Absolute Proper Motions and Galactic Structure

Terry Girard, Yale University

There are strong motivations for measuring stellar proper motions directly with respect to external galaxies. Such absolute proper motions are critical to our understanding of the kinematic structure of our own Galaxy, as simple star counts alone have proven incapable of satisfactorily disentangling its multi-component nature.

During the second half of this century, a handful of photographic surveys was undertaken specifically to produce absolute proper motions with respect to galaxies. The earliest of these, the Lick Northern Proper Motion program (NPM) and the Pulkova program, were begun in the 1940's. Two decades later, a southern counterpart to the Lick NPM was initiated. The Yale/San Juan Southern Proper Motion program (SPM) uses the 51-cm double astrograph at Cesco Observatory in El Leoncito, Argentina in order to provide, in combination with the Lick NPM data, absolute proper motions over the full sky. Other groups with photographic, absolute proper-motion programs include those at Kiev and at Potsdam (using the Tautenberg Schmidt telescope). In addition to these primarily astrograph programs, there are currently various groups in the process of measuring the wide-field Palomar and U.K. Schmidt surveys, both original and present epochs, with a goal, among others, of providing vast numbers of absolute proper motions. We wish these groups the best of success in dealing with this difficult plate material and formidable data analysis task!

Midway through the closing decade of this century, we are a breath away from seeing the benefits of the highly successful HIPPARCOS astrometric satellite. The proper motions from HIPPARCOS, while not absolute, offer milliarcsecond accuracy on a rigid reference frame over the entire sky. Indeed, the NPM/SPM data may provide the best means of putting the HIPPARCOS proper motions on an extragalactic inertial system, increasing even more their already immense value.

We present here an overview of the SPM program and its current status. A significant effort has been devoted to the input catalog selection and preparation, with special attention to including sufficient numbers of anonymous stars in order to address questions of Galactic kinematics. Plate measuring for a pilot program, covering approximately 1000 square degrees around the South Galactic Pole, has been completed. During the astrometric reductions of these initial plates, significant systematic effects were discovered, caused by the photographic astrometrist's evil archenemy - magnitude equation. The means by which we are able to correct for these magnitude effects will be outlined. Finally, a preliminary comparison between the SPM results and a Galactic structure and kinematics model will be given to illustrate the manner in which the NPM/SPM data will eventually help constrain detailed models of our Galaxy.