1. Probing Cosmic-Ray Acceleration and Propagation with H3+ Observations
Nick Indriolo, Brian D. Fields, &
Benjamin J. McCall
University of Illinois at Urbana-Champaign
Image credit: Gerhard Bachmayer

2. Collaborators Takeshi Oka – University of Chicago
Tom Geballe – Gemini Observatory
Tomonori Usuda – Subaru Telescope
Miwa Goto – Max Planck Institute for Astronomy
Geoff Blake – California Institute of Technology
Ken Hinkle – NOAO

3. Cosmic Ray Basics Energetic charged particles and nuclei
Thought to be primarily accelerated in supernova remnants
Diffuse throughout the interstellar medium along magnetic field lines
Generally assumed that the cosmic-ray spectrum is uniform in the Galaxy

4. 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, 252
4 - Kneller, J. P., Phillips, J. R., & Walker, T. P. 2003, ApJ, 589, Spitzer, L., Jr., & Tomasko, M. G. 1968, ApJ, 152, – Indriolo, N., Fields, B. D., & McCall, B. J. 2009, ApJ, 694, 257

5. Interactions with the ISM
Ionization and excitation of atoms and molecules
CR + H  CR’ + p + e-
CR + H2  CR’ + H2+ + e-
Spallation of ambient nuclei and of heavier cosmic rays
CR + [C,N,O]  CR’ + [Li,Be,B] + fragments

6. Interactions with the ISM
Excitation of nuclear states, resulting in gamma-ray emission
CR + 12C  CR’ + 12C*  12C + 4.44
CR + 16O  CR’ + 16O*  16O + 6.13
Production of mesons (+, -, 0) during inelastic collisions
CR + H  CR’ + H + 0
 + 

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. Pionic Gamma-Rays & Supernova Remnants

9. Pionic Gamma-Rays & Supernova Remnants
VERITAS gamma-ray map of IC 443:
Acciari et al. 2009, ApJ, 698, L133

10. Pionic Gamma-Rays & Supernova Remnants
HESS gamma-ray map of W 28
Aharonian et al. 2008, A&A, 481, 401
Fermi-LAT gamma-ray map of W 28
Abdo et al. 2010, ApJ, 718, 348

11. Pionic Gamma-Rays & Supernova Remnants
Supernova remnants accelerate hadronic cosmic rays
Ekin > 280 MeV
Abdo et al. 2010, ApJ, 718, 348

12. Tracing Lower-Energy Cosmic Rays
Formation of molecular ion H3+ begins with ionization of H2
CR + H2  H2+ + e- + CR’
H2+ + H2  H3+ + H
Cross section for ionization increases as cosmic-ray energy decreases, so H3+ should trace MeV particles

13. H3+ Chemistry Formation Destruction Steady state in diffuse clouds
CR + H2  H2+ + e- + CR’
H2+ + H2  H3+ + H
Destruction
H3+ + CO  HCO+ + H2 (dense clouds)
H3+ + e-  H2 + H or H + H + H (diffuse clouds)
Steady state in diffuse clouds

14. Calculating the Ionization Rate
Sheffer et al. 2008, ApJ, 687, 1075
N(H2) from N(CH)
xe from C+;
Cardelli et al. 1996, ApJ, 467, 334
nH from C2;
Sonnentrucker et al. 2007, ApJS, 168, 58

15. Observations
Transitions of the 2  0 band of H3+ are available in the infrared
R(1,1)u: m; R(1,0) : m
R(1,1)l : m; Q(1,1) : m
Q(1,0) : m; R(3,3)l : m
Weak absorption lines (typically 1-2%) require combination of a large telescope and high resolution spectrograph

16. Instruments/Telescopes
IRCS: Subaru
CGS4: UKIRT
NIRSPEC: Keck II
Phoenix: Gemini South
CRIRES: VLT UT1

17. Select H3+ Spectra
Crabtree et al. 2010, ApJ, submitted

18. Current Survey Status
Searched for H3+ in about 50 diffuse cloud sight lines
Detected absorption in 20 of those
Column densities range from a few times 1013 cm-2 to a few times 1014 cm-2
Inferred ionization rates of 2–810-16 s-1, with 3 upper limits as low as 710-17 s-1
Dame et al. 2001, ApJ, 547, 792

19. Implications
Variations in the ionization rate suggest that the cosmic-ray spectrum may not be uniform at lower energies
If true, the cosmic-ray flux should be much higher in close proximity to the site of particle acceleration
Search for H3+ near the supernova remnant IC 443

20. Target Sight Lines HD 43703 ALS 8828 HD 254755 HD 43582 HD 254577

21. Results
Indriolo et al. 2010, ApJ, in press

22. HD 43703
ALS 8828 HD
HD 43582 HD
HD 43907

23. Results Either ζ2 is large, or xenH is small N(H3+) ζ2 (1014 cm-2)
ALS 8828
4.4
16±10 HD
2.2
26±16 HD
< 0.6
< 3.5
HD 43582
< 0.8
< 9.0
HD 43703
< 5.7
HD 43907
< 2.1
< 40
Either ζ2 is large, or xenH is small

24. Case 1: Low electron density
By taking an average value from C+, have we overestimated the electron density?
xe decreases from ~10-4 in diffuse clouds to ~10-8 in dense clouds
C2 rotation-excitation and CN restricted chemical analyses indicate densities of cm-3 (Hirschauer et al. 2009)
Estimated values of x(CO) are ~10-6, much lower than 3×10-4 solar system abundance of carbon

25. Case 2: High Ionization Rate
How can we explain the large difference between detections and upper limits?
Cosmic-ray spectrum changes as particles propagate
Perhaps ALS 8828 & HD sight lines probe clouds closer to SNR
Torres et al. 2008, MNRAS, 387, L59
Spitzer & Tomasko 1968, ApJ, 152, 971

26. Propagation & Acceleration
MHD effects
May exclude lower-energy particles from entering denser regions
Damping of Alfvén waves may limit time spent in denser regions
Acceleration effects
In models of diffusive shock acceleration, the highest energy particles escape upstream while the others are advected downstream (into the remnant)

27. Applications
With sufficient spatial coverage (i.e. sight lines), it may be possible to track particle flux in supernova remnants
This may be useful in constraining particle acceleration/escape efficiency in models
Allow for better constraints on the interstellar cosmic-ray spectrum

28. Summary
H3+ has been detected in 20 of ~50 diffuse cloud sight lines studied, and ionization rates range from 0.7–810-16 s-1
Ionization rates inferred near IC 443 are ~210-15 s-1, suggesting that the supernova remnant accelerates a large flux of low-energy cosmic rays
Propagation effects and proximity to the acceleration site may cause non-uniformity in the cosmic-ray spectrum

29. Future Work Continue survey of H3+ in diffuse cloud sight lines
Search for H3+ near more supernova remnants interacting with the ISM
Where possible, perform necessary ancillary observations (H2, CH, CO, C, C+) to constrain sight line properties