SCIENTIFIC ABSTRACT

FU Orionis stars (FUORs) represent a rare and unique class of pre-main-sequence
objects of which only about nine members are known.  Most probably they consist
of a T Tauri-like star surrounded by a very luminous accretion disc currently in
a state of very high mass accretion (approx.  10**-4 Msun/yr).  We propose to
measure the spectral energy distributions of all known FUORs and FUOR candidates
between 3.6 and 160 microns.  With the addition of IRAS and ground-based
measurements, the time evolution of these discs can be studied.  These studies -
combined with a comparison with other young stars - will lead to a much better
understanding of the properties of FUOR discs, the physical processes in
accretion discs, and hence discs around young stars in general.

OBSERVATION SUMMARY

We propose to measure the spectral energy distributions of almost all of the
classic FUORs - BBW 76, FU Ori, L1551-IRS5, RNO1B, V1057 Cyg, V1515 Cyg, V1735 
Cyg, and V346 Nor - from 4.8 to 100 microns using the PHT-P photometer.
Some important objects have been left out due to object complexity (Z CMa) or
to spacecraft time limitations (e.g. SVS 13) and will have to be observed
during the normal open observing periods.

A uniform aperture of 180" has been chosen for these bright objects so that
peaking the position will be unnecessary and to insure that the same region of 
the sky is observed in all filters.  The filters have been chosen so that each 
effective wavelength is approx. twice/half that of the previous/next filter, 
insuring good coverage of the spectral energy distribution.  The 4.8 micron 
filter is important in order to compare ground-based with ISO fluxes.  The other
filters are similar to the IRAS bandpasses - a necessary condition for the 
detection of any changes between the two epochs.

The total times for the performance of each "ON" and "OFF" - 586 and 426s,
respectively - were calculated using the ISOPHOT observing time estimator and a 
uniform exposure time of 32s for each of these bright sources.  The times for 
the concatenated "OFF" AOT's are realistic given the fact that they are all
very close to the "ON" AOT's. The resulting total times for each source ("ON" 
and "OFF") are then 1012s. The total integration times for the 6/5 sources 
(12/10 AOT's for the "ON" and "OFF" measurements) for the spring/fall launch 
date is 6072/5060s (1.69/1.41 hrs), an increase of 22/27% from the Phase 2 
estimates.  This difference is due to the correction of the exposure times (from
8 or 16 to 32s) and could only be partially mitigated by a decrease in the 
number of filters (from 9 to an absolute minimum of 5).  Further savings are 
possible if the proposal were to be changed from PH03 to PH19 (sparce maps) 
sometime this spring (when such are supported by the PGA!.