Session 44 - Double Stars.
Display session, Thursday, January 08
Exhibit Hall,

## [44.06] The Mass-Luminosity Relation at the End of the Main Sequence

T. Henry (Harvard-Smithsonian CfA), O. Franz, L. Wasserman (Lowell Obs.), G. F. Benedict, P. Shelus (U. Texas), P. Ianna (U. Virginia), J. D. Kirkpatrick (IPAC Caltech), D. McCarthy (U. Arizona)

The empirical mass-luminosity relation at optical wavelengths is presented for objects with masses 0.07 to 0.20 M_ødot. Using Fine Guidance Sensor #3 on the Hubble Space Telescope (HST) in the transfer function scan mode, we have resolved several very low mass binaries for the first time at optical wavelengths. The observed multiple systems are within 20 parsecs, have orbital periods less than 20 years, and have at least one component with mass less than 20% that of the Sun. The systems included in this 20-20-20 Sample'' are relatively faint (typically V = 10 to 15) and have separations less than 1\arcsec, making them difficult targets for ground-based resolution efforts at optical wavelengths. Yet, these systems provide our only knowledge of the mass-luminosity relation near the end of the main sequence, and yield critical information about the transition region between stars and brown dwarfs.

The magnitude differences are measured to \pm0.10 mag or better in most cases. When combined with infrared speckle measurements, the components can now be characterized comprehensively from 0.5 to 2.2 \mum. Improved parallaxes from Hipparcos or HST, and orbital elements from the HST/infrared speckle effort (the first reliable orbits in many cases) are yielding masses accurate to 5%, something never before done for the lower main sequence. The optical mass-luminosity relation defined by the HST data reaches to M_V = 18.6, mass = 0.074 M_ødot, and provides a powerful empirical test for discriminating the lowest mass stars from high mass brown dwarfs at wavelengths shorter than 1\mum.