The Milky Way contains between 100 million and one billion neutron stars, left behind after the supernova explosions that distributed heavy elements throughout the Galaxy. Understanding the distribution and properties of these stars is important for understanding Galactic chemical evolution, as well as for constraining the dynamics of supernova explosions and for illuminating the formation and life-cycles of various type of exotic compact binary systems. The most direct constraints on the neutron star distribution come from measurements of parallaxes and proper motions. In most cases, these measurements can be made only for the subclass of neutron stars that are luminous at radio wavelengths: the radio pulsars. Even then, the closest radio pulsars are a hundred times the distance of the closest stars, and the effects of parallax and proper motion are correspondingly smaller. Only about 1% of pulsars have measured parallaxes and only about 10% of pulsars have even crudely measured proper motions. Here, Thorsett and Dewey will continue their highly successful interferometry program, using the Very Large Array and the Very Long Baseline Array to make very precise astrometric measurements of a large sample of pulsars and address several important questions. The investigation will, over the next three years, measure the birth velocity distribution of neutron stars with sufficient accuracy to constrain both the high and low velocity tails (needed to understand supernova asymmetries and retention of neutron stars in globular clusters, respectively), and will improve understanding of local pulsar distances and the local pulsar birthrate. This plan is based primarily on observations that are either underway or already approved. The investigations will include student training and support, with project data forming the basis of a Ph.D. thesis. The student and the principle investigators will also maintain formal connections and participate in activities with the Exploratorium science museum and the COSMOS summer program. Co-investigator Dewey is also Education and Public Outreach Director for the UCSC branch of Calspace, providing further outreach opportunities. External science investigations will also be enabled by both web and traditional publication of the data gathered here. These include identification of the primary target lists for The Gamma-ray Large Area Space Telescope and other satellite missions, and interpretation of multiwavelength pulsar emission data. Comparison of the local pulsar birthrate to the massive star birthrate, constrains the properties of the progenitors of Type II supernovae. Pulsar velocity measurements not only constrain models of their birth events, and are important input parameters to population synthesis studies of binary systems. The observations made to identify calibration sources for the Very Long Baseline Array surveys comprise a large survey sample of 21-cm snapshots of extragalactic sources.
