Toward the HIPPARCOS link: astrometry of stars and extragalactic sources in the field of the spiral galaxy M51.

Abstract

We present absolute proper motions of 14 Hipparcos stars from ground based photographic astrometry in a 2x2deg field around the spiral galaxy Messier 51. These are part of a catalogue of 340 stars with B<=16, for which positions, proper motions, and B magnitudes have been derived from a series of 16 photographic plates. Due to large epoch differences of up to 88 years between the observations, the proper motion accuracy is typically <=1mas/a. An extragalactic calibration of the proper motion zero-point is achieved by measuring the fictitious proper motions of bright extragalactic objects in the field. For this purpose, a cross-correlation technique was developed. The resulting precision of the proper motion link to an extragalactic frame is estimated as 1.0mas/a. Measurements of double stars and high proper motion stars are also discussed.