International School for Advanced Studies Via Beirut n 2/4, 34014 Trieste (Italy)
Ecole Normale Superieure Laboratoire de Radioastronomie Millimétrique 24, Rue Lhomond, 75231 Paris cedex (France)
In the post-recombination epoch, most of the structure formation scenarios involve gravitational instability which leads to large primordial clouds which, thereafter collapse. Because the protocloud temperature increased with contraction, a cooling mechanism was crucial to the first generation structure formation by lowering pressure opposing gravity, i.e., by allowing continued collapse of Jeans unstable protoclouds. Many authors have examined this problem introducing molecular coolants. Lahav (1986) elaborated a very simple description of the evolution of a protocloud with a three-phases model and with as the main cooling agent. More recently, Puy & Signore (1995), from this simple description, but with a more complete chemistry (primordial , HD and LiH molecules) considered the three phases of the protoclouds supposed to be initially spherical: i) a linear evolution which approximately follows the expansion, ii) a turn around epoch () when the protocloud reaches its maximum value, and iii) a non-linear evolution or the collapse of the protocloud. Therefore, adopting the Inhomogeneous Big Bang nucleosynthesis model with , h Km Mpc and and the molecular abundances calculated in Puy et al. (1993) as the initial conditions of the collapse phase, Puy & Signore (1995) have examined the beginning of the collapse of protoclouds of masses 10 and 10 M.
Table 1 shows, at the turn around redshift (redshift at the beginning of the collapse), the dynamical conditions at the beginning of the collapse phase. The initial relative abundances, for the primordial molecules, are: , , .
Table: Initial conditions for different masses M (in M of protoclouds at with and (in Kelvins) are respectively the temperature of the radiation and of the matter at , (in 10 cm) the number density, (in 10) the maximum radius and (in 10 s) the free fall time, Km s Mpc, , .
The curves of the Figure 1 show the evolution of a 10 M cloud, and those of the Figure 2 the evolution of a 10 M, describes the adiabatic temperature of the collapse (i.e., without the thermal influence of the molecules). The left curves concern the evolution of abundance relative to the initial abundance. For this range of protocloud masses, and LiH abundances increase. The HD abundance decreases, due to the destructive collision with . This simple model of collapse does not consider the opacity effect of these molecules (at the end of the collapse phase). The treatment of these questions must be analyzed through a thermal approach of these phases; this last point is in progress.
In conclusion, this work shows a modification of the chemical abundances during the gravitational collapse of a protocloud. Finally, this chemical approach of a gravitational collapse suggests a strategy for the detection of primordial molecules. The excitation of the rotational level of these molecules could offer a possibility of a detectable signature, and the possible constraints on the abundances of light elements.
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Puy D. et al. 1993 A&A, 267, 337
Puy D. 1996, A&A, in press