ERIC HERBST

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Low-Temperature Gas-Phase Surface
Reactions in Interstellar
Clouds

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

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Dense Interstellar Cloud Cores
10 K
10(4) cm-3
Molecules seen in IR absorption and radio emission
H2 dominant
sites of star formation
Cosmic rays create weak plasma
Fractional ionization lt 10(-7)
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Cosmic Elemental Abundances

• H 1
• He 6.3(-2)
• O 7.4(-4) 1.8(-4)
• C 4.0(-4) 7.3(-5)
• N 9.3(-5) 2.1(-5)
• S 2.6(-5) 8.0(-8)
• Si 3.5(-5) 8.0(-9)
• Fe 3.2(-5) 3.0(-9)

• Dust/gas 1 by mass
• Gas-phase abundances of heavy elements in clouds
  reduced.

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Some Fractional Abundances in TMC-1

• CO 1(-4)
• HCN 2(-8)
• C4H 9(-8)
• HCO 8(-9)
• c-C3H2 1(-8)
• HC9N 5(-10)

• OH 2(-7)
• NH3 2(-8)
• HC3N 2(-8)
• N2H 4(-10)
• HNC 2(-8)
• O2 lt 8(-8)

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Water, CO, CO2
small grains and PAHs
Water ice 10(-4) of Gas density
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H2 e
Cosmic ray
O
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Efficient Low T Gas-PhaseReactions

1. Ion-molecule reactions
2. Radiative association reactions
3. Dissociative recombination reactions
4. Radical-radical reactions
5. Radical-stable reactions

Ea 0
Exothermic
In areas of star formation, reactions with
barriers occur.
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Ion-Molecule Reactions

• Experimental evidence down to a few K
• Rate coefficients explained by classical
  capture models in most but not all instances.
• ion-non polar (Langevin case)

cm3 s-1
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Ion-mol. Rx. (cont)

• Ion-polar

more complex state-specific models
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Remaining Questions

• 1) Why are some reactions slow?

2) Is there a quantum limit?
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Radiative Association
, size, bond engy
Few ion trap measurements by Gerlich, Dunn down
to 10 K
What is the 0 K limit?
What about competitive channels?
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Dissociative Recombination Reactions
Studied in storage rings down to zero relative
energy products measured for approx.10 systems
n0.5, 1.5
Some systems studied H3, HN2, HCNH, H3O,
NH4, CH5 ,CnHm
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QUESTION

• How large must ions be before the dominant
  process becomes radiative recombination?
  statistical trapping
• Answer via statistical theories (RRKM) 20-30
  atoms?????

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Radical-radical Reactions
Detailed capture models by Clary, Troe
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RADICAL-NEUTRAL RX (CONT)
CN C2H2 ? HCCCN H
YES
C C2H2 ? C3H H
YES
CCH HCN ? HCCCN H
NO
Barrier cannot be guessed!!
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Attachment
If enough large molecules with large electron
affinities present, electrons may not exist! Note
no competitive fragmentation channels.
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FORMATION OF GASEOUS WATER
H2 COSMIC RAYS ? H2 e
Elemental abundances C,O,N 10(-4) CltO
Elemental abundances C,O,N 10(-4) CltO
H2 H2 ? H3 H H3 O ? OH
H2 OHn H2 ? OHn1 H H3O e ?
H2O H OH 2H, etc
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FORMATION OF HYDROCARBONS
H3 C ? CH H2 CHn H2 ? CHn1
H n1,2 CH3 H2 ? CH5 hn CH5
e ? CH4 H (5) ? CH3
2H (70) CH5 CO ? CH4 HCO
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Solved kinetically thermodynamics useless!
t0 atoms except for H2
Latest network osu.2003 contains over 300
rapid neutral-neutral reactions. Rate
coefficients estimated by Ian Smith and others
for many of these. Verification needed!!
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Chemistry imperfect!!
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Nature of Solution for a homogeneous,
time-independent cloud
early time if O- rich
fi
0.1
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Small species (CO) Large species (HC9N)
Time (Myr)
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Nature of Solution for a homogeneous,
time-independent cloud
early time if O- rich
fi
Found in pre-stellar cores
accretion
Small species (CO) Large species (HC9N)
0.1
10
Time (Myr)
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Low Temperature Surface Chemistry on Amorphous
Surfaces

• 1) Mechanisms (diffusive Langmuir-Hinshelwood,
  Eley-Rideal, hot atom, impurity site)
• 2) Dependence on size, mantle, fluffy nature,
  energy parameters
• 3) Rate equations vs. stochastic treatments
• 4) non-thermal desorption (cosmic rays)

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Edes
Ediff
physisorption
(diffusion)
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Desorption Diffusion
for heavies
Desorption via evaporation and cosmic-ray heating.
kdiff khop/N N is the number of binding sites
For H, tunneling can occur as well.
H diffuses the fastest and dominates the
chemistry.
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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)
??????????  
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Experiments on cold surfaces

• Vidali et al. Formation of H2 on silicates,
  carbon, and amorphous ice LH mechanism
  characterized and energies obtained formation of
  CO2 energy partitioning of hydrogen product
  (also UCL group)
• Ediff(H, olivine) 287 K Ediff(H, carbon) 511
  K
• But whole analysis of data has been questioned by
  others, who feel that both tunneling and some
  chemisorption sites are involved!!!!!
• Hiraoka et al. Formation of ices (CH4, H2O,NH3,
  H2CO)
• Watanabe et al. Formation of methanol
• Danish group formation of H2

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

• Unfortunately, with more than one reactive
  surface species, one must compute joint
  probabilities

so that
the computations require significant computing
power. It is necessary to impose cutoffs on the
ni and the total number of surface species
considered.
More simple fix modified rate method
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New Gas-Grain Stochastic-Deterministic Model

• Stantcheva Herbst (2004)
• Gas-phase chemistry solved by deterministic rate
  equations, while surface chemistry solved by
  solution of master equation. Some results quite
  different from total deterministic approach.

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RESULTS surfaces

• From observations of grain mantles, the dominant
  species in the ice are water, CO, CO2, and
  occasionally methanol.
• The models at 10 K and a gas density of 10(4)
  cm-3 are able to reproduce the high abundance of
  water, seem to convert CO into methanol too
  efficiently, and tend to underestimate the amount
  of CO2. Results sensitive to density.
• The modified rate method reproduces the master
  equation approach at 10 K, but the normal rate
  method can be in error.

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Results from Stantcheva Herbst (2004)
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CO
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Agreement in TMC-1
Gas-phase species
Roberts Herbst 2002
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Some Conclusions

• 1) Low-temperature chemistry in interstellar
  clouds (both gas-phase and surface) partially
  understood only.
• 2) Chemistry gives us many insights into the
  current state and history of sources
• 3) More work on cold chemistry is clearly
  needed to make our mirror into the cosmos more
  transparent.