the abundances of gaseous h 2 o and o 2 in dense cloud cores eric herbst & helen roberts the...
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The abundances of gaseous H2O and O2 in dense cloud cores
Eric Herbst & Helen Roberts
The Ohio State University
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CURRENT GAS-PHASE MODEL NETWORKS
4,000 reactions; 10-20% "studied"; 400 species through 13 atoms in size
elements: H, He, N, O, C, S, Si, Fe, Na, Mg, P, Cl elemental abundances: “low metal”
photodestruction: external, internal (via cosmic rays)
Successes for quiescent cores:
(1)Reproduces 80% of abundances including ions, radicals, isomers
(2)Predicts strong deuterium fractionation
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106 sites
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TYPES OF SURFACE REACTIONS
REACTANTS: MAINLY MOBILE ATOMS AND RADICALS
A + B AB associationH + 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 ??????????
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MODELLING DIFFUSIVE SURFACE CHEMISTRY
Rate Equations
d N H / d t = k a c c n H - k e v a p N H - K H - H N H N H 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
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Rates of Diffusion
• Standard astrochemical (e.g. Hasegawa et al. 1991) for silicates
• Versions for amorphous carbon and for water ice• Slow H (P1): H slowed down to olivine (carbon)
value of Pirronello et al. (1997)• Slow (P2): all other species slowed proportionally• All networks contain evaporation and cosmic-ray
desorption; some contain photo processes
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MORE ACCURATE METHODS FOR SURFACE
RATES• Modified rate approach – available but
semi-empirical; used here and by a few other groups.
• Stochastic methods – soon to be available
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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). Current status: not yet programmed for large models
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Some predicted gas-phase abundances (10 K; 104 cm-3)
P2 Energies
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Some predicted surface
abundances (10 K; 104 cm-3)
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TMC-1
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SWAS UPPER LIMITS WRT H2
• H2O
• O2
• 7.0(-08)• 3.2(-06) (Odin claims
7.7(-08) towards ammonia)
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Overall and particular agreement: pure gas-phase (low metals)
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Same but with C/O = 1
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Percentage agreement for gas-grain models
2nd peak despite depletion
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Agreement for specific species
Is late-time CO depletion serious???
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L134N
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SWAS UPPER LIMITS WRT H2
• H2O
• O2
• 3.0(-07)• 3.4(-06) (Odin claims
1.7(-07) towards ammonia)
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Percentage agreement for gas-grain models
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Ageement for specific species
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Source: Oph
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SWAS VALUES WRT H2
• H2O
• O2
• 3.0(-09)• <3(-07) (Odin claims
<9.3(-08) towards a)
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Gas-phase abundances for P2, 20 K, 105 cm-3
P1 similar at 15 K
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Specific agreement
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Same with amorphous carbon grains
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CONCLUSIONS
• Current generation of our gas-grain models gives best agreement for water and oxygen at long times for 10 K sources
• Chemistry and physics of desorption critical and poorly known
• Depletion at long times from gas in agreement with results on pre-stellar cores including deuterium fractionation