The XPM catalogue as a realization of the ICRS in optical and near-infrared ranges of wavelengths

P. N. Fedorov 1 V. S. Akhmetov 1 V. V. Bobylev 0 G. A. Gontcharov 0 0 Central Astronomical Observatory at Pulkovo of RAS , Pulkovskoye chaussee 65/1, 196140 Saint-Petersburg, Russia 1 Institute of Astronomy of Kharkiv National University , Sums'ka 35, 61022 Kharkiv, Ukraine A B S T R A C T The final version of the XPM catalogue, created by the authors, contains about 314 million stellar positions and absolute proper motions and covers the whole celestial sphere without gaps in the magnitude range 10m < B < 22m. The analysis of stellar positions and proper motions of the XPM catalogue is presented. The coordinate axes, defined by the XPM catalogue, were shown to have an ambiguity of rotation relative to the LQAC quasars and ICRF2 sources less than 0.2 mas yr1. It is concluded that the XPM catalogue is the independent realization of the ICRS in the optical and near-infrared wavelengths in the sense of obtaining of proper motions. The absolute proper motions of XPM stars were compared with the similar data from the HCRF modern catalogues. The proper motions of stars in these catalogues were shown to have the appreciable random and systematic errors. 1 I N T R O D U C T I O N In this paper, we present the complete study, the XPM catalogue, of stellar absolute proper motions. It has been sent to the Centre de Donnes astronomiques de Strasbourg and will soon be available to users. The brief description of the catalogues format is presented in this paper. It is well known that the existing catalogues of stellar absolute proper motions cover the celestial sphere only partially. The Southern hemisphere has particularly poor coverage: there is just one catalogue of the absolute proper motions for the zone to the south of 45 in declination, which covers only about 720 deg2 near the southern pole. At the same time, the catalogues of the stellar absolute proper motions are very important for astrometry since they potentially provide, in a quite wide range of stellar magnitudes, a coordinate system that does not rotate relative to extragalactic objects, with no need to use any intermediate systems. If a catalogue of stellar absolute proper motion covers the whole celestial sphere, then the global quasi-inertial coordinate system is produced, which is fixed at J2000.0 by the kinematic method. On the other hand, the data of these catalogues are very important in stellar astronomy, namely in studying the kinematics of the Galaxy, for example within the OgorodnikovMilne model (Milne 1935; Ogorodnikov 1932). Since the mass determinations of radial velocities are not numerous as yet, the proper motions are just one E-mail: (PNF); (VSA); (VVB); (GAG) extensive source of the data for kinematic studies of the Galaxy. In this case, as was pointed out by du Mont (du Mont 1977), it is important to use the data for both hemispheres because the 3D OgorodnikovMilne model gives the most adequate results if only the proper motions of stars more or less evenly distributed over the whole celestial sphere are used. The conception of using coordinates of galaxies as a zero-point for direct binding of stellar proper motions was proposed by Gerasimovich and Dneprovsky (Dneprovsky & Gerasimovic 1932). They suggested creating of the Catalogue of Faint Stars (KSZ) with the use of extragalactic nebulae to determine the stellar absolute proper motions. We use the term absolute proper motions to describe proper motions with the zero-point, which is determined via a reference system defined by positions of galaxies. Such stellar absolute proper motions are free from any influence of precession and can be used to refine it, as well as to study kinematics of various stellar groups in the Galaxy. The idea has been supported by many observatories in the world. As a result, in the subsequent years three most extensive programs of determining stellar absolute proper motions were realized: the Catalogue of Faint Stars (KSZ), the Lick Northern Proper Motion (NPM), the Yale/San Juan Southern Proper Motion (SPM). Within the framework of the KSZ, a huge work has been fulfilled to combine 20 initial catalogues and to investigate the master catalogue named GPM (Rybka & Yatsenko 1997). This catalogue contains 54 463 stars from 8 to 15.5 m in 185 regions from +90 to 25 in declination. The mean accuracy of proper motions is 0.008 arcsec yr1. A bit later, another catalogue of stellar absolute proper motions was created in Pulkovo PUL2 (Bobylev, Bronnikova & Shakht 2004). Principal results of the NPM program were published as two catalogues NPM1 and NPM2. The NPM1 catalogue is described by Klemola, Jones & Hanson (1987) and Klemola, Hanson & Jones (1994). It contains 148 940 stars in 899 fields in the magnitude range 8m < B < 18m. The average number of galaxies in a field is 80. The NPM2 catalogue contains 232 062 stars in 347 fields near the galactic equator (Hanson et al. 2004). These fields practically do not contain galaxies; therefore the NPM2 stellar positions and proper motions are obtained in the HCRF frame (Perryman et al. 1997). The SPM program is the extension of the NPM into the Southern hemisphere. Principal results of the SPM program are published as three catalogues: SPM2 (Platais et al. 1998), SPM3 (Girard et al. 2004) and SPM4 (van Altena et al. 2009). The SPM2 catalogue contains about 287 000 stars. The SPM3 catalogue is the extension of the HCRF frame to fainter objects. It contains positions, proper motions and photographic B and V magnitudes for about 10.7 millions stars. The SPM4 catalogue contains positions, proper motions and B, V magnitudes for 103 319 647 stars and galaxies from southern pole to 20 in declination. It should be noted that although the authors of the SPM4 call the obtained proper motions as absolute ones, they are, in fact, relative. This is due to the fact that they are not based on the extragalactic objects directly, but are linked to the zero-point specified by the HCRF frame. The random errors of the proper motions of the aforementioned catalogues depend on stellar magnitude and vary from 2 to 10 mas yr1, whereas the accuracy of the absolute calibration is 1 to 5 mas yr1. The final version of the XPM catalogue (Fedorov, Myznikov & Akhmetov 2009, hereafter Paper I; Fedorov et al. 2010, hereafter Paper II) contains about 314 million stellar positions and absolute proper motions and covers the whole celestial sphere without gaps in the magnitude range 10m < B < 22m. The absolute proper motions were derived from the Two-Micron All-Sky Survey (2MASS) Point Sources (Skrutskie et al. 2006) and USNO-A2.0 catalogue (Monet 1998) positions with a mean epoch difference of about 45 yr for the Northern hemisphere and about 17 yr for the Southern one. The catalogues have the densest data sets and the large difference of epochs suitable for solving this task, since positions in other catalogues, including the USNO-B1 (Monet et al. 2003), are given usually for the epoch J2000. The ra (...truncated)