High signal-to-noise optical spectrophotometry of a sample of field subluminous B stars drawn largely from the Palomar Green ultraviolet excess survey is analyzed with a new grid of model atmospheres and synthetic spectra. The stellar effective temperatures, surface gravities, and photospheric helium abundances are determined simultaneously from a detailed analysis of hydrogen and helium absorption line profiles. The derived temperatures and gravities place the subluminous B stars in the theoretical H-R diagram along and bounded below by theoretical sequences of the zero-age extended horizontal branch, lending strong support to the hypothesis that these stars are composed of helium-burning cores of ∼0.5 $M(⊙) overlain by very thin layers of hydrogen (≲0.02 M(⊙)). Various scenarios for their past evolutionary history are examined in the context of their probable future evolution into white dwarfs of lower than average mass. The derived distances above the Galactic plane support a scale height for the population of z₀ = 285 pc, consistent with the identification of their progenitor stars as members of the old disk population. Radial velocities of sdB and sdO stars are analyzed to infer their kinematic characteristics. The results for the sdB stars are inconclusive, but for the sdO stars the results also are consistent with the population belonging to the older part of the thin disk.

Thesis (Ph. D.)--Ohio State University, 1976. / Includes vita. Includes bibliographical references (leaves 74-78).

Thesis--Wisconsin. / Vita. Includes bibliographical references (leaves 118-119).

We have completed a search for low luminosity companions, including high mass brown dwarfs, to all M dwarfs known within eight parsecs of the sun, and north of -25°. We found six new companions orbiting the survey stars. The masses of the six new secondaries fall between 0.39 and 0.05 M(⊙). Three of the new companions, G208-44B, GL 623B and LHS 1047B, and one previously known secondary in the survey, Ross 614B, are brown dwarf candidates with masses ∼80 Jupiters (0.08 M(⊙)), the dividing line between stars and brown dwarfs. In addition, we provide infrared photometry at J, H and K for all 99 survey members, and spectral types on standard system for half. Analysis of the entire sample indicates that 50% of the stars in the more distant half of the survey volume remain undetected, as is supported by the steadily growing M dwarf census over the last 45 years. The binary fraction of M dwarfs, 30-40%, is lower than that of earlier type main sequence stars, and there are more companions to M dwarfs found between 1 and 10 AU than in any other decade interval. We find that the luminosity function of the lowest mass stars is flat or rising to the end of the main sequence, and that the mass function undoubtedly rises to the stellar/substellar break. We illustrate that the resolution of close binaries is crucial if accurate luminosity and mass functions are to be determined. Finally, we estimate 0.02 M(⊙)/pc³ to be the amount of mass contributed by M dwarfs to the galactic mass. Based upon new mass-luminosity relations developed at infrared wavelengths using a sample of stars with well-determined masses between 1.2 and 0.08 M(⊙), we are able to define empirically the end of the main sequence. We present absolute magnitudes, colors and spectral types for objects at the theoretical lowest stellar mass. Using these relations, we conclude that a few brown dwarfs may have already been discovered. (Abstract shortened with permission of author.)

As any introductory astronomy student knows, M dwarfs are the most common stars in the Galaxy and are the faintest of the core hydrogen burners. A comprehensive study of these faint objects is crucial to our understanding of the stellar composition of the Galaxy and necessary for a more complete knowledge of the transition between main sequence M stars and their slightly less massive counterparts, the brown dwarfs, which never achieve hydrogen burning in their cores. In this thesis, a spectroscopic catalog of 125 K and M dwarfs is first presented. This catalog covers the wavelength range from 6300 to 9000 Å, near where these objects emit most of their light. Eight of these spectra, covering classes M2 through M9, are combined with infrared spectra from 0.9 to 1.5 μm to create a second catalog. The two sets of spectra are used to search for temperature-sensitive atomic lines and molecular bands, which are then used in fitting the observed spectra to a sequence of theoretical models. As a result, a new temperature scale for M dwarfs is determined, and this scale is more accurate than previous determinations which have depended on blackbody energy distributions. The sequence of spectra is also used to compare the spectrum of the brown dwarf candidate GD 165 B to known M dwarfs. Furthermore, the spectral catalog is used in an attempt to separate the spectra of faint companions from their M dwarf primaries in systems where the two objects are too close for conventional spectroscopy to resolve the individual components. A survey for faint M dwarfs is also launched using the data acquired through the CCD/Transit Instrument (CTI) on Kitt Peak, Arizona. Follow-up spectroscopy is presented for 133 of these objects, and several more very late M dwarfs are identified. This spectroscopy combined with photometric data from the CTI are used to construct a luminosity function for M dwarfs which is in excellent agreement with determinations from previous surveys. Finally, possible avenues for future work are discussed. These include spectroscopic follow-up of the reddest of Luyten's proper motion objects--the first results from which have uncovered, in just twelve observations, two objects of type M7 and one of type M8, among the coolest objects yet recognized. Future searches, such as an all-sky survey for objects of extremely high proper motion, are also outlined.

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