  In this Letter we propose a new view on UX Orionis type variability. The idea
is based on the earlier proposal by various authors that UXORs are
nearly-edge-on disks in which hydrodynamic fluctuations could cause clumps of
dust and gas to cross the line of sight. However, because the standard disk
models have a flaring geometry, it is mostly the outer regions of the disk that
obscure the star. The time scales for such obscuration events would be too long
to match the observed time scales of weeks to months. Recent 2-D
self-consistent models of Herbig Ae/Be protoplanetary disks (Dullemond et al.
2002,2003 henceforth D02/DD03), however, have indicated that for Herbig Ae/Be
star disks there exists, in addition to the usual flared disks, also a new
class of disks: those that are fully self-shadowed. Only their puffed-up inner
rim (at the dust evaporation radius) is directly irradiated by the star, while
the disk at larger radius resides in the shadow of the rim. For these disks
there exist inclinations at which the line of sight towards the star skims the
upper parts of the puffed-up inner rim, while passing high over the surface of
outer disk regions. Small hydrodynamic fluctuations in the puffed-up inner rim
could then be held responsible for the extinction events seen in UXORs. If this
idea is correct, it makes a prediction for the shape of the SEDs of these
stars. It was shown by D02/DD03 that flared disks have a strong far-IR excess
and can be classified as `group I' (in the classification of Meeus et al.
2001), while self-shadowed disks have a relatively weak far-IR excess and are
classified as `group II'. Our model therefore predicts that UXORs belong to the
`group II' sources. We show that this correlation is indeed found within a
sample of 86 Herbig Ae/Be stars.
