G. S. Stringfellow 1, T. Prusti 2, & A. Brown 2
1 Center for Astrophysics and Space Astronomy, University of Colorado at Boulder, USA
2 ISO Science Operations Center, Vilspa, Spain
EXORs, nemad after the prototype EX Lupi, are a very small
subclasss of classical T Tauri stars (CTTs) which experience
rather major optical outbursts. Outbursts of the EXORs are intermediate between
the long lived and more energetic FU Ori type outbursts (FUORs)
and the more modest rapid accretion variability of the CTTSs. EXORs undergo
large amplitude outbursts as a direct result of enhanced mass accretion
from their circumstellar disk, where the energy liberated
results in a brightening of 3-5 magnitudes in the UV and optical.
Direct accretion to the stellar surface can be via a boudary layer
or magnetic coupling between the disk and the star. The outbursts play a major
role in
modifying the circumstellar environment, and initiate the formation of winds,
jets, H-H objects, and dispersal of the disk, and likely influence
the formation and evolution of giant planets and brown dwarfs. Exactly how the
disk replenishes itself between the outbursts and the underlying mechanism that
triggers the outbursts remains a mystery. First results of an ISO
program investigating the replenishment and evolution of the accretion
disk in EXOR systems and their impact on the above issues will be discussed.
ISO observed EX Lupi at four epochs during 1997. Analysis of
the spectral energy distribution (3.6 to 200 
of the accretion
disk, chemical composition, and the temporal evolution between the epochs
will be presented.