June 19, 2008University of Illinois at Urbana-Champaign 1 Constraining the Low-Energy Cosmic Ray Spectrum 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 - Nath, B. B., & Biermann, P. L. 1994, MNRAS, 267, Hayakawa, S., Nishimura, S., & Takayanagi, T. 1961, PASJ, 13, Valle, G., Ferrini, F., Galli, D., & Shore, S. N. 2002, ApJ, 566, Kneller, J. P., Phillips, J. R., & Walker, T. P. 2003, ApJ, 589, Spitzer, L., Jr., & Tomasko, M. G. 1968, ApJ, 152, this study

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 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

8 Leaky Box Model Ionization Spallation Escape

9 Leaky Box Model Broken power law in momentum Produces an ionization rate of 1× s -1, much lower than the value inferred from H 3 + Try a spectrum with more flux at low energies by changing power law index below 200 MeV p 1.8 p -2.6

10 Matching Observations Float the spectral index to match inferred ζ 2 Choose a low energy cutoff (2 MeV) We find a relationship of p -2.0 is required to produce an ionization rate of 3.6× s -1 This gives 8.6× s -1 assuming a 10 MeV cutoff (dense cloud) Cravens, T. E., & Dalgarno, A. 1978, ApJ, 219, 750

11 Effects on Light Elements B B Be Li Li CalculatedMeteoritic * Abundances ( w.r.t. H) * Lemoine, M., Vangioni-Flam, E., & Cassé, M. 1998, ApJ, 499, 735

12 Effects on Gamma Rays Interstellar gamma ray lines at 4.4 MeV & 6.13 MeV have not been observed We predict a diffuse Galactic flux of ~6×10 -8 cm -2 s -1 deg -2 for both lines This is below the detection limit of current gamma ray telescopes such as Integral

13 Energy Requirements We can also calculate the rate at which all of the particles in our spectrum lose energy The result is 0.15×10 51 ergs century -1 The standard supernova energy is ergs, and the standard supernova rate is 3 per century in the Galaxy, so this energy requirement is well within the Galactic budget

14 Conclusions Leaky box propagated spectrum is unable to account for ionization rate Possible to generate an ionization rate of about 4× s -1 given the correct power law index (p -2 ) at low energies This spectrum is in rough agreement with light element abundances, and is not inconsistent with gamma ray observations Required input energy is available from supernovae

15 Future Work Use more advanced cosmic ray models including re-acceleration effects Consider variation of the cosmic ray spectrum in space and time Continue observations to put better constraints on the ionization rate in various environments

16 Acknowledgments Brian Fields The McCall Group