The Implications of a High Cosmic-Ray Ionization Rate in Diffuse Interstellar Clouds
-
Upload
teagan-vazquez -
Category
Documents
-
view
63 -
download
0
description
Transcript of The Implications of a High Cosmic-Ray Ionization Rate in Diffuse Interstellar Clouds
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
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
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