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Denis Puy
International School for Advanced Studies
Via Beirut n
2/4, 34014 Trieste (Italy)
Monique Signore
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.
ERBCHBICT941163) of the
European Community.
Lahav O. 1986, MNRAS, 220, 259
Puy D. et al. 1993 A&A, 267, 337
Puy D. 1996, A&A, in press