INTERSTELLAR MATTER NEAR THE PLEIADES .4. THE WAKE OF THE PLEIADES THROUGH THE INTERSTELLAR-MEDIUM IN TAURUS

A mosaic of IRAS co-added images covering 8.7-degrees x 4.3-degrees re veals a large emission ''cavity'' with bright rims that extends some 5 -degrees eastward from the Pleiades. This is a novel interstellar stru cture, unrelated to outflows or ionizing radiation from high-mass star s. Instead, it is a region pressurized by the soft-ultraviolet radiati on of the cluster, most likely through photoelectric heating of the am bient interstellar medium. The IRAS emission cavity delineates the wak e of the Pleiades as it moves supersonically through the interstellar medium. Evidence for the dynamical interaction that produces the wake exists in the morphology of the gas and dust, the orientation of inter stellar polarization, and especially in the velocities of the atomic a nd molecular gas. The magnitude of radial velocity shifts near the clu ster provide constraints on the orientation of the gas flow, and hence on the space motion of the gas. These data suggest that the interacti on involves generation of a shock wave. Consideration of possible mech anisms for the cluster-cloud interaction identifies photoelectric heat ing as the most likely agent and leads to the suggestion that transver se expansion of heated gas near the cluster plays a crucial role in dr iving the shock. The analysis indicates that the interstellar matter i n the Pleiades is undergoing a chance collision with the cluster at ap proximately 18 km s-1; it is unrelated to the birth of the cluster. Th e wake orientation offers an alternative and independent approach to t he cloud motion. Tracing the cloud trajectory back to an origin in Gou ld's Belt with transverse velocity as a free parameter yields a less p recise but virtually identical result for the space motion as the anal ysis of the kinematics. The orbit reconstruction suggests that the clo ud originated approximately 15 Myr ago in an energetic event near l = 60-degrees and low positive b. The present-day position of the hypothe sized source exhibits a local minimum in molecular gas and a 21 cm mor phology that suggests a blow-out of gas into the Galactic halo, but no recognized star cluster or stellar association exists there. The puls ar PSR 1919+21, which has a spin-down age of 15 Myr but no detected pr oper motion, lies tantalizingly nearby. The orientation of the Pleiade s wake by itself implies that this material is kinematically unrelated to the star-forming complexes in Taurus and Perseus, in spite of prox imity in space and similarity in radial velocity. Review of data for t he larger Taurus/Perseus region leads to a schematic view of the spati al structure of the interstellar medium in this region, and to some fi nal remarks on the evolution of the local system of gas and young star s.