Direct Detection of Brown Dwarf Companions of Nearby Stars

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This thesis presents the first direct detection of a substellar companion of a star other than the Sun. This object, a called Gliese 229B, presented a unique opportunity to characterize low-temperature dwarfs for the first time. The discovery and initial spectrum of Gliese 229B show that the object must be substellar based on its intrinsic luminosity of 6.4 x 10-6 L☉ and its cool surface temperature, 900 K. Detailed study of Gliese 229B includes extensive photometric measurements from 0.5 to 12 µm, high signal-to-noise ratio spectroscopy from 0.84 to 5.0 µm and the detection of 0.1 yr -1 of orbital motion. These results are presented in Chapters 2 and 3. A detailed review of science leads to a complete and scientifically meaningful definition of the classes planet and brown dwarf in Chapter 1. After the discovery of Gliese 229B, which was found in a survey for companions of young stars, we began an extensive search for companions in orbit about all known stars within 8 pc of the Sun and with δ > -35°. The search includes optical coronagraphic and infrared direct imaging of these stars, conducted on the Palomar 60 and 200 telescopes respectively. The search was designed to find companions of each star without color bias. While the search revealed no other companions of these stars, it did uncover 6 new stellar companions. The sensitivity limits of the survey permit the detection of dwarfs up to four magnitudes fainter than Gliese 229B around 90% of the stars. The sensitivity is, however, not-uniform spatially or from star to star. This limits our ability to make strong statements about the prevalence of companions of nearby stars. The survey does have sensitivity to all stellar companions between 3 and 30 from the survey stars, however. Chapter 5 describes related work on very low-mass stars in the Pleiades star cluster. This optical spectroscopy involved trying to find a member of the Pleiades by identifying lithium absorption features. We revealed no members of the Pleiades but did discover two members with ages much younger than the age of the Pleiades (120 Myr). We provide an adequate explanation for the presence of these stars. The three appendices summarize work conducted in association with the Palomar Adaptive Optics System. The work includes (1) the detailed study and characterization of the deformable mirror, (2) invention of a fast calibration scheme for the deformable mirror and (3) the design and implementation of the fast steering mirror.