  The solar system's Oort cloud can be perturbed by the Galactic tide and by
individual passing stars. These perturbations can inject Oort cloud objects
into the inner parts of the solar system, where they may be observed as the
long-period comets (periods longer than 200 years). Using dynamical simulations
of the Oort cloud under the perturbing effects of the tide and 61 known stellar
encounters, we investigate the link between long-period comets and encounters.
We find that past encounters were responsible for injecting at least 5% of the
currently known long-period comets. This is a lower limit due to the
incompleteness of known encounters. Although the Galactic tide seems to play
the dominant role in producing the observed long-period comets, the non-uniform
longitude distribution of the cometary perihelia suggests the existence of
strong -- but as yet unidentified -- stellar encounters or other impulses. The
strongest individual future and past encounters are probably HIP 89825 (Gliese
710) and HIP 14473, which contribute at most 8% and 6% to the total flux of
long-period comets, respectively. Our results show that the strength of an
encounter can be approximated well by a simple proxy, which will be convenient
for quickly identifying significant encounters in large data sets. Our analysis
also indicates a smaller population of the Oort cloud than is usually assumed,
which would bring the mass of the solar nebula into line with planet formation
theories.
