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 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
