Résumé :
In recent years, the chemistry of dense cores has been revisited with the
discovery of a plethora of multiply deuterated molecules and of large
depletions of CO. It is now firmly established that condensation of stable
species onto dust grains leads to systematic molecular
differentiation in starless cores. Recent chemical calculations
suggest that at high densities a "complete freezeout" occurs and H3+ and
its deuterated isotopologues become the only tracers of the
molecular gas. As the density threshold for the complete freezeout is time
and model dependent, good observational constraints are needed. The
submillimeter ground state lines of deuteroammonia provide new
opportunities to study the physics and chemistry of cold, dense ISM.
Planned absorption studies with the HIFI instrument on Herschel will give
insights into the physics and chemistry of foreground clouds on the lines
of sight toward bright submillimeter continuum sources,
where many complex molecules have already been detected.
