Proper motions and natures of First Byurakan Survey blue stellar objects

Abstract

A new method for combined calculations of proper motions (PMs) based on accurate measurements of POSS1 and POSS2 epoch plates is given. The positional accuracy of various surveys and catalogues is estimated, and statistical weights for each of them are established. To achieve the best positions, weighted averaging of direct measurements on DSS1/DSS2, and data from APM, MAPS, USNO-A2.0, USNO-B 1.0 and GSC 2.3.2 catalogues were used. The rms accuracy of positions achieved for POSS1 is 119 mas in each coordinate and 168 mas total, and for POSS2 it is 69 mas in each coordinate and 98 mas total. Using these accurate positions and the large separation in years between POSS1 and POSS2, we calculated the best possible PMs: 3.9 mas yr -1 in each coordinate and 5.5 mas yr -1 total. We developed methods to control and exclude the accidental errors that appear in any survey. We compared and combined our PMs with those given in USNO-B 1.0, SDSS DR7 and Tycho-2 catalogues and obtained even better results: 3.0 mas yr -1 in each coordinate and 4.2 mas yr -1 total PM. This approach was applied to the First Byurakan Survey blue stellar objects, containing significant numbers of white dwarfs and subdwarfs. In total, 640 objects were revealed with PM ≥ 10 mas yr -1 , the detection limit for this method, and an electronic table of these objects is given. For more confident PMs, we adopt the limit 20 mas yr -1 (333 objects). Adopting a 50 km s -1 upper limit for tangential velocities, we calculated maximum distances and absolute magnitudes and estimated luminosity types for these objects, obtaining 185 probable white dwarfs (M > 8 mag), 69 possible white dwarfs (6 < M < 8 mag), and 42 candidate subdwarfs/white dwarfs (3 < M < 6 mag). Given that we rediscovered 141 genuine white dwarfs among the classified objects, the success rate for the discovery of white dwarfs is estimated as 83 per cent, and as 38 per cent for our probable and possible white dwarfs, respectively.