Fundamental Systems of Positions and Proper Motions

Abstract

radio sources, a process that requires the determination of precise radio positions by means of radio techniques and of optical positions of faint objects with large reflectors. Moreover, it now seems to be possible to establish a frame of reference by means of absolute determina­ tions of positions of radio sources. Another example is the measurement of proper motions of stars with respect to galaxies, where initial results have been obtained during the past few years. In view of these promising new developments, it appears appropriate to ask whether the traditional fundamental system of stellar positions and proper mo­ tions can be extended so that it fulfills the requirements of a reference frame for positions of faint objects, whether stars, galaxies, or optical counterparts of radio sources, or whether the conventional system should be replaced by a reference system represented by extragalactic objects. The purpose of this review is to present, first, an analysis of the present status of the fundamental system represented by positions and proper motions of bright stars, and of possible improvements of the system and its extension to faint ob­ jects. Recent observations relevant to the improvement and extension of the sys­ tem will be discussed and desiderata delineated. Second, a discussion of the measurement of proper motions with respect to galaxies and of determinations of precession will be presented. Finally, the significance of recent advances in the determination of absolute positions of radio sources in the light of the properties of the traditional fundamental system will be discussed. In concentrating on problems arising from the establishment of a funda­ mental reference system, we find that several highly interesting projects involving differential measurements cannot be considered. An example is the search for high proper motion stars for the purpose of finding nearby stars. Such surveys do not require measurements with respect to a well-defined reference frame. Blink surveys of plates taken with the 48-inch Palomar Schmidt deserve men­ tion; these have yielded large proper motions of objects down to apparent mag­ nitude ,...., 21. Among the recent results of blinking are the lists of subluminous stars and suspected white dwarfs published by Luyten (1970a , b).