Session 43 - Binary Stars.
Display session, Tuesday, January 16
North Banquet Hall, Convention Center

## [43.06] The Astrometric Binary W1062: Initial Visual'' Orbit from Astrometry with the HST Fine Guidance Sensors in the Transfer Function Scan Mode

O. G. Franz, L. H. Wasserman (Lowell Obs.), T. J. Henry (STScI), A. J. Bradley (Allied-Signal Aerospace Co.), G. F. Benedict, R. L. Duncombe, P. D. Hemenway, W. H. Jefferys, B. McArthur, E. Nelan, P. J. Shelus, D. Story, A. L. Whipple (UTx), L. W. Fredrick (UVa), W. F. van Altena (Yale)

The M-dwarf W1062 = GL748 [V=11.1; RA = 19:12:08.6, Dec = +02:53:36 (2000)] was first discussed as an astrometric binary by R. S. Harrington (1977, PASP 89, 214) and S. L. Lippincott (1977, Astron. J. 82, 925). It was first resolved and confirmed at visible wavelengths with HST-FGS (Franz et al. 1994, BAAS 26, 1464) near apastron at separations of 0.2032 and 0.2034 arcsec. We have obtained four additional observations near periastron covering a position angle interval of 50 deg at separations of 0.0923 to 0.0887 arcsec. These six HST-FGS measures yield the first visual'' orbit of W1062 with P = 2.423 \pm 0.055 yr and a = 0.1462 \pm 0.0007 arcsec. The largest residual in separation is 2.0 mas; the others lie between 0.5 and 0.1 mas. Position angle residuals are 1.25 to 0.38 deg. Using the absolute trigonometric parallax p = 0.1008 arcsec (C. C. Dahn et al. 1982, Astron. J. 87, 419), we derive a total mass of M = 0.52 \pm 0.03\ M_ødot. The uncertainty is based on the formal errors of the orbital elements only and does not include the parallax error. Our 1995 observations are part of an ongoing project of HST-FGS astrometry combining transfer function mode and position mode relative to a set of local reference stars. Once complete, these observations will yield barycentric orbits and thus masses of the individual components free of any assumptions and with an expected accuracy of 5 percent or better. This work was supported in part by NASA under grant NAG5-1603 to UTexas. Support for this work was provided also by NASA through grant number GO-06047.03-94A from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555.