The PPM database is the Catalog of Positions and Proper Motions. It combines the two catalogs for PPM North and PPM South, the Bright Stars Supplement to PPM, and the 90,000 Stars Supplement to the PPM. The PPM North list gives positions and proper motions of 181731 stars north of -2.5 degrees declination. PPM South gives positions and proper motions of 197179 stars south of about -2.5 degrees declination. The star density of PPM South is slightly higher than that of its northern counterpart and the accuracy of the present-epoch positions is roughly twice that in the north. A number of bright stars are missing from the PPM and PPM South Star Catalogs. The Bright Stars Supplement included here makes the PPM catalogs complete down to V=7.5 mag. For this purpose it adds all missing stars brighter than V=7.6 mag that could be found in published star lists. Their total number is 321. Only 5 of them are brighter than V=3.5 Since its appearance in 1966, the SAO Catalogue has been the primary source for stellar positions and proper motions. Typical values for the rms errors are 1 arcsec in the positions at epoch 1990, and 1.5 arcsec/century in the proper motions. The corresponding figures for the AGK3 Catalog in the northern hemisphere are 0.45 arcsec and 0.9 arcsec/century. Common to both of these catalogues is the fact that proper motions are derived from two observational epochs only, and that positions are nominally in the B1950/FK4 coordinate system. The PPM Star Catalogue (Roeser and Bastian, 1991, Bastian et al., 1993; for a short description see Roeser and Bastian, 1993) effectively replaced these catalogues by providing more precise astrometric data for more stars on the J2000/FK5 coordinate system. Compared to the SAO Catalogue the improvement in precision is about a factor of 3 on the northern and a factor of 6 to 10 on the southern hemisphere. In addition, the number of stars is increased by about 50 percent. Typical values for the rms errors on the northern hemisphere are 0.27 arcsec in the positions at epoch 1990, and 0.42 arcsec/century in the proper motions. On the southern hemisphere PPM is much better, the corresponding figures being 0.11 arcsec and 0.30 arcsec/century. The improvement over the SAO Catalogue was made possible by the advent of new big catalogues of position measurements and by the inclusion of the century-old Astrographic Catalogue (AC) into the derivation of proper motions. The AC contains roughly four million stars that are not included in PPM. For most of them no precise modern-epoch position measurements exist. Thus it is not yet possible to derive proper motions with PPM quality for all AC stars. But among the 4 million there is a subset of some 100,000 CPC-2 stars that are not included in PPM. These stars constitute the 90,000 Stars Supplement to PPM, and can be identified from their PPM Number having a value between 700001 and 789676. This database was updated by the HEASARC in October 1999 based on ADC/CDS catalogs I/146 (PPM - North), I/193 (PPM - South), I/206 (Bright Stars Supplement to PPM), and I/208 (90,000 Stars Supplement to PPM). This is a service provided by NASA HEASARC .
