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

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)

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 H2(g) & CO(g) - ices & complex species?

Photos from NASA-APOD Archive

Physical Conditions

• nH, Tg – dense cloud ~105-6 yr

• AV, Td – more gradual change

Photodesorption Rates

CO:

Direct photodesorption

Temperature dependent

CO2:

Photodissociation / desorption

Temperature, coverage dependentN2:

Direct photodesorption only with CO

**indicates non-thermal**

H2O:

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 AV ~ 3-4

OH(s) + H(s) -> H2O(s)

CO(g) -> CO(s)

Ice Composition

CO(s) + OH(s) -> CO2(s)

CO2(s) + h -> CO(s) + O(s)

CO(s) + O(s) -> CO2(s)

EA = 290 K (Roser et al. 2001)

= 130 K (Ruffle & Herbst 2001)

CO2 Ice

Td0=20 K

EA= 130 K

EA=290 K

Td0=15 K

EA= 130 K

EA=290 K

Complex Gas Molecules

H2S NH3

HC3N

14 species

6 species

17-29 species

Conclusions• Ice composition:

H2O : CO : CO2 ~ observed abundances

AV Threshold

CH4 minimized

• Governed by photodesorption

• Gas phase molecules – 3 distinct evolution stages

Acknowledgements

• Rob Garrod

• Herma Cuppen & Karin Oberg

• MWAM ’08 Organizing Committee