Session 60 - Multiple Stars.
Display session, Wednesday, June 12
Great Hall,

## [60.03] A Visual Binary'' Orbit of the Low-Luminosity Hyad VA351

O. G. Franz, L. H. Wasserman (Lowell Obs.), 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)

VA351=G7-203 \V=13.2, (B-V)=1.5; RA=04:25:13.4, Dec=17:16:06 (2000)\ was first resolved as a 0.051-arcsec pair on 1994 d45 in the course of our search for binaries among faint members of the Hyades cluster conducted with HST Fine Guidance Sensor #3 in the Transfer Function Scan mode (Franz et al. 1994, BAAS 26, 929). With four additional measures obtained during 1994 d296 through 1996 d78, our data now cover a position-angle interval of 156 deg. From these five HST-FGS measures we have computed a first visual'' orbit of VA351 with P=3.36 \pm 0.12\ y and a=0.069 \pm 0.001 arcsec. Using a cluster parallax p=0.021 arcsec we derive a total mass M=1.8 \pm 0.3 M_ødot, a value clearly too large for a pair of near-equal M-dwarfs. It may, however, be in accord with the conclusion by Latham (1955, private comm.), based upon radial velocities, that VA351 is a triple system. Spectra we have obtained at McDonald Observatory and at Lowell Observatory appear to provide independent evidence for the presence of additional components. Under a proposal approved for HST Cycle 6 we expect to obtain six additional FGS observations which will combine transfer function mode with position mode relative to local reference stars. These observations, together with the data now in hand, will yield true barycentric orbits and thus masses of the resolved components of VA351 with an expected accuracy of five percent or better. Moreover, the combination of HST-FGS astrometry and radial velocity measures should permit the conclusive analysis of this low-luminosity Hyad system. This work is supported by NASA under grant NAG5-1603 to UTexas.