Abstract.
The FU Orionis (FUor) or EX Orionis (EXor) phenomenon has attracted
increasing attention in recent years and is now accepted as a crucial
element in the early evolution of low-mass stars. FUor and EXor
eruptions of
young stellar objects (YSOs) are caused by strongly enhanced accretion
from
the surrounding disk. FUors display optical outbursts of &sim 4 mag or
more and
last for several decades, whereas EXors show smaller outbursts
(Δ m ∼ 2 - 3 mag) that last from a few months to a few years
and
may occur repeatedly. Therefore, FUor/EXor eruptions represent a rare
but
very important phenomenon in early stellar evolution, during which a
young
low-mass YSO brightens by up to several optical magnitudes. Hence,
long-term
observations of this class of eruptive variables are important to design
theoretical models of low-mass star formation. In this paper, we present
recent results from our long-term monitoring observations of three rare
types
of eruptive young variables with the 2-m Himalayan Chandra
Telescope (HCT)
and the 2-m IUCAA Girawali Observatory (IGO) telescope.
Key words:stars: formation -- stars: pre-main-sequence -- stars: variables: other
ISM: clouds -- (ISM:) reflection nebulae