H 3 Abundances Modeled with a Variable Cosmic

H 3+ Abundances Modeled with a Variable Cosmic Ray Ionization Rate Paul Rimmer and Eric Herbst Ohio State University

Outline • Differing cosmic ray ionization • A possible reason – a cosmic ray ionization rate varying as the column density • ζ-Per with a variable cosmic ray ionization rate

Differing Cosmic Ray Ionization Rates • Dense cloud observations of various molecules suggest a cosmic ray ionization rate of ζ ~ 10 -17 s-1 • Diffuse cloud observations strongly suggest a ζ ~ 10 -15 – 10 -16 s-1 N Indriolo, TR Geballe, T Oka, BJ Mc. Call - Ap. J, 2007 F Le Petit, E Roueff, E Herbst - A&A, 2004

A Possible Explanation for the Differences • Cosmic rays lose energy and are effectively shielded from regions with higher column density • The primary causes for energy loss are: – Ionizing collisions – Excitation collisions – Elastic collisions – Deflection by Alfvén Waves

Cosmic Ray Spectrum

ζ as a Function of Column Density • We numerically determine ζ(NH) using a one dimensional Monte Carlo simulation. • A simple analytical model fits this simulation within a factor of two for wide values of hydrogen density of 100 – 107 cm-3, and B ~ 0 – 10 -3 Gauss.

Plot of ζ(NH)

H 3+, the best measure of ζ in diffuse clouds • Protonated molecular hydrogen is formed by the reaction chain: • Its destruction is independent of cosmic ray ionization.

PDR Model of Zeta-Per using the Variable ζ

Further Work • The model should be expanded to three dimensions. • Elastic and excitation collisions should also be considered to see if they are significant. • The effect of Alfvén waves should be considered for regions outside the aforementioned ranges. • Other regions where H 3+ is observed should be modeled using ζ(NH).

Thank You • Eric Herbst • Ben Mc. Call and Nicholas Indriolo for sharing their H 3+ data and for their correspondence • Evelyn Roueff for her computational resources • Tom Millar for helpful conversations