Dissociative Recombination in Space by i301aw

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									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
      dNH/dt = kaccnH - kevapNH - KH-HNHNH
      Advantages gas-phase and grain
                  chemistry are coupled in
                  time-dependent
                  calculations

      Problems averages obtained only
                Accurate if large numbers
                of reactive species on
                grains; reality is that
               small numbers may exist
                 especially for H
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)
    Advantages modifies practical rate
        method to approach Monte Carlo
    Problems     semi-empirical, no way to
       prove correct in all situations


New models: Caselli, Ruffle, Roberts, Herbst
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
     Call two random numbers – which lead to
 increment of time and which process occurs
 during that time. Repeat procedure.


      Advantages       takes discrete nature of
      grains into account; formally exact
       Problems so far limited to simple
      systems with fixed gas-phase
     abundances
PROBABILISTIC MASTER
EQUATION
   dNH/dt = kaccnH - kevapNH - KH-HNHNH



    replaced when NH << 1 by a series of
    coupled equations for Pn(H):

    <NH> =    n Pn(H)

    dP0(H)/dt = ……….
                   PROTOSTELLAR CORES


Current Picture: surface chemistry during late previous
stage.



              H     O     CO        H2CO   + isotopomers
                                      CO2
        D                              CH3OH
                                          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
     dense (giant) molecular clouds



                                                    organic molecules

   core                                                   H
                                                              2
       4 -3
 n = 10 cm
T = 10 K                                                      PDR’s


embedded
stars                                                    hot
                                                         ionized
                                                         gas
                                                         HII region
                              protoplanetary disk


           studied in millimeter-wave and IR

								
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