The Water D/H Ratio in Molecular Outflows in Orion BN/KL Shiya Wang Astronomy Department, University of Michigan Edwin A. Bergin (U. of Michigan) René

The Water D/H Ratio in Molecular Outflows in Orion

BN/KL

Shiya WangAstronomy Department, University of Michigan

Edwin A. Bergin (U. of Michigan) René Plume (U. of Calgary)

Midwest Astrochemistry Meeting, Nov. 7-8, 2008

Interstellar and Cometary Ices

H2OCOCO2

CH3OHCH4

OCS

1002 - 204 - 243 - 1020.4

1005 - 73 - 200.3 - 50.2 - 1.20.2

ISM(ices) Comets(1AU)

Langer et al 1999, PPIV

Deuterium Fractionation of Water

OH

H2O

H3O+

O

O

H2

e-

e-H3

+

HD

H2

H2D+

cosmic ray

H2DO+

O

OD

HDO

e-

O

e-

e-

HD

High TLow T

HDO/H2O > HD/H2 ~ 3 x 10-5

Water Origin in Molecular Outflows

Orion BN/KL High-T shock chemistry

Supersonic flows compress and heat the surrounding material and change the chemistry

shocks with velocities between 15 and 30 km/s will produce large amounts of water vapor (Kaufmann & Neufeld 1996) O + H2 OH + H OH + H2 H2O + H

Grain surface evaporation

Grain-grain collisions and sputtering in shocks can vaporize mantles

CSO HDO 464 GHz Emission in Orion BN/KL

T A* (K)

VLSR (km/s)

Offset in Arc-Seconds

Plateau (18km/s flow): X(H2O) ~ 4 x 10-6

(Persson et al. 2007) HDO/H2O ~ 0.005

3 Stage Chemical Model3 Stage Chemical Model

Pre-Shock

Tgas = Tdust

= 10 - 30 K106 years

Shock

vs = 10 - 40 km/sTgas= 500 - 3000 KTdust = 10, 500 K100 years

Post-Shock

Tgas = Tdust = 30 K

106 years

Stage 1 Stage 2 Stage 3Stage 3

Bergin, Neufeld, & Melnick 1998Kaufmann & Neufeld 1996

• constant density• visual extinction

shock <----- -----> post-shock

Outflow age ~ 3000 years with 20 km/s, 0.04pc

shock <----- -----> post-shock

Outflow age ~ 3000 years with 20 km/s, 0.04pc

• Enhanced water abundance will persist for ~105 yrs, whereupon water freezes onto grain surfaces

• Two cases: (1) all the shocked water is produced by shock

chemistry via gas-phase reactions of O and OH with H2

(2) all atomic oxygen is locked on grains in the form of H2O, which then evaporates from the grain surfaces when shocked.

• Observed water abundance in Orion BN/KL low-velocity outflows can be produced in shocks (with shock velocity ~20 km/s and preshock gas temperature ~ 20 K), without grain evaporation contributions

Summary