Formation and Evolution of Molecules Behind Shocks GEORGE HASSEL Dept. of Physics, The Ohio State University Eric Herbst (Ohio State), Ted Bergin (U. Michigan) SATURDAY, NOVEMBER 8, 2008 MWAM08

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

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

Shock Chemistry Model Hydrodynamical 1 point model (Bergin et al. 2004) Diffuse ISM -> Shock -> Dense cloud? Formation of H 2 (g) & CO(g) - ices & complex species? Photos from NASA-APOD Archive

Physical Conditions n H, T g – dense cloud ~ yr A V, T d – more gradual change

Photodesorption Rates CO: Direct photodesorption Temperature dependent CO 2 : Photodissociation / desorption Temperature, coverage dependent N 2 : Direct photodesorption only with CO **indicates non-thermal** H 2 O: Photodissociation / desorption Oberg et al. 2007, in prep.

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

Ice – Photodesorption

Ice Composition Formation at A V ~ 3-4 OH(s) + H(s) -> H 2 O(s) CO(g) -> CO(s)

Ice Composition CO(s) + OH(s) -> CO 2 (s) CO 2 (s) + h -> CO(s) + O(s) CO(s) + O(s) -> CO 2 (s) E A = 290 K (Roser et al. 2001) = 130 K (Ruffle & Herbst 2001)

CO 2 Ice T d0 =20 K E A = 130 K E A =290 K T d0 =15 K E A = 130 K E A =290 K

Complex Gas Molecules H2SH2SNH 3 HC 3 N 14 species 6 species species

Conclusions Ice composition: H 2 O : CO : CO 2 ~ observed abundances A V Threshold CH 4 minimized Governed by photodesorption Gas phase molecules – 3 distinct evolution stages

Acknowledgements Rob Garrod Herma Cuppen & Karin Oberg MWAM 08 Organizing Committee