AAS Meeting #193 - Austin, Texas, January 1999
Session 102. Binary Stars
Display, Saturday, January 9, 1999, 9:20am-4:00pm, Exhibit Hall 1

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[102.07] VA351 - A Quadruple Hyad Containing a White Dwarf?

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

VA351 = H346 = GH7-203 [ V = 13.2, B-V = 1.5, M5V; RA = 04:25:13.2, DEC = 17:16:06 (2000)] is by kinematic, photometric, and spectroscopic evidence a member of the Hyades cluster. It was first resolved with HST-FGS3 into two near-equal components of 51 mas separation in a search for binaries among faint Hyades cluster members (Franz et al. 1994, BAAS 26, 929). Orbital analysis based upon twelve FGS3 measures now available shows compelling evidence of third-body motion with a period p = 0.5692 {±} 0.0041 yr and a semimajor axis {\alpha} = 0.0044 {±} 0.0006 arcsec. The dynamical mass derived for the system on the basis of the orbit of its resolved components (P = 5.263 {±} 0.112 yr; a = 0.0820 {±} 0.0016 arcsec) and the Hipparcos mean cluster distance d = 46.34 pc is 1.98 M_{\odot}. Assuming that VA351 lies at the cluster boundary nearest us at a tidal radius of 10pc (Brown et al. 1997, ESA Symp. "Hipparcos", ESA SP-402), we can lower its total dynamic mass to 0.96 M_{\odot}. However, the spectroscopic and photometric properties of VA351 are incompatible with such a small distance. Assuming the true mass lies between these two values, we will demonstrate that it cannot be acccounted for by plausible triple systems. A hierarchical quadruple system, on the other hand, consisting of one close pair of unequal M-dwarfs and of a second pair containing an M-dwarf in near-contact with a white dwarf, can readily yield a total mass within the stated range. Each of these two pairs represents one component of the resolved orbital system. We will show that the pair of M-dwarfs easily accounts for the observed perturbation, while the near-contact MD/WD pair is almost certainly responsible for observed H{\alpha} emission variability on a time scale of hours.

This work is based on observation 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.

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