Post on 27-Mar-2015
November 8, 2008 MWAM 08 1
The Implications of a High Cosmic-Ray Ionization Rate
in Diffuse Interstellar Clouds
Nick Indriolo, Brian D. Fields, Benjamin J. McCallUniversity of Illinois at Urbana-
Champaign
2
Cosmic Ray Basics
• Charged particles (e-, e+, p, α, etc.) with high energy (103-1019 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 + H2 → H2+ + e- + CR
• H2+ + H2 → H3
+ + H
– Spallation: light element isotopes• [p, α] + [C, N, O] → [6Li, 7Li, 9Be, 10B, 11B]
– 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, 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 ζ2=4×10-16 s-1
• Given a cosmic ray spectrum and cross section, the ionization rate can be calculated theoretically
dEEEhigh
low
E
E)()(4
Indriolo, N., Geballe, T. R., Oka, T., & McCall, B. J. 2007, ApJ, 671, 1736
7
Results from Various Spectra
3b40aObservations
0.90.9Herbst & Cuppen
2.73.6Valle et al.
1.01.3Kneller et al.
34260Nath & Biermann
0.70.7Spitzer & Tomasko
96165Hayakawa et al.
4.31.4Propagated
ζ2 (dense)ζ2 (diffuse)Spectrum
Cosmic-Ray Ionization Rate (ζ2×10-17 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
8
p-2.7
p0.8
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
ζ2 (dense)ζ2 (diffuse)Spectrum
Cosmic-Ray Ionization Rate (ζ2×10-17 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
Ratio Solar Systema
Propagated
Power Law
Carrot
1010×6Li/H
1.5 1.3 8.2 2.5
1010×7Li/H
19 1.9 18 5.8
1010×9Be/H
0.26 0.33 0.59 0.35
1010×10B/H
1.5 1.3 2.5 1.4
1010×11B/H
6.1 2.8 6.4 3.2
6Li/9Be 5.8 4.0 13.9 7.110B/9Be 5.8 3.9 4.3 4.0
a – Anders, E. & Grevesse, N. 1989 Geochim. Cosmochim. Acta, 53, 197
12
Gamma-Ray Results
2.45.90.4106.13 MeV
3.08.30.9104.44 MeV
CarrotPower LawPropagatedINTEGRALaEnergy
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 1051 erg of mechanical energy
• The carrot spectrum requires 0.18×1051 erg per century, while the broken power law requires 0.17×1051 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