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
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
Data Reduction
R(1,0)
R(1,1)u
7
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
8
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
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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