Comparison of XPM and UCAC4 catalogues in the galactic coordinate system

Abstract

This paper presents the results of the comparison of the galactic reference frames realized by the catalogues XPM and UCAC4. Based on about 40 million stars common to both catalogues, the systematic differences of the galactic coordinates and proper motions have been derived for 12 magnitudes in 0.5m width bins with the mean J‐values from 10m.25 to 15m.75. The systematic differences were represented by vector spherical harmonics, with the magnitude equation taken into consideration. The mutual orientation of the frames was found to be at the level of 10 mas. It is concluded that these differences are negligible in comparison with the accuracy of the implementation of the standard galactic coordinate system MAS 1958. We investigated two features of the XPM catalogue. First, unlike the HCRF and UCAC4 catalogues, whose proper motions are tied to the quasars and galaxies, the XPM catalogue implements a reference system based only on galaxies. Second, the XPM catalogue has two systems of proper motions—XPMx and XPMp, referred to the two galaxy sub‐catalogues of the project 2MASS—PSC and XSC. The study of the differences XPMx−XPMp showed that they are free of the magnitude equation. The speed of relative rotation of XPMx over XPMp was found to be ω=0.453±0.003 mas/year, which exceeds the residual rotation of the HCRF (0.25 mas/year). Analysis of systematic differences XPMx−UCAC4 and XPMp−UCAC4 showed that both frames XPMx and XPMp have an appreciable rotation speed relative to the UCAC4 (hence the ICRF), especially large (up to 2 mas/year) for the brightest stars in our range. This shows that a relatively high speed of rotation of the two frames XPMx and XPMp with respect to the UCAC4 is a consequence of the transition from a combined “quasar‐galaxy” to a purely “galactic” reference system. It is shown that the systematic difference between the proper motions of stars can be interpreted within the Ogorodnikov–Milne kinematic model of the velocity field.