Spectroscopic and photometric studies of main sequence M stars and a search for late-type dwarfs in the solar vicinity.

Abstract

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.