Nearby Red Dwarfs and Their Dance Partners: Characterizing More Than 2000 Single and Multiple M Dwarfs Near the Sun

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

Distance Dependent Offsets between Parallaxes for Nearby Stars and Gaia DR1 Parallaxes

We use 612 single stars with previously published trigonometric parallaxes placing them within 25 pc to evaluate parallaxes released in Gaia's first data release (DR1). We find that the Gaia parallaxes are, on average, $0.24 \pm 0.02$ mas smaller than the weighted mean trigonometric parallax values for these stars in the solar neighborhood. We also find that the offset changes with distance out to 100 pc, in the sense that the closer the star, the larger the offset. We find no systematic trends in the parallax offsets with stellar $V$ magnitude, $V-K$ color, or proper motion. We do find that the offset is roughly twice as large for stars south of the ecliptic compared to those that are north.Comment: 14 pages, 2 figures, 3 tables, accepted for publication in ApJ Letter. The table 1 is available in its entirety in a machine-readable form in the online journa

Active Stars in the Spectroscopic Survey of Mid-to-Late M Dwarfs Within 15pc

We present results from the volume-complete spectroscopic survey of 0.1-0.3M$_\odot$ M dwarfs within 15pc. This work discusses the active sample without close binary companions, providing a comprehensive picture of these 123 stars with H${\alpha}$ emission stronger than -1\unicode{xC5}. Our analysis includes rotation periods (including 31 new measurements), H${\alpha}$ equivalent widths, rotational broadening, inclinations, and radial velocities, determined using high-resolution, multi-epoch spectroscopic data from the TRES and CHIRON spectrographs supplemented by photometry from TESS and MEarth. Using this volume-complete sample, we establish that the majority of active, low-mass M dwarfs are very rapid rotators: specifically, 74$\pm$4% have rotation periods shorter than 2 days, while 19$\pm$4% have intermediate rotation periods of 2-20 days, and the remaining 8$\pm$3% have periods longer than 20 days. Among the latter group, we identify a population of stars with very high H${\alpha}$ emission, which we suggest is indicative of dramatic spindown as these stars transition from the rapidly to slowly rotating modes. We are unable to determine rotation periods for six stars and suggest that some of the stars without measured rotation periods may be viewed pole-on, as such stars are absent from the distribution of inclinations we measure; this lack notwithstanding, we recover the expected isotropic distribution of spin axes. Our spectroscopic and photometric data sets also allow us to investigate activity-induced radial-velocity variability, which we show can be estimated as the product of rotational broadening and the photometric amplitude of spot modulation.Comment: Accepted for publication in AJ; 18 pages, 12 figures, 3 table

The Solar Neighborhood. XXXIV. A Search for Planets Orbiting Nearby M Dwarfs using Astrometry

Astrometric measurements are presented for seven nearby stars with previously detected planets: six M dwarfs (GJ 317, GJ 667C, GJ 581, GJ 849, GJ 876, and GJ 1214) and one K dwarf (BD $-$10 3166). Measurements are also presented for six additional nearby M dwarfs without known planets, but which are more favorable to astrometric detections of low mass companions, as well as three binary systems for which we provide astrometric orbit solutions. Observations have baselines of three to thirteen years, and were made as part of the RECONS long-term astrometry and photometry program at the CTIO/SMARTS 0.9m telescope. We provide trigonometric parallaxes and proper motions for all 16 systems, and perform an extensive analysis of the astrometric residuals to determine the minimum detectable companion mass for the 12 M dwarfs not having close stellar secondaries. For the six M dwarfs with known planets, we are not sensitive to planets, but can rule out the presence of all but the least massive brown dwarfs at periods of 2 - 12 years. For the six more astrometrically favorable M dwarfs, we conclude that none have brown dwarf companions, and are sensitive to companions with masses as low as 1 $M_{Jup}$ for periods longer than two years. In particular, we conclude that Proxima Centauri has no Jovian companions at orbital periods of 2 - 12 years. These results complement previously published M dwarf planet occurrence rates by providing astrometrically determined upper mass limits on potential super-Jupiter companions at orbits of two years and longer. As part of a continuing survey, these results are consistent with the paucity of super-Jupiter and brown dwarf companions we find among the over 250 red dwarfs within 25 pc observed longer than five years in our astrometric program.Comment: 18 pages, 5 figures, 4 tables, accepted for publication in A

Observations of Binary Stars with the Differential Speckle Survey Instrument. V. Toward an Empirical Metal-Poor Mass-Luminosity Relation

In an effort to better understand the details of the stellar structure and evolution of metal poor stars, the Gemini North telescope was used on two occasions to take speckle imaging data of a sample of known spectroscopic binary stars and other nearby stars in order to search for and resolve close companions. The observations were obtained using the Differential Speckle Survey Instrument, which takes data in two filters simultaneously. The results presented here are of 90 observations of 23 systems in which one or more companions was detected, and 6 stars where no companion was detected to the limit of the camera capabilities at Gemini. In the case of the binary and multiple stars, these results are then further analyzed to make first orbit determinations in five cases, and orbit refinements in four other cases. Mass information is derived, and since the systems span a range in metallicity, a study is presented that compares our results with the expected trend in total mass as derived from the most recent Yale isochrones as a function of metal abundance. These data suggest that metal-poor main-sequence stars are less massive at a given color than their solar-metallicity analogues in a manner consistent with that predicted from the theory