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[102.08] The Low-mass Binaries Gl 791.2 and Gl 623 - Parallaxes and Mass Estimates from HST Fringe-tracking (POS mode) Interferometry

G. F. Benedict, B. J. McArthur (McDonald Obs., U. Texas), O. G. Franz, L. H. Wasserman (Lowell Obs.), T. J. Henry (Harvard-SAO), E. P. Nelan (STScI), W. H. Jefferys (U. Texas), R. L. Duncombe (Aerospace Eng., U. Texas), P. J. Shelus (McDonald Obs., U. Texas), P. D. Hemenway (U. Rhode Island), D. Story (Jackson \& Tull), A. L. Whipple, A. J. Bradley (Allied-Signal Aerospace), Wm. F. van Altena (Yale U.), L. W. Fredrick (U. Virginia)

With fifteen epochs of data spanning 1.7y from Fine Guidance Sensor 3 (one of three white-light interferometers on HST) we have obtained a parallax (\pi=0\farcs1113±\farcs0006, d=8.96± 0.05 pc) and perturbation orbit for Gl791.2A. This distance and an apparent brightness (V=13.06 ±.02, Leggett 1992. ApJS, 82, 351) imply M_V= 13.30 for component A. From the recent lower main sequence Mass-Luminosity relationship of Henry et al. (1998, ApJ, in press) M_A=0.16 ±0.017 M_\sun. The perturbation orbit for Gl 791.2A yields a sum of masses M_A + M_B = 0.256 ±0.003 M_\sun, hence for GL 791.2B, M_B = 0.093±.017 M_\sun, where virtually all the error is from the M-L fit. \begin{center} \begin{tabular}{|c|c|} \hline \multicolumn{2}{|c|}{Gl 791.2A Perturbation Orbit} \\ \hline\hline \multicolumn{1}{|c|}{Parameter} & \multicolumn{1}{c|}{Value} \\ \hline \alpha&0\farcs0332±0\farcs0005\\ P&1.471±0.008 y\\

e&0.534±0.021\\

\hline \end{tabular} \end{center}

The FGS 3 fringe-scanning (TRANS) mode provides \Delta V = 3.56±.23, allowing us to place Gl 791.2B in a sparsely-populated region of the lower main sequence mass-luminosity relationship, one of the few points defining the lower main sequence mass-luminosity relationship.

We discuss a preliminary parallax and perturbation orbit for Gl 623. Observations covering a complete period are required to improve those results.

This research had the support of NASA Grants NAS5-1603, GO-06036.01-94A, and GO-07491.01-97A.

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