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-University of Chicago, Yerkes Observatory
3-National Optical Astronomy Observatories (NOAO)
June 24, 2009
International Symposium on Molecular Spectroscopy

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 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
ζHe α3
10830 Å A

4. Steady State Analysis ζHe neα1 neα3 A A neα3 ζHe neα1 ζHe neα1 A neα3
Ionization ni nm ng
ζHe
neα1
neα3 A A
neα3
ζHe
neα1
ζHe
neα1 A
neα3 A
neα3
ζHe
neα1

5. Is it Observable? Wλ = 0.47 mÅ Nm = 9.5×108 cm-2 S/N ~ 1000
Using reported values for diffuse molecular cloud sight lines, we compute the expected He* column density and equivalent width of the Å line
Wλ = 0.47 mÅ
Nm = 9.5×108 cm-2
S/N ~ 1000
Indriolo et al. 2007, ApJ, 671, 1736
R. Porter 2009, private communication
Woodworth & Moos 1975, Phys. Rev. A., 12, 2455
Anders & Grevesse 1989, Geo. Cosmo. Acta, 53, 197
Bohlin et al. 1978, ApJ, 224, 132
Hata & Grant 1981, J. Phys. B, 14, 2111

6. Observations
Selected a favorable diffuse molecular sight line (high ionization rate, bright J magnitude, large NH, “well behaved” stellar He lines) HD
63 minutes on target with Phoenix on Gemini South

7. Data Processing

8. Spectrum
S/N~700
McCall et al. 2002, ApJ, 567, 391

9. More Ways to Remove He+ He+ + H → He + H+ 13.6 eV He+ + H2 → He + H2+
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→O+C+
He (3S1)
H2→H+H+
H2 → H2+
H → H+
He (1S0)

10. Branching Fraction Redux
0.40
0.08
10-6
Rate coefficients from UMIST Database for Astrochemistry

11. Direct Excitation to Triplet Manifold
He + e- → He* + e-

12. A More Complete Analysis
Ionization α3
3S1 ζ α1 A δ H H2 CO
1S0

13. Ask Again: Is it Observable?
Wλ = 0.47 mÅ
Nm = 9.5×108 cm-2
S/N ~ 1000
S/N ~ 1500
Nm = 6.3×108 cm-2
Wλ = 0.31 mÅ
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

14. 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 ζHe

15. University of Kentucky
Acknowledgments
Ryan Porter
University of Kentucky