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

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

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

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

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 10830 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

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

Data Processing

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

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)

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

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

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

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

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

University of Kentucky Acknowledgments Ryan Porter University of Kentucky