1. Surface Chemistry: New Methods, New Results
ERIC HERBST
DEPARTMENTS OF PHYSICS AND ASTRONOMY
THE OHIO STATE UNIVERSITY

4. 106 sites

5. TYPES OF SURFACE REACTIONS
REACTANTS: MAINLY MOBILE
ATOMS AND RADICALS
A B  AB association
H H  H2
H X  XH (X = O, C, N, CO,
etc.)
WHICH CONVERTS
O  OH  H2O
C  CH  CH2  CH3  CH4
N  NH  NH2  NH3
CO  HCO  H2CO  H3CO  CH3OH
X + Y  XY ??????????

6. MODELLING DIFFUSIVE SURFACE CHEMISTRY
Rate Equations

7. Rates of Diffusion Standard astrochemical (e.g. Hasegawa et al. 1991)
Slow H (P1): H slowed down to olivine (carbon) value of Pirronello et al. (1997)
Slow (P2): all other species slowed proportionally
With P1/2, H2 formation efficient only in narrow T range dependent on surface

8. MODIFIED RATE APPROACH
Slows diffusive rate coefficients so that
rate of reaction does not exceed rate of accretion
See Caselli et al. (1998)
New models: Caselli, Ruffle, Roberts, Herbst

9. (COLD CLOUDS; silicate grains)
GRAIN MANTLE GROWTH
(COLD CLOUDS; silicate grains)

10. % Agreement in TMC-1
Gas-phase species
Roberts & Herbst 2002

11. TMC-1: COMPARISON WITH SWAS
H2O CO O2
Roberts and Herbst 2002

12. STOCHASTIC METHODS
Based on solution of master equation, which is a kinetic-type equation in which one calculates not concentrations but probabilities that certain numbers of species are present. Can solve directly (Hartquist, Biham) or via Monte Carlo realization (Charnley).

13. MONTE CARLO APPROACH

14. PROBABILISTIC MASTER EQUATION

15. PROTOSTELLAR CORES
Current Picture: surface chemistry during late previous stage. CO O H
H2CO
+ isotopomers
CO2
CH3OH D
H2O ice
Shock, heat disrupt mantle:gas-phase chemistry ensues!

16. RE: rate equation
MR: modified rate
MC: Monte Carlo
ME: master eq.
“High density”
O, CO > H

18. Dear David,
The very best of luck in your new state of active “retirement.”
With best wishes, Eric