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**Session 54 - Large Scale Structure.**

*Display session, Tuesday, January 16*

*North Banquet Hall, Convention Center*

## [54.08] Hydrogen Molecules and the Radiative Cooling and Fragmentation of Cosmological Sheets

*P. Anninos, M. L. Norman (LCA, NCSA)*
We extend previous studies of
nonlinear hydrodynamical effects on the fragmentation of cosmological
sheets in a dark matter dominated universe
by allowing for the formation of hydrogen molecules. This is
accomplished by solving a reaction flow system
in nonequilibrium for the baryonic
fluid that includes 27 chemical reactions and 9 separate species:
H, H^+, He, He^+, He^++, H^-, H_2^+, H_2, and e^-.
Several one-dimensional calculations are performed for different
initial data parameterized by the perturbation wavelength
\lambda_1 along the collapsing direction.
Initial wavelengths in the range 1 to 10 Mpc
corresponding to average shock velocities of 9 to 110 km/s are considered.
The higher velocity shocks produce higher concentrations of molecular
hydrogen ranging from
n_H2=5.8\times 10^-2 cm^-3
with a mass fraction n_H2/n=2.8\times 10^-3 for the 10 Mpc
case to 2.5\times 10^-7 cm^-3 and
1.5\times 10^-5 for \lambda_1=1 Mpc.
The gas for those shocks (namely \lambda_1 > 1 Mpc)
that produces large concentrations of H_2 then cools further through
the vibrational/rotational excitation of the molecules.
Because the cooling is isobaric, the accompanying increase in density together
with the drop in temperature combine to collapse the gas
to smaller volumes and to reduce the Jeans mass by factors
up to 10^3 for \lambda_1=10 Mpc (dropping from
9\times 10^6 M_ødot when H_2 is neglected to 9\times10^3 M_ødot).
Hence, the faster moving shocks are likely to
fragment into smaller units that may be associated with
massive stars. The fragmentation process is
investigated with two-dimensional simulations for the case
\lambda_1=4 Mpc.
We confirm predictions from the 1D studies
regarding the size and mass estimates of fragments.

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