AAS 197, January 2001
Session 4. The Milky Way Galaxy
Display, Monday, January 8, 2001, 9:30am-7:00pm, Exhibit Hall

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[4.18] Astrometry with the Macho Data Archive

A.J. Drake, C. Alcock, K.H. Cook, S.L. Marshall, C.A. Nelson, P. Popowski (LLNL), T.S. Axelrod, R.A. Allsman, K.C. Freeman (ANU), M. Geha (UCSC), D. Minniti (UPC), A.C. Becker, C.W. Stubbs (Washington), D.P. Bennett (Notre Dame), D. Alves (STSI), K. Griest, T. Vandehei (UCSD), P.J. Quinn (ESO), W. Sutherland (Oxford), D.L. Welch (McMaster), MACHO Collaboration

We present the preliminary findings of our astrometric study of stellar motions along the lines of sight of the LMC and the Galactic bulge. By performing astrometry with images spanning seven years we find that we are able to detect space motions as small as 0.006\arcsec at a 1 sigma level. We also find that we are able to select stars with proper motions as small as 0.03\arcsec/yr from five years of photometry by using the characteristic shape of their light curves. This shape is seen as a decreasing flux baseline with increasing scatter and is due to the proper motion of the object relative to its initial centroid location where the photometry is performed. By selecting such light curves we have discovered and measured motions for ~150 new high proper motion (HPM) stars in 502\arcdeg from amongst the millions of stars observed in these fields. These objects have proper motions as high as 0.45\arcsec/yr, brightnesses ranging from V ~ 13 to V ~ 19, and V-R colours from 0.3 to 1.3. These are the first HPM stars to be discovered in these locations because of the severe crowding conditions. Based on object colours, magnitudes, and motions, most of these objects are M-dwarfs. However, a few are likely to be disk white dwarfs. For several of the HPM objects the signature of parallax can be seen within their light curves. For these stars we have measured trigonometric parallaxes using a few hundreds of the best observations of each.

HPM stars such as these, in the foreground of densely crowded stellar fields, may play an important role in future observations of astrometric microlensing events with the Space Interferometry Mission (SIM), since it is possible to predict lensing events caused by them many years in advance.

The author(s) of this abstract have provided an email address for comments about the abstract: ajd@mso.anu.edu.au

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