ERIC HERBST

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Gas and Dust (Interstellar)
Astrochemistry

• ERIC HERBST
• DEPARTMENTS OF PHYSICS, CHEMISTRY AND
  ASTRONOMY
• THE OHIO STATE UNIVERSITY

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Efficient Low T Gas-PhaseReactions

• Ion-molecule reactions
• Radiative association reactions
• Dissociative recombination reactions
• Radical-radical reactions
• Radical-stable reactions

Ea 0
Exothermic
In areas of star formation, reactions with
barriers occur.
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Important New Studies

• Dissociative recombination product branching
  fractions such as
• N2H e ? NH N (Geppert et al.)
  determination of H3 e rate!! (McCall et al.)
• Radical-stable reactions Reactions of C, CN,
  CCH with hydrocarbons at low temperatures (Rowe,
  Smith, Sims)

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Chemistry/history imperfect heterogeneous
New uncertainty analyses
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Pre-stellar Cores

• Salient features cold, collapsing. With some
  heavy species depleted towards center extreme
  deuterium fractionation.
• Fit most simply with gas-phase accretion models
  including shell structure (Roberts et al. 2004)
  hydrodynamic models coming on line.

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PDR Models

• Multi-slab gas-phase models with careful
  radiative transfer useful for an assortment of
  diffuse dense heterogeneous objects, especially
  close to stars (z Persei, Orion bar, Horsehead
  nebula)
• Diffuse sources H3 problem (Le Petit et al.)
• Dense sources Cannot quite reproduce abundance
  of large molecules detected recently (Teyssier et
  al.)
• Protoplanetary disks

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Edes
Ediff
physisorption
(diffusion)
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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 (CO O ? CO2)
??????????  
D atoms react in same manner as H atoms
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MODELLING DIFFUSIVE SURFACE CHEMISTRY
Rate Equations
The rate coefficient is obtained by
Method accurate if Ngt1
Biham et al. 2001
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Master Equation

• FfS (atoms per grain per second)
• Aa/S (fraction of grain surface per second)
• n0,1,2...

First combined gas-grain model with master
equation published this year.
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More Detail on H2 Formation

• Is the master equation approach exactly right? NO

Subtle problem is that back diffusion to sites
already occupied is ignored. Hence, efficiency of
H2 formation may be too high.
New Monte Carlo method can treat amorphous and
irregular grains!
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Rough Olivine
also shows enhanced efficiency at higher T
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Protostellar Cores

• Much more complex than pre-stellar case star
  formation, winds, and shocks well advanced.
• Heat from star formation evaporates grain mantles
  close to star.
• Strong deuterium fractionation for methanol
  thought to arise from grain processes in previous
  cold era involving D atoms.

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HOT MOLECULAR CORES

• Hot cores are regions of warm, quiescent gas near
  high-mass star-forming regions. Temperatures are
  100-300 K and densities are typically 107 cm-3.
  They are associated with a variety of saturated
  gas-phase organic molecules methanol, ethanol,
  acetaldehyde, methyl formate, acetic acid,
  glycolaldehyde, ethylene oxide, dimethyl ether,
  and possibly diethyl ether, glycine, and ethyl
  methyl ether.

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Hot Molecular Cores, cont.

• The standard chemical model produces complex
  species in the gas following desorption of
  methanol as grains begin to heat up as a result
  of star formation.
• Much of the gas-phase chemistry has not been
  studied in the laboratory.

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Ab Initio Calculations
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TWO EXPERIMENTS

• 1) SIFT AT HANSCOM AF BASE
• dominant product cluster ion (high density)
• 2) ICR AT WATERLOO, CANADA
• dominant product CH3OCH2 (low density)
• CONCLUSION no major channel to produce
  protonated methyl formate. New approaches needed
  for hot core chemistry?