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

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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. - PowerPoint PPT Presentation

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

  • Variability of the Cosmic-Ray Ionization Rate in Diffuse Molecular CloudsNick Indriolo1, Thomas R. Geballe2, Takeshi Oka3, & Benjamin J. McCall11 University of Illinois at Urbana-Champaign2 Gemini Observatory3 University of Chicago

    International Symposium on Molecular Spectroscopy

  • MotivationsThe cosmic-ray ionization rate () is an important parameter in cloud modelingThe cosmic-ray flux is typically assumed to be uniform in the GalaxyInferred values of 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

  • BackgroundFormationCR + H2 CR + H2+ + e-H2+ + H2 H3+ + HDestructionH3+ + e- H2 + H or 3HAssume steady state

  • ObservationsSelect various diffuse molecular cloud sight linesSearch for R(1,0) and R(1,1)u lines of H3+ near 3.67 mCGS4 on UKIRT34 sight lines to date (19 detections, 15 non-detections)

  • Data Reduction

  • Ionization Rate vs. Hydrogen ColumnIndriolo, 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, 391R=-0.14

  • Well Constrained Upper Limits Oph: 2 < 1.610-16 s-1 Sco: 2 < 1.210-16 s-1S/N ~ 400S/N ~ 700

  • Ionization Rate and Galactic Coordinatessouthern skyAKARI 9 m all-sky survey (JAXA/ESA)

  • Small Scale VariationsSight 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: 2=2.410-161.310-16 s-1HD 168607: 2=1.310-160.710-16 s-1

  • ConclusionsNo conclusive correlation between the ionization rate and column density in diffuse molecular cloudsH3+ detections are more likely at lower Galactic latitudesThe ionization rate may be higher in the Galactic plane where the acceleration sources (SNR, OB associations) are more highly concentrated

  • Future WorkData from more diffuse clouds sight lines are currently being processedSearch for H3+ and compute the ionization rate near a cosmic-ray acceleration source2 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