li, 9 Be, 10 B, 11 B] Nuclear excitation: gamma rays [p, ] + [C, O] [C *, O * ] (4.4, 6.13 MeV) Slide 4 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 Slide 5 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, 252 5 - Hayakawa, S., Nishimura, S., & Takayanagi, T. 1961, PASJ, 13, 184 6 - Nath, B. B., & Biermann, P. L. 1994, MNRAS, 267, 447 Points AMS Collaboration, et al. 2002, Phys. Rep., 366, 331 Slide 6 6 Motivations Recent results from H 3 + give an ionization rate of 2 =410 -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 Slide 7 7 Results from Various Spectra 3b3b 40 a Observations 0.9 Herbst & Cuppen 2.73.6 Valle et al. 1.01.3 Kneller et al. 34260 Nath & Biermann 0.7 Spitzer & Tomasko 96165 Hayakawa et al. 4.31.4 Propagated 2 (dense) 2 (diffuse)Spectrum Cosmic-Ray Ionization Rate ( 2 10 -17 s -1 ) 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 Slide 8 8 p -2.7 p 0.8 p -2.0 Add Flux at Low Energies p -4.3 f=0.01 Slide 9 9 High Flux Results 340Observations 2.637Carrot 8.636Broken Power Law 2 (dense) 2 (diffuse)Spectrum Cosmic-Ray Ionization Rate ( 2 10 -17 s -1 ) This is no surprise, as these spectra were tailored to reproduce the diffuse cloud ionization rate results Slide 10 10 Carrot Construction Slide 11 11 Light Element Results RatioSolar System a PropagatedPower LawCarrot 10 10 6 Li/H1.51.38.22.5 10 10 7 Li/H191.9185.8 10 10 9 Be/H0.260.330.590.35 10 10 10 B/H1.51.32.51.4 10 10 11 B/H6.12.86.43.2 6 Li/ 9 Be5.84.013.97.1 10 B/ 9 Be5.83.94.34.0 a Anders, E. & Grevesse, N. 1989 Geochim. Cosmochim. Acta, 53, 197 Slide 12 12 Gamma-Ray Results 2.45.90.4106.13 MeV 3.08.30.9104.44 MeV CarrotPower LawPropagatedINTEGRAL a Energy a Teegarden, B. J., & Watanabe, K. 2006, ApJ, 646, 965 Diffuse Gamma-Ray Flux from the Central Radian (10 -5 s -1 cm -2 rad -1 ) Slide 13 13 Energy Constraints There are approximately 32 supernovae per century, each releasing about 10 51 erg of mechanical energy The carrot spectrum requires 0.1810 51 erg per century, while the broken power law requires 0.1710 51 erg per century Both are well within constraints Slide 14 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 Slide 15 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 Slide 16 16 Acknowledgments Brian Fields The McCall Group Slide 17 17 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