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Session 77 - Binary Stars.
Display session, Wednesday, January 15
Separate spectra have been acquired of the components of the low-mass binary L722-22A,B. Using the Hubble Space Telescope Faint Object Spectrograph in the same manner described in BAAS 27,1341 for Ross 614A,B a small aperture was placed on each star, excluding the light from the other. L722-22 was discovered to be a binary by Ianna (1988, AJ 95,1226) from a small photographic photocentric orbit found in parallax observations. The ground-based work indicated L722-22B might have a mass in the brown-dwarf range, at 0.06 M_\sun which motivated the FOS observations. However, current HST astrometric work indicates L722-22B is at the 0.1 M_\sun level (Taff, Hershey Space Telescope Astrometry Team 75th Meeting Report, Apr 1996).
Ground based CCD spectra of M dwarf standards have been provided to us by J. Davy Kirkpatrick in the 6300 to 8500Årange. Apart from the telluric lines the FOS spectra interpolate very closely into the ground-based series across this spectral range. A classification program has been written which defines a series of spectral interval ratios, does fits for the indices of the standards as a function of spectral subtype across the M3 to M7 range of standards, and inverts the fits for the four unknown spectra of Ross 614A,B and L722-22A,B. The internal formal error of the mean from the series of indices is a small fraction of a spectral subtype. The spectral types of L722-22A and B are found to be earlier by about 3/4 of a spectral subtype than Ross 614A and B, respectively.
The HST astrometry and spectroscopy yield points for these 4 binary members which lie in a very narrow locus in the mass-spectral type plane and imply that single stars of types dM6 and later, have masses less than 0.08 M_\sun, presumably substellar.
Support for this work was provided by NASA through grant number 06048 from 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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