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

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
2
Cosmic Ray Basics

• Charged particles (e-, e, p, a, 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, a C, N, O ? 6Li, 7Li, 9Be, 10B, 11B
• Nuclear excitation gamma rays
• p, a C, O ? C, O ? ? (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, 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
6
Motivations

• Recent results from H3 give an ionization rate
  of ?2410-16 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
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, AAL,
358, L79
8
Add Flux at Low Energies
9
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
Gamma-Ray Results
13
Energy Constraints

• There are approximately 32 supernovae per
  century, each releasing about 1051 erg of
  mechanical energy
• The carrot spectrum requires 0.181051 erg per
  century, while the broken power law requires
  0.171051 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
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, AA, 40, 91