November 8, 2008MWAM 081 The Implications of a High Cosmic-Ray Ionization Rate in Diffuse Interstellar Clouds Nick Indriolo, Brian D. Fields, Benjamin.

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