The abundances of gaseous H2O and O2 in dense cloud cores

Eric Herbst & Helen Roberts

The Ohio State University

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

106 sites

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 ?????????? 

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

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

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

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

Some predicted gas-phase abundances (10 K; 104 cm-3)

P2 Energies

Some predicted surface

abundances (10 K; 104 cm-3)

TMC-1

SWAS UPPER LIMITS WRT H2

• H2O

• O2

• 7.0(-08)• 3.2(-06) (Odin claims

7.7(-08) towards ammonia)

Overall and particular agreement: pure gas-phase (low metals)

Same but with C/O = 1

Percentage agreement for gas-grain models

2nd peak despite depletion

Agreement for specific species

Is late-time CO depletion serious???

L134N

SWAS UPPER LIMITS WRT H2

• H2O

• O2

• 3.0(-07)• 3.4(-06) (Odin claims

1.7(-07) towards ammonia)

Percentage agreement for gas-grain models

Ageement for specific species

Source: Oph

SWAS VALUES WRT H2

• H2O

• O2

• 3.0(-09)• <3(-07) (Odin claims

<9.3(-08) towards a)

Gas-phase abundances for P2, 20 K, 105 cm-3

P1 similar at 15 K

Specific agreement

Same with amorphous carbon grains

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