Session 93 - Binary and Variable Stars.
Oral session, Friday, January 09
Monroe,

## [93.02] HST Fine Guidance Sensor Discovery of a Third Component in the Low-Mass System Wolf 922 (Gliese 831)

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

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.