1. International Symposium on Molecular Spectroscopy
Variability of the Cosmic-Ray Ionization Rate in Diffuse Molecular Clouds
Nick Indriolo1, Thomas R. Geballe2, Takeshi Oka3, & Benjamin J. McCall1
1 – University of Illinois at Urbana-Champaign
2 – Gemini Observatory
3 – University of Chicago
June 24, 2009
International Symposium on Molecular Spectroscopy

2. Motivations
The cosmic-ray ionization rate (ζ) is an important parameter in cloud modeling
The cosmic-ray flux is typically assumed to be uniform in the Galaxy
Inferred values of ζ range over a few orders of magnitude ( s-1)
What does this imply about the acceleration sources of cosmic rays?

3. Accelerators
Supernova Remnants
OB Associations

4. Background Formation Destruction Assume steady state
CR + H2 → CR + H2+ + e-
H2+ + H2 → H3+ + H
Destruction
H3+ + e- → H2 + H or 3H
Assume steady state

5. Observations Select various diffuse molecular cloud sight lines
Search for R(1,0) and R(1,1)u lines of H3+ near 3.67 μm
CGS4 on UKIRT
34 sight lines to date (19 detections, 15 non-detections)

6. Data Reduction
R(1,0)
R(1,1)u

7. Ionization Rate vs. Hydrogen Column
Indriolo, N., Geballe, T. R., Oka, T., & McCall, B. J. 2007, ApJ, 671, 1736
McCall, B. J., Geballe, T. R., Hinkle, K. H., & Oka, T. 1998, Science, 279, 1910
McCall, B. J., et al. 2002, ApJ, 567, 391

8. Well Constrained Upper Limits
S/N ~ 400
S/N ~ 700
ζ Oph: ζ2 < 1.6×10-16 s-1
ο Sco: ζ2 < 1.2×10-16 s-1

9. Ionization Rate and Galactic Coordinates
southern sky
AKARI 9 μm all-sky survey (JAXA/ESA)

10. Small Scale Variations
Sight lines HD and HD are separated by 1’ on the sky.
At a distance of 1400 pc, this corresponds to 0.4 pc separation.
Both sight lines show 2 cloud components and have comparable ionization rates.
HD : ζ2=2.4×10-16±1.3×10-16 s-1
HD : ζ2=1.3×10-16±0.7×10-16 s-1

11. Conclusions
No conclusive correlation between the ionization rate and column density in diffuse molecular clouds
H3+ detections are more likely at lower Galactic latitudes
The ionization rate may be higher in the Galactic plane where the acceleration sources (SNR, OB associations) are more highly concentrated

12. Future Work
Data from more diffuse clouds sight lines are currently being processed
Search for H3+ and compute the ionization rate near a cosmic-ray acceleration source
2 nights with NIRSPEC on Keck and 2 nights with IRCS on Subaru to observe sight lines that probe molecular material near the SNR IC 443

13. Acknowledgments