  After its formation, a young star spends some time traversing the molecular
cloud complex in which it was born. It is therefore not unlikely that, well
after the initial cloud collapse event which produced the star, it will
encounter one or more low mass cloud fragments, which we call "cloudlets" to
distinguish them from full-fledged molecular clouds. Some of this cloudlet
material may accrete onto the star+disk system, while other material may fly by
in a hyperbolic orbit. In contrast to the original cloud collapse event, this
process will be a "cloudlet flyby" and/or "cloudlet capture" event: A
Bondi-Hoyle-Lyttleton type accretion event, driven by the relative velocity
between the star and the cloudlet. As we will show in this paper, if the
cloudlet is small enough and has an impact parameter similar or less than
$GM_{*}/v^2_\infty$ (with $v_\infty$ being the approach velocity), such a flyby
and/or capture event would lead to arc-shaped or tail-shaped reflection
nebulosity near the star. Those shapes of reflection nebulosity can be seen
around several transitional disks and FU Orionis stars. Although the masses in
the those arcs appears to be much less than the disk masses in these sources,
we speculate that higher-mass cloudlet capture events may also happen
occasionally. If so, they may lead to the tilting of the outer disk, because
the newly infalling matter will have an angular momentum orientation entirely
unrelated to that of the disk. This may be one possible explanation for the
highly warped/tilted inner/outer disk geometries found in several transitional
disks. We also speculate that such events, if massive enough, may lead to FU
Orionis outbursts.
