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Session 93 - Binary and Variable Stars.
Oral session, Friday, January 09
Monroe,
The M--dwarf astrometric binary W922 = GL831 \V = 12.00, B--V = 1.67; RA = 21:31:18.5, Dec = -09:47:27 (2000)\ was first resolved at visible wavelengths with HST---FGS3 in the Transfer Function scan (TRANS) mode (Franz et al. 1994, BAAS 26, 1464). This observation and nine additional FGS measures now yield a well--defined ``visual'' binary orbit with period P = 1.9330 \pm 0.0086 years and semi axis--major a = 0.1440 \pm 0.0023 arcsec. With the ground-based parallax \pi = 0.1258 \pm 0.0023 arcsec and a fractional mass from the infrared brightness ratio and mass--luminosity relation, the binary orbit yields primary and secondary component masses M_A = 0.26 M_\sun and M_B = 0.14 M_\sun, respectively. However, two TRANS observations made nearly one year apart, gave excessively large and grossly systematic residuals when analyzed as binary--star functions, but were cleanly and easily resolved into three components. W922 is evidently a triple system. With a magnitude difference \Delta m = 2.27 \pm 0.03 mag measured through the F583W filter, the absolute visual magnitude of the brightest component becomes M_V(A) = 12.6 and that of the secondary M_V(B) \sim 15. The third component, detected only twice and at magnitude differences of 3.6 and 2.8 mag below the primary, is probably a flare star of very low mass and luminosity, located at or near the end of the main sequence.
This work is based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5--26555.
