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

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

3 Background Cosmic rays have several impacts on the interstellar medium, all of which produce some observables –Ionization: molecules CR + H 2 H e - + CR H H 2 H H –Spallation: light element isotopes [p, α] + [C, N, O] [ 6 Li, 7 Li, 9 Be, 10 B, 11 B] –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, Spitzer, L., Jr., & Tomasko, M. G. 1968, ApJ, 152, Kneller, J. P., Phillips, J. R., & Walker, T. P. 2003, ApJ, 589, 217 Shading – Mori, M. 1997, ApJ, 478, Valle, G., Ferrini, F., Galli, D., & Shore, S. N. 2002, ApJ, 566, Hayakawa, S., Nishimura, S., & Takayanagi, T. 1961, PASJ, 13, Nath, B. B., & Biermann, P. L. 1994, MNRAS, 267, 447 Points – AMS Collaboration, et al. 2002, Phys. Rep., 366, 331

6 Motivations Recent results from H 3 + give an ionization rate of ζ 2 =4× 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 Results from Various Spectra 3b3b 40 a Observations 0.9 Herbst & Cuppen Valle et al Kneller et al Nath & Biermann 0.7 Spitzer & Tomasko Hayakawa et al Propagated ζ 2 (dense)ζ 2 (diffuse)Spectrum Cosmic-Ray Ionization Rate (ζ 2 × 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

8 p -2.7 p 0.8 p -2.0 Add Flux at Low Energies p -4.3 f=0.01

9 High Flux Results 340Observations 2.637Carrot 8.636Broken Power Law ζ 2 (dense)ζ 2 (diffuse)Spectrum Cosmic-Ray Ionization Rate (ζ 2 × s -1 ) This is no surprise, as these spectra were tailored to reproduce the diffuse cloud ionization rate results

10 Carrot Construction

11 Light Element Results RatioSolar System a PropagatedPower LawCarrot × 6 Li/H × 7 Li/H × 9 Be/H × 10 B/H × 11 B/H Li/ 9 Be B/ 9 Be a – Anders, E. & Grevesse, N Geochim. Cosmochim. Acta, 53, 197

12 Gamma-Ray Results MeV 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 )

13 Energy Constraints There are approximately 3±2 supernovae per century, each releasing about erg of mechanical energy The carrot spectrum requires 0.18×10 51 erg per century, while the broken power law requires 0.17×10 51 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 Acknowledgments Brian Fields The McCall Group

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