## Physics and Astronomy Dissertations

12-17-2015

Dissertation

#### Degree Name

Doctor of Philosophy (PhD)

#### Department

Physics and Astronomy

Todd J. Henry

David Charbonneau

Douglas R. Gies

Gary Hastings

Sebastien Lepine

Harold A. McAlister

Russel J. White

#### Abstract

This dissertation presents the results of a study to (1) determine the census of the nearby southern M dwarf stellar population via three types of distances and (2) determine the multiplicity rate of nearby M dwarfs using two different search methodologies.

The first part of this work reports three types of distance calculations (photographic, photometric, and trigonometric) for 1748 southern M dwarfs. Distances were estimated for 500 red dwarfs using photographic plate BRI magnitudes from SuperCOSMOS, while estimates were made for 667 stars using CCD VRI magnitudes. Both BRI and VRI were combined with 2MASS infrared JHK magnitudes. Distances for an additional 581 red dwarfs were derived from trigonometric parallaxes, 124 of which were measured for the first time during this work.

For the second portion of this thesis, an all-sky sample of 1122 M dwarfs, known via trigonometric parallaxes to lie within 25 pc of the Sun, was surveyed for stellar companions at separations of 2" to 600". I-band images using primarily the CTIO/SMARTS 0.9m and the Lowell 42in telescopes were obtained in order to search these systems for companions at separations of 2" to 180". A complementary reconnaissance of wider companions to 600" was also done via blinking SuperCOSMOS BRI images.

We find a stellar multiplicity fraction of 27.4 $\pm$ 1.3% for M dwarfs. Using this new gauge of M dwarf multiplicity near the end of the stellar main sequence, we calculate a multiplicity fraction of 30.1% for stellar systems of all types, implying that most systems are single. We find a peak in the separation distribution of the companions at 26 AU, i.e., distances on the scale of our Solar System, with a weak trend of smaller projected separations for lower mass primaries. A hint that M dwarf multiplicity may be a function of age/composition was revealed, with faster moving (and generally older) systems being multiple slightly less often. We calculate that at least 16% of M dwarf mass is made up of the stellar companions of multiple systems. Finally, we show that the mass function for M dwarfs increases to the end of the main sequence.

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