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

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?

Accelerators Supernova Remnants OB Associations

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

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)

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

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

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

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

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

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

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

Acknowledgments