Chemistry of Protoplanetary Disks with Grain Settling and Lyman α Radiation

1. Chemistry of Protoplanetary Disks with Grain Settling and Lyman α Radiation • Jeffrey Fogel, Tom Bethell and Edwin Bergin • University of Michigan

2. Disk Structure

3. Disk Structure

4. Disk Structure

5. 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

6. 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

7. 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 Tau Blue = TW Hya dashed = scaled ISRF

8. 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

9. CO, ε = 1

10. CO, ε = 0.01

11. Dust Settling

12. Ly α approximation, ε = 0.01

13. 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

14. Photodissociation • Directly calculate from UV field and species cross sections: • Most codes use • an approximation: