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M.W. McElwain (University of California Los Angeles), D.W. Koerner (University of Pennsylvania), J.D. Kirkpatrick (California Institute of Technology), I.N. Reid (Space Telescope Science Institute), P.R. Allen (University of Pennsylvania), G.R. Murphy (University of Maryland)
We present the results of a deep infrared search for substellar companions to low-luminosity dwarfs. K-band imaging of a sample of late M and L dwarfs was carried out at the Keck telescope down to a limiting magnitude of mK = 20. Companions were distinguished from background stars by common proper motion as identified in a double-epoch study with a 1 to 3 year timeline. We found no companions at separations of 1'' to 15'' in a sample of 90 targets. We are testing this result further with an IRTF survey of a larger sample over a wider field of view. Preliminary results of the latter are also presented here. Four close companions were detected in the Keck survey with luminosities similar to the primaries. Angular separations of 0.3'' to 0.5'' corresponded to linear separations of 5-10 AU, assuming trigonometric parallaxes recently obtained by USNO. This result accords well with the number of similar-luminosity companions detected in a recent HST survey of low-luminosity dwarfs (Reid et al. 2001). The detection rate of both studies falls short of that for earlier spectral types, but sensitivity to high luminosity contrast was reduced at these separations. High-contrast companions may in fact be abundant at the shorter separations. Thus we can conclude only that companions to low-luminosity dwarfs are absent at the separations for which they are most abundant in earlier spectral types (~30 AU for G dwarfs). This signifies either a lower companion rate overall for low-luminosity dwarfs, or a separation distribution peaked closer to the primary.
The author(s) of this abstract have provided an email address for comments about the abstract: mcelwain@astro.ucla.edu