Cool fronts originated by H2 formation and supported by non saturated thermal conduction in the pregalactic gas, are analyzed. The pressure (p2), number density(n2), temperature (T2) and flow velocity (v2) behind the front are found as functions of the temperature ahead the cool front T1 and the intake Mach number M1. Compression behind the cool front occur for both, supersonic and subsonic intake flows providing that M1 is larger than a threshold value, the exact value of which depends on T1. But strongly compressed subsonic flows are left for larger values of M1. Quasi-isobaric cool fronts (p2/p1 ≈ 1) occur when the ratio n1/n2 is closed to the maximum value, where the compressional branch just emerges, beyond which the pressure of the flow behind the front increases when n1/n2 decreases, i.e. for denser subsonic flows behind the cool front. Implications of the above results on the formation of cool condensations in the primordial gas are outlined.

Introduction

Previous studies (Field 1965, Yoneyama 1973, Ibáñez & Parravano 1983, Fall & Rees 1985, Corbelli & Ferrara 1995, Puy et al. 1998) have showed that thermal instability can originate cool condensations in hot plasmas. Also it is believed that at large scales such cold structures are the precursors of the gravitational instability, because if a thermal instability is triggered, in cool regions the temperature decreases and the density increases, i.e. the Jeans mass (∼ T3/2ρ−1/2) could decrease below the value of the actual mass and therefore such regions should gravitationally collapse likely forming stars, globular clusters and galaxies.