Cluster proper motion programs utilizing large astrometric reflectors

Abstract

The prominent rôle now being played by open clusters in studies of stellar evolution indicates a need for more extensive and precise knowledge of the proper motions of members of such systems. Indeed, it is likely that the solutions to many problems in this field must depend predominantly upon them. This article briefly reviews recent investigations of proper motions of stars in open clusters and describes several recent advances in our knowledge of the nature of open clusters resulting mainly through accurate photometry of stars in cluster fields. It amplifies the outline of a large-scale program for the determination of cluster proper motions with the 61-inch Astrometric Reflector of the U.S. Naval Observatory ( Guetter and Upgren, 1966). This telescope and its recently inaugurated parallax and proper motion program have been described by Strand (1966) and by Worley (1966a), respectively. The significance of the greater magnitude range accessible to the 61-inch telescope in terms of recent studies of the evolution of stars in clusters of all ages is discussed. Particular attention is given to recent theoretical and observational conclusions which maintain that fainter stars form an extended corona about the central regions of open clusters as defined by the brighter members.