  With 2D hydrodynamical simulations of disks perturbed externally by stars,
brown dwarfs or planets we investigate possible scenarios that can account for
the spiral structure in circumstellar disks. We consider two scenarios, spiral
structure driven by an external bound planet or low mass star and that excited
by a previous stellar close encounter or flyby. We find that both scenarios
produce morphology similar to that observed in the outer disks of HD 141569A
and HD 100546; moderately open 2-armed outer spiral structure. The outer
two-armed spiral structure observed in the disk of HD 141569A is qualitatively
reproduced with tidal perturbations from its companion binary HD 141569B,C on a
prograde orbit near periapse. Our simulation accounts for the outer spiral
arms, but is less successful than the secular model of Augereau and Papaloizou
at matching the lopsidedness or asymmetry of the disk edge at 300AU. The disk
has been previously truncated by the tidal force from the binary. A bound
object (stellar or planetary) is unlikely to explain the spiral structure in HD
100546. A co-eval planet or brown dwarf in the disk of sufficient mass to
account for the amplitude of the spiral structure would be detectable in NICMOS
and STIS images, however existing images reveal no such object. A previous
encounter could explain the observed structure, provided that the encounter
occurred less than a few thousand year ago. The object responsible for causing
the spiral structure must then be within a few arcminutes of the star. However,
the USNO-B proper motion survey reveals no candidate object. Moreover, the
probability that a field star encountered HD 100546 in the past few thousand
years is very low.
