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P.J. Lowrance (UCLA Division of Astronomy)
We present the results of a coronagraphic survey of nearby, young stars with the Near-Infrared Camera and Multi-Object Spectrometer (NICMOS) on the Hubble Space Telescope to search for very low-mass companions. Substellar objects are predicted to be more luminous when younger, and therefore easier to detect. Using independently determined ages and distances for the target stars, the masses of detected secondaries can be ascertained from their infrared magnitudes and theoretical evolutionary tracks.
With the coronagraph on Camera 2 and the F160W filter (1.4-1.8um), we searched the region 0.4"- 4" around 50 stars ranging from spectral type A to M, concentrating on 30 G and K stars. Our detection limit was improved by the subtraction of two images taken within the same orbit, centered on the star, but with a telescope orientation that differed by 30 degrees. The lower mass limit of a detectable companion depends on the brightness and age of the target star as well as the angular separation between star and companion. For several of our targets that are 10-50 Myr old, this lower mass limit was as low as a few Jupiter masses.
The results include two low-mass stellar companions, two very likely brown dwarfs (located 100 AU and 180 AU from their primaries) and several brown dwarf candidates currently being followed up. We add these two likely substellar companions to the the three already known: GD165B (30 AU) (Becklin & Zuckerman 1988), GL229B (70 AU)(Nakajima et al 1995) and G196-3 (300AU)(Rebolo et al 1998). It has been conjectured that brown dwarfs with mass 10-80 MJup are rare at separations less than 5 AU and greater than 200 AU. Although subject to small number statistics, these results hint that the semi-major axis distribution of brown dwarf and late M dwarf secondaries may differ since the latter are found throughout a wide range of semi-major axes.
This work is supported in part by NASA grants NAG 5-4688 to UCLA and NAG 5-3042 to the University of Arizona NICMOS Instrument Design Team. This talk is based on observations obtained with the NASA/ESA Hubble Space Telescope at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc. under NASA contract NAS 5-26555.