Surface Chemistry: New Methods, New Results

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Presentation transcript:

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

106 sites

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 ??????????

MODELLING DIFFUSIVE SURFACE CHEMISTRY Rate Equations

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

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

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

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

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

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).

MONTE CARLO APPROACH

PROBABILISTIC MASTER EQUATION

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!

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

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