Transcript He2009OSU_v3.ppt

Can We Use Metastable Helium to Trace
the Cosmic-Ray Ionization Rate?
Nick Indriolo1, L. M. Hobbs2,
K. H. Hinkle3, & B. J. McCall1
1-University of Illinois at Urbana-Champaign
2-University of Chicago, Yerkes Observatory
3-National Optical Astronomy Observatories (NOAO)
June 24, 2009
International Symposium on
Molecular Spectroscopy
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Motivations
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Molecules are predominantly ionized
by cosmic rays
The flux of ionizing cosmic rays is
unobservable at the Earth
Current estimates using other
tracers range from 10-18-10-15 s-1
Physical processes associated with
He* are thought to be very simple
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Why Use Metastable Helium?
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Helium is abundant
and ubiquitous in
the ISM
Relatively simple
atomic structure
The proposed path
to populate the 3S1
state is extremely
simple
α3
10830 Å
ζHe
A
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Steady State Analysis
Ionization
ni
neα3
nm
ζHe
neα1
A
ng
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Is it Observable?
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Using reported values for diffuse molecular cloud
sight lines, we compute the expected He* column
density and equivalent width of the 10830 Å line
Nm = 9.5×108 cm-2 Wλ = 0.47 mÅ
S/N ~ 1000
Anders & Grevesse 1989, Geo. Cosmo. Acta, 53, 197
Indriolo et al. 2007, ApJ, 671, 1736
Bohlin et al. 1978, ApJ, 224, 132
R. Porter 2009, private communication
Hata & Grant 1981, J. Phys. B, 14, 2111
Woodworth & Moos 1975, Phys. Rev. A., 12, 2455
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Observations
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Selected a favorable diffuse
molecular sight line (high ionization
rate, bright J magnitude, large NH,
“well behaved” stellar He lines)
HD 183143
63 minutes on target
with Phoenix on
Gemini South
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Data Processing
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Spectrum
S/N~700
McCall et al. 2002, ApJ, 567, 391
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More Ways to Remove He+
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He+ + H → He + H+
 13.6 eV
He+ + H2 → He + H2+
 15.4 eV
He+ + H2 → He + H + H+
 18.1 eV
He+ + CO → He + O + C+
 22.5 eV
He+ + e- → (3S1)He
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He+
CO→O+C+
He (3S1)
H2→H+H+
H2 → H2+
H → H+
4.8 eV
He+ + e- → (1S0)He

24.6 eV
He (1S0)
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Branching Fraction Redux
0.40
0.08
10-6
Rate coefficients from UMIST
Database for Astrochemistry
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Direct Excitation to Triplet Manifold
He + e- → He* + e-
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A More Complete Analysis
Ionization
α3
3S
1
ζ
α1
A
H
H2
CO
δ
1S
0
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Ask Again: Is it Observable?
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Nm = 9.5×108 cm-2
Wλ = 0.47 mÅ
S/N ~ 1000
Nm = 6.3×108 cm-2
Wλ = 0.31 mÅ
S/N ~ 1500
The predicted S/N necessary for a 3σ
detection is not much worse than our
original prediction
With b=0.08 and δ=ζ/3, excitation by
secondary electrons is the dominant path to
producing metastable helium in diffuse
molecular clouds
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Conclusions
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Obtaining the S/N necessary to
detect interstellar metastable He
will be a challenge for current
instrument/telescope combinations
The pathway to the metastable
state is much more complicated
than previously thought
Excitation to the triplet manifold by
secondary electrons greatly
complicates the computation of ζHe
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Acknowledgments
Ryan Porter
University of Kentucky
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