Formation and Evolution of Molecules Behind Shocks

1. 0 Formation and Evolution of Molecules Behind Shocks GEORGE HASSEL Dept. of Physics, The Ohio State University ghassel@mps.ohio-state.edu Eric Herbst (Ohio State), Ted Bergin (U. Michigan) SATURDAY, NOVEMBER 8, 2008 MWAM’08

2. 0 OVERVIEW • Can shocks form dense clouds from diffuse ISM? (Bergin et al. 2004) • Replicate molecular observations? • Ice composition with AV (Whittet et al. 2007, Nummelin et al. 2001) • Complex gas molecules: L134N, TMC-1 (Ohishi et al 1992, Wakelam, Herbst & Selsis 2007)

3. 0 OSU GAS-GRAIN NETWORK Modifications: • Read shock hydro results as input • Compute dust temperature • New / modified rates: • Photodesorption • Eley-Rideal • CO + O surface barrier Grain Model: r = 0.1 mm Spherical, silicate grains Rate equations – no stochastic methods

4. 0 Shock Chemistry Model • Hydrodynamical 1 point model (Bergin et al. 2004) • Diffuse ISM -> Shock -> Dense cloud? • Formation of H2(g) & CO(g) - ices & complex species? Photos from NASA-APOD Archive

5. 0 Physical Conditions • nH, Tg – dense cloud ~105-6 yr • AV, Td – more gradual change

6. 0 Photodesorption Rates CO: Direct photodesorption Temperature dependent CO2: Photodissociation / desorption Temperature, coverage dependent N2: Direct photodesorption only with CO **indicates non-thermal** H2O: Photodissociation / desorption Oberg et al. 2007, in prep.

7. 0 Ice – No Photodesorption Ices: Whittet et al. 2007, CO(g): Ohishi et al 1992 (TMC-1, L134N)

8. 0 Ice – Photodesorption

9. 0 Ice Composition • Formation at AV~ 3-4 OH(s) + H(s) -> H2O(s) CO(g) -> CO(s)

10. 0 Ice Composition CO(s) + OH(s) -> CO2(s) CO2(s) + hn -> CO(s) + O(s) CO(s) + O(s) -> CO2(s) EA = 290 K (Roser et al. 2001) = 130 K (Ruffle & Herbst 2001)

11. 0 CO2 Ice Td0=15 K EA= 130 K Td0=20 K EA= 130 K EA=290 K EA=290 K

12. 0 Complex Gas Molecules HC3N 17-29 species H2S NH3 6 species 14 species

13. 0 Conclusions • Ice composition: • H2O : CO : CO2 ~ observed abundances • AV Threshold • CH4 minimized • Governed by photodesorption • Gas phase molecules – 3 distinct evolution stages

14. 0 Acknowledgements • Rob Garrod • Herma Cuppen & Karin Oberg • MWAM ’08 Organizing Committee

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