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A.R. Upgren, J.T. Lee, E.W. Weis (Yale Observatory)
In 1982 a working group of I.A.U. Commission 24 was established in order to provide parallax standard fields (IAU Transactions, XVIIIB,127 1982). Three of these fields for regular trigonometric parallax observation are centered on open clusters; the Pleiades, Praesepe and IC4756. Very few studies on IC4756 have been made; one is by Herzog and Sanders (AAPS, 19, 211 1975). The Van Vleck Observatory began normal parallax observations of IC4756 with its 0.5m astrometric refractor in 1980. A few observations were also obtained in 1963. Using Yale PDS machine, Lee has measured two of these early plates and two from 1997-98. The proper motion differences among the stars from different plate pair solutions are about 0.0008"/yr, and the mean proper motion of member stars is about 0.003"/yr, with respect to the mean motion of the field stars. The epoch difference of 34 years appears sufficient for accurate measures of the internal motion of the member stars. This cluster has also been observed with the 1.5m reflector of the U.S. Naval Observatory and the 0.65m McCormick Observatory refractor. These observations may also become available for the motion study.
The author(s) of this abstract have provided an email address for comments about the abstract: jlee@astro.yale.edu