Can%20We%20Use%20Metastable%20Helium%20to%20Trace%20the%20Cosmic-Ray%20Ionization%20Rate?

1
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-Uni
versity of Chicago, Yerkes Observatory 3-National
Optical Astronomy Observatories (NOAO)
2
Motivations

• 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

3
Why Use Metastable Helium?

• Helium is abundant and ubiquitous in the ISM
• Relatively simple atomic structure
• The proposed path to populate the 3S1 state is
  extremely simple

4
Steady State Analysis
5
Is it Observable?

• Using reported values for diffuse molecular cloud
  sight lines, we compute the expected He column
  density and equivalent width of the 10830 Å line

S/N 1000
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Observations

• 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/N700
McCall et al. 2002, ApJ, 567, 391
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More Ways to Remove He

• 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
• 4.8 eV
• He e- ? (1S0)He
• 24.6 eV

He
CO?OC
He (3S1)
H2?HH
H2 ? H2
H ? H
He (1S0)
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Branching Fraction Redux
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
a3
3S1
a1
?
A
d
H
H2
CO
1S0
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Ask Again Is it Observable?

• The predicted S/N necessary for a 3s detection is
  not much worse than our original prediction
• With b0.08 and d?/3, excitation by secondary
  electrons is the dominant path to producing
  metastable helium in diffuse molecular clouds

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Conclusions

• 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
  ?He

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Acknowledgments
Ryan Porter University of Kentucky
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More Ways to Remove He

• He H ? He H
• H ? H uses 13.6 eV
• He H2 ? He H2
• H2 ? H2 uses 15.4 eV
• He H2 ? He H H
• H2 ? H H uses 18.1 eV
• He CO ? He O C
• CO ? O C uses 22.5 eV
• He ? (1S0)He frees 24.6 eV
• He ? (3S1)He frees 4.8 eV