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)

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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

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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

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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

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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

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p-2.7

p0.8

p-2.0

Add Flux at Low Energies

p-4.3

f=0.01

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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

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Carrot Construction

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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

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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)

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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

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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

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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

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Acknowledgments

Brian Fields

The McCall Group

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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