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Variability of the Cosmic-Ray Ionization Rate in Diffuse Molecular Clouds

Variability of the Cosmic-Ray Ionization Rate in Diffuse Molecular Clouds. Nick Indriolo 1 , Thomas R. Geballe 2 , Takeshi Oka 3 , & Benjamin J. McCall 1. 1 – University of Illinois at Urbana-Champaign 2 – Gemini Observatory 3 – University of Chicago. Motivations.

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Variability of the Cosmic-Ray Ionization Rate in Diffuse Molecular Clouds

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  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-Champaign 2 – Gemini Observatory 3 – University of Chicago 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 (10-18-10-15 s-1) • What does this imply about the acceleration sources of cosmic rays?

  3. Supernova Remnants OB Associations Accelerators

  4. Background • Formation • 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. R(1,1)u R(1,0) Data Reduction

  7. Ionization Rate vs. Hydrogen Column R=-0.14 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 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

  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

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