Résumé :
The initial conditions for star formation are an essential
input to codes describing the formation and evolution of a
protostar. It is plausible that these initial conditions are
hydrostatic or at least involving only subsonic velocity
fields. It is difficult to imagine any convincing
theoretical derivation of what has been called the
"pivotal state" (i.e. the density and magnetic field distribution
as well as the velocity field just prior to dynamical collapse)
though that has not prevented people trying. A better approach
may be to identify and define such "pre-protostellar cores"
observationally. In this seminar, I will describe the attempts
we have made to resolve this problem as well as the problems
we have encountered. One particular problem is the depletion
of molecules containing heavy elements onto the dust grain
surfaces and the consequence of such depletion for the physical
conditions in the dense regions which may be close to collapse.
I will describe our determinations of temperature and
dust grain emissivity in two well studied objects : L1544
and B68. I will also describe a recent survey we have made
searching for cores which appear to be close to the "pivotal
state". Finally, I will briefly describe the recent detection
by van der Tak and colleagues of H2D+ in L1544 and comment on
what one can conclude about the electron density and hence
the ambipolar diffusion time scale.
