Variability of the Cosmic-Ray Ionization Rate in Diffuse Molecular Clouds
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June 24, 2009 International Symposium on Molecular Spectroscopy
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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-Champaign2 – Gemini Observatory3 – University of Chicago
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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 (10-18-10-15 s-1)
• What does this imply about the acceleration sources of cosmic rays?
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Accelerators
Supernova Remnants
OB Associations
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Background
• Formation– CR + H2 → CR + H2
+ + e-
– H2+ + H2 → H3
+ + H
• Destruction– H3
+ + e- → H2 + H or 3H
• Assume steady state
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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)
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Data Reduction
R(1,0)
R(1,1)u
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Ionization Rate vs. Hydrogen Column
Indriolo, N., Geballe, T. R., Oka, T., & McCall, B. J. 2007, ApJ, 671, 1736McCall, B. J., Geballe, T. R., Hinkle, K. H., & Oka, T. 1998, Science, 279, 1910McCall, B. J., et al. 2002, ApJ, 567, 391
R=-0.14
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Well Constrained Upper Limits
ζ Oph: ζ2 < 1.6×10-16 s-1
ο Sco: ζ2 < 1.2×10-16 s-1
S/N ~ 400
S/N ~ 700
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Ionization Rate and Galactic Coordinates
southern sky
AKARI 9 μm all-sky survey (JAXA/ESA)
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Small Scale Variations• Sight lines HD
168625 and HD 168607 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 168625: ζ2=2.4×10-
16±1.3×10-16 s-1
HD 168607: ζ2=1.3×10-
16±0.7×10-16 s-1
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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
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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
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Acknowledgments