(1a) Radiative Shocks and Hydrogen Molecules in Pregalactic
Gas: The Effects of Postshock Radiation
Kang and Shapiro (1992)
generalized and substantially improved their previous calculations of
radiative shocks in pregalactic gas. They solved the hydrodynamical
conservation equations, along with the rate equations for nonequilibrium
ionization, recombination, and molecule formation and the equation of
radiative transfer, for steady state shocks in a gas of primordial composition.
Such shocks waves are relevant to a wide range of theories of galaxy
and primordial star formation. These calculations self-consistently included
the effects of the diffuse post-shock emission as well as a possible external
radiation flux on the postshock flow and the preshock ionization levels.
They confirmed the previous result by
Shapiro and Kang (1987),
which had not taken account of the diffuse flux,
that the shocked gas cools faster than it can
recombine and, as a result, is able to form an H_2 concentration as high
as 10^-3 via the formation of H- and H_2+ intermediaries
due to the enhanced nonequilibrium ionization fraction at 10^4K.
With such an H_2 concentration, the gas cools by rotational-vibrational
line excitation of H_2 molecules to well below the canonical final
temperature of 10^4K for a molecule-free gas without metals. This
cooling below 10^4K significantly lowers the characteristic
gravitational fragment mass estimated for shocks relative to the value if the
gas cooling stops at 10^4K.
If H_2 cools the postshock gas from 10^4K to 10^2K, for
example, this lowers the fragment mass by the square of the temperature ratio,
a factor of 10^4!
These calculations showed that, as the level of the external
radiation flux is increased, the formation of and cooling by H_2
molecules can be inhibited and delayed. Finally, the detailed radiation
spectra and ionizing photon number fluxes emergent form these
shocks were presented.