Abstract:
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Despite recent progress, both the earliest stages of individual protostellar
collapse and the origin of the global stellar initial mass function (IMF) are
poorly understood.  Since pre-stellar condensations and young protostars have
$T_{bol} \simlt 30$~K and emit the bulk of their luminosity in the
80--350~$\mu$m band, a large far-infrared and submillimeter space telescope
such as FIRST (to be launched by ESA in 2007) is needed to make further
advances in this area.  In particular, FIRST will provide a unique probe of the
energy budget and temperature structure of pre/proto-stellar condensations.
With an angular resolution at 85-300~$\mu$m comparable to, or better than, the
largest ground-based millimeter radiotelescopes, the two imaging instruments of
FIRST (i.e., PACS and SPIRE) will make possible deep, unbiased surveys for such
condensations in all the nearby ($d \simlt 1$~kpc) molecular cloud complexes of
the Galaxy.  These surveys are already considered among the top scientific
priorities of the SPIRE and PACS core observing programmes.  They will provide,
for the first time, the mass and luminosity functions of complete samples of
cold pre-stellar condensations, comprising thousands of objects down to
substellar masses.  This should greatly help improve our understanding of the
fragmentation origin of the IMF.  In addition, follow-up spectroscopy at high
resolution with the HIFI heterodyne instrument (and at medium resolution with
the PACS and SPIRE spectrometers) will give quantitative constraints on the
dynamical and chemical states of the most interesting condensations identified
in the photometric surveys.