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

Paul Rimmer and Eric HerbstOhio State University

For the Lorentz Center Workshop

Cosmic Ray Ionization Rate

• The cosmic ray ionization rate is a per second ionization rate from protonsζH: H + CRP -> H+ + e- + CRP

ζH2: H2 + CRP -> H2+ + e- + CRP

ζ H2 ≈ 1.53 ζH

The Different Cosmic Ray Spectra

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.

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.

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.

• 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.

The Effect of ζ on Interstellar Chemistry: OH+, H2O+, H3O+

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

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.

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.

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.

Results for the Horsehead Nebula: Hydrocarbons

Results for the Horsehead Nebula: HCO+ and HC3N

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.

Thanks to:

• Eric Herbst• Evelyne Roueff• Oscar Morata• Nick Indriolo• Ben McCall• Ewine van Dishoeck• John Black• Tom Cravens