Young stellar objects are usually divided in several classes according
to the wavelength at which their energy distribution peaks, roughly
corresponding to their age. The transition of these stars from a system
in which the strongly accreting central pre-main sequence star is heavily
embedded in its natal cloud to a system in which most of this cloud has
been accumulated into a circumstellar disk and the accretion on the central
object has diminished by several orders of magnitude is particularly
interesting, since current theories predict that this is the onset of
planet formation in these disks. It is expected that the study of the
physical phenomena occurring during the different phases can provide us
with valuable new insights in this process, which, in the case of the
solar system, eventually lead to the formation of inhabitable planets.
Intermediate-mass pre-main sequence stars, recognizable as Herbig Ae/Be
systems, play a crucial role in these studies, since the evolution of
their disks can be followed up to later ages due to their greater
luminosity than the low-mass T Tauri stars.
Here we ask for discretionary time to complete our ISO study of the
transition of intermediate-mass YSO's from the phase at which they start
clearing most of their natal cloud to their appearance as Herbig Ae/Be
stars surrounded by protoplanetary disks. The goal of these new observations
is twofold: (1) investigate the connection between solid-state and gas-phase
phenomena in a small number of young objects in which we have found evidence
for strong outflow activity, (2) detect the cool molecular hydrogen from
the circumstellar disk in a small number of more evolved objects. The
combination of these new spectra with our data on these objects from the
central program PWESSELI.YSOAEBE proposal will give considerable enhanced
value to these spectra and form a unique data-set of immense value for
the study of star formation for years to come.