June 24, 2009 International Symposium on Molecular Spectroscopy

1

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

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 (10-18-10-15 s-1)

• What does this imply about the acceleration sources of cosmic rays?

3

Accelerators

Supernova Remnants

OB Associations

4

Background

• Formation– CR + H2 → CR + H2

+ + e-

– H2+ + H2 → H3

+ + H

• Destruction– H3

+ + e- → H2 + H or 3H

• Assume steady state

)()H()H( 322 ennkn e

HH

H32

)()H(2

n

en

N

n

f

kN e

)H(2)H(

)H(2

2

2

nn

nf

)H(2)H( 2H nnn

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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