|
Description
The NPM2 Catalog is the second part of the Lick Northern Proper Motion (NPM) program to measure absolute proper motions, on an inertial system defined by distant galaxies, for nearly 400,000 stars over a blue apparent magnitude range from 8 to 18. There are 1246 6x6degree fields in the NPM survey (to declination -23 degrees). The NPM2 Catalog covers the 28% of the northern sky lying near the plane of the the Milky Way and contains some 232,000 stars in the 347 NPM fields remaining after the 1993 NPM1 Catalog (149,000 stars in 899 fields away from the Milky Way). Each NPM field was photographed with the 51cm Carnegie Double Astrograph at two epochs between 1947 and 1988. The mean first and second epochs are 1950 and 1977; the average epoch difference is 27 years. The first-epoch plates were taken in the blue only; both blue and yellow plates were taken at the second epoch. For NPM2, the plates were scanned by the Precision Measuring Machine (PMM) at the US Naval Observatory, Flagstaff. From the PMM scans, 120,000 faint (B>14) stars were chosen anonymously for the NPM astrometric reductions and for statistical studies of stellar motions. The NPM2 catalog also contains 92,000 bright (B<14) positional reference stars, mostly from the Tycho-2 Catalogue, and 35,000 stars chosen for astronomical interest from Klemola's "Lick Input Catalog of Special Stars". (These categories overlap). Details of the NPM2 star selection, data reductions, and catalog compilation will be presented in a paper being prepared for the Astronomical Journal. The completed version of the NPM2 Catalog totals 232,062 stars from all 347 NPM2 fields; it supersedes the September 2002 preliminary version (295 fields). Together with the NPM1 Catalog (Cat. I/199, or http://www.ucolick.org/~npm/NPM1/) the NPM2 Catalog completes the Lick Northern Proper Motion program after more than a half-century of work by three generations of Lick Observatory astronomers. The NPM2 catalog gives J2000 positions computed for the catalog epoch 2000, and is ordered in 108 one-degree declination zones from +83 degrees to -23 degrees. Following the convention of the NPM1 catalog, each NPM2 star has an NPM2 "name" (e.g. +83.0001) reflecting the declination zone and a running number in right ascension order within the zone. The NPM2 Catalog is also available as 108 separate files, one for each declination zone from +83 degrees (z+83n) to -23 degrees (z-23n), from http://www.ucolick.org/~npm/NPM2/zones/ Each star's entry includes the absolute proper motion and blue (B) magnitude. For 98.5% of the stars the B-V color is also given. Other data given for each star are: the original mean epoch, a stellar class code, the number of NPM fields on which the star was measured, and discrepancy flags for proper motion, and photometry. Tycho-2 numbers are given for the primary positional reference stars (40% of NPM2); for these stars the Tycho B,V photometry, transformed to Johnson B and B-V, has been averaged with the NPM2 photographic photometry, with appropriate weights. ACRS and Hipparcos numbers are also given for NPM2 stars selected from those catalogs. The RMS precision of the NPM2 individual proper motions is about 0.6"/cent (6mas/yr) in each coordinate, comparable to the NPM1 errors. The NPM2 relative proper motions in each field were reduced to absolute (ICRS system) using an average of 370 Tycho-2 stars per NPM2 field. Magnitude-dependent systematic errors for the brightest (8<B<12) NPM2 stars were removed in the same reductions. The RMS accuracy of the proper motion zero point in each field is about 0.05"/cent (0.5mas/yr) in each coordinate. The RMS position errors at the NPM2 catalog epoch 2000 average about 0.2" (200mas) in each coordinate, due mostly to the accumulated proper motion error from original plate epochs (average 1968) to 2000. The pure positional errors at the original epochs average 0.08" (80mas) in each coordinate. The RMS errors for the NPM photographic photometry average about 0.18mag in B, and 0.20 mag in B-V. We thank the National Science Foundation for its long-term support of the NPM program. The NPM2 phase was supported by NSF grants AST-9530632 and AST-9988105. We thank the Yale Southern Proper Motion group (W. van Altena, I. Platais, and T. Girard) for their help in developing software to process the PMM plate scans.
|
