  We reexamine a previously proposed model for thermal pressure acceleration of
collimated outflows in young stellar objects (YSO). We are motivated by new
results from recent X-ray observations of YSO. These show that there is
essentially no difference between the properties of X-ray emission from YSO
with and without outflows, imposing quite severe constraints on models based on
magnetic launching of jets. In our scenario the magnetic fields are weak, and
serve only to recollimate the outflow at large distances from the source. We
perform time scale estimates and an analytical calculation of the acceleration
of gas originating in the boundary layer (BL) of an accretion disk. By applying
global energy conservation considerations we find that the mass escaping the
system is compatible with observations. A crucial ingredient of the proposed
model is that the accreted material is strongly shocked, and then cools down on
a time scale longer than its ejection time from the disk. By using appropriate
properties of accretion disks boundary layers around YSO, we show that the
conditions for the proposed scenario to work are reasonably met. We find also
that the thermal acceleration mechanism works only when the accretion rate in
YSO accretion disk is large enough and the `alpha' parameter of the disk small
enough - otherwise the cooling time is too short and significant ejection does
not take place. This result appears to be compatible with observations as well
as theoretical considerations.

Astrophysics