The Effect of Various Cosmic Ray Flux-Spectra on PDR Chemistry

1. The Effect of Various Cosmic Ray Flux-Spectra on PDR Chemistry Paul Rimmer and Eric Herbst Ohio State University For the Lorentz Center Workshop

2. Cosmic Ray Ionization Rate • The cosmic ray ionization rate is a per second ionization rate from protons ζH: H + CRP -> H+ + e- + CRP ζH2: H2 + CRP -> H2+ + e- + CRP ζ H2 ≈ 1.53 ζH

3. The Different Cosmic Ray Spectra

4. Cosmic Rays as a Dial • Most ions, and many neutral species increase with ζ. • Some species decrease with ζ. • Turn up the ionization too much and abundances will decrease.

5. The Effect of ζ on Interstellar Chemistry: H3+ and HCO+ • H3+ is a very direct tracer for ζ. Formed by: H2+ + H2 -> H3+ + H • HCO+ can also be a direct tracer for ζ: H3+ + CO -> HCO+ + H2 We use a time-dependent single-point gas-phase model with the OSU ‘08 Network.

6. The Effect of ζ on Interstellar Chemistry: Hydrocarbons • The formation of C2H, C3H2, and C4H is very complex, involving many formation pathways. • Since multiple formation pathways are dependent on ζ, C2H, C3H2, and C4H abundances scale with ζ.

7. The Effect of ζ on Interstellar Chemistry: OH+, H2O+, H3O+ • The value of ζ has a major affect on the abundances of OH+, H2O+, H3O+. • OH+ and H2O+both increase with ζ, even past 10-14 s-1. • H3O+ decreases with ζ.

8. What’s Special about 10-14 s-1? • Why do most abundances eventually decrease with an increasing ζ? • Because, as the electron fraction rises, electron recombination increases, and this depletes the ions. • Why at 10-14 s-1? • At this point, atomic hydrogen becomes ionized very rapidly, increasing the electron fraction substantially. • What about OH+ and H2O+? • Formation by H+ + O -> O+ + H O+ + H2 -> OH+ + H

11. Different values of ζ in a single object: Cosmic ray transport • Protons Only • Energy loss by ionization • Energy loss by excitation • Energy loss by Alfven waves • ζ as a function of depth.

12. Different values of ζ in a single object: Good Candidates • Horsehead Nebula, Orion KL, any edge-on nebula. Other candidates? • We will examine the Horsehead Nebula more carefully as an ideal candidate for column-dependent ζ. • Not as simple as previous examples: Number density and temperature have a major impact on species abundances, and these change with column as well.

13. The Horsehead Nebula: Parameters and Initial Conditions • ζ, nH and T are all column-dependent. • The initial UV radiation field is taken to be χ = 60 in Draine Units. • Initial abundances are standard Cold Core abundances.

14. Results for the Horsehead Nebula: Hydrocarbons

15. Results for the Horsehead Nebula: HCO+ and HC3N

16. Concluding Remarks • Species trace with ζ up to ~10-14 s-1, and then most drop down, or level off, because of ion destruction with electrons. • A high column-dependent cosmic ray ionization rate helps explain hydrocarbon abundances in the Horsehead Nebula. • But for the Horsehead Nebula, especially right at the edge, cosmic rays are not the whole story. Other mechanisms, like possibly PAH fragmentation, are necessary to explain observations. • Nevertheless, the column-dependence of ζ should be determinable in the Horsehead (and maybe other PDRs) with ALMA-level angular resolution.

17. Thanks to: • Eric Herbst • Evelyne Roueff • Oscar Morata • Nick Indriolo • Ben McCall • Ewine van Dishoeck • John Black • Tom Cravens