Chemistry of Protoplanetary

Disks with Grain Settling and Lyman

α Radiation

Jeffrey Fogel, Tom Bethell and Edwin BerginUniversity of Michigan

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Disk Structure

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Disk Structure

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Disk Structure

Stellar Radiation Field

•UV Excess due to shock

•most models used a scaled up ISRF

•Full UV field from D’Alessio et al

•Analytical scattering calculation from Bergin et al. 2003

Gullbring et al 2000

Dust•Major heating agent in the disk due

to absorption

•SED depends on the dust grain settling

- ε = dust-to-gas ratio in the upper layers as compared with the ISM

•Observations by D’Alessio et al. 2006 indicate that the median SED in Taurus fits ε ~ 0.01

Lyman α• Dominates UV radiation field - in TW Hya

carries 75% of FUV flux (Herczeg et al 2004)

• Important for chemistry. e.g. HCN and H2O will be dissociated by Ly α photons -- CN is not (Bergin et al 2003).

Bergin et al. 2003

Red = BP TauBlue = TW Hya

dashed = scaled ISRF

Chemical Network• 639 Species

• 5902 Reactions

• Herbst’s gas-phase network plus:

- Photodissociation:

- X-ray ionization (Glassgold et al. 1997)

- UV Photolysis induced by X-rays (Gredel et al. 1989, Aikawa & Herbst 2001)

- Grain Reactions (freeze-out, evaporation, cosmic-ray desorption, photodesorption)

- H2, CO self-shielding

CO, ε = 1

CO, ε = 0.01

Dust Settling

Ly α approximation, ε = 0.01

Conclusions• Important to include true stellar UV field

and calculate photodissociation rates from it

•Dust settling and Lyman α radiation play significant roles in the chemistry of protoplanetary disks

•Next step is to calculate column densities and show what can be observed now and in the future with ALMA

• Directly calculate from UV field and species cross sections:

• Most codes use

•an approximation:

Photodissociation