Distances, Masses, Radii, and Metallicities of the Small Stars in the Solar Neighborhood

Abstract

Data from the NASA Kepler spacecraft indicate that small planets are common around the smallest main sequence stars (M dwarfs). Rocky planets transiting M dwarfs will be the best targets for atmospheric characterization with the next generation of scientific instruments. Ground based transit and radial velocity surveys, and the upcoming NASA TESS mission are expected to reveal the transiting terrestrial exoplanets that are nearest to the Sun. Understanding these worlds requires that we first understand their host stars. In this thesis, I present better estimates of the distances, masses, radii, and metallicities of these target stars.

I used data from the MEarth-North transit survey to obtain trigonometric distances to 1507 mid-to-late M dwarfs with a precision of 5 milliarcseconds. I use these distance measurements to obtain better estimates of the masses and radii of these stars than available from photometry alone, and prioritize targets to monitor at high cadence for transiting planets. I find that the M dwarf census in the northern hemisphere is mostly complete to a distance of 25 parsecs for stars of spectral type M5.5V and earlier, and mostly complete for stars earlier than M7.0V out to 20 parsecs.

I present calibrated MEarth optical photometry of 1844 MEarth-North targets with a typical precision of 1.5%. By combining these measurements with trigonometric distances, spectroscopic metallicities, and extant near infrared (NIR) magnitudes, I derive a color-magnitude-metallicity relation with a precision of 0.1 dex. I find that the median metallicity for Solar Neighborhood M dwarfs is [Fe/H] = -0.030 +/- 0.008, indistinguishable from the solar neighborhood G dwarfs.

I present the MEarth-South discovery of LP 661-13, a low-mass double lined eclipsing binary system with an orbital period of 4.7 days. I determine the component masses to be 0.3050 +/- 0.0056 and 0.1937 +/- 0.0027 M_sun and the radii to be 0.3192 +/- 0.0037 and 0.2159 +/- 0.0061 R_sun. While each component is marginally consistent with stellar models, the sum of the radii is well constrained and is inflated 5% compared to stellar models, which cannot be ascribed to metallicity or age effects. LP 661-13 joins the small sample of low-mass stars with precisely measured masses and radii that serves as a robust test of models of fully convective dwarf stars.

I present calibrated griz photometry of 150 of the MEarth-North target stars with a typical precision of 1%. I find that no combination of griz filters alone can reliably determine the metallicity of an M dwarf. However, interpolation in the (g-i, i-K) color plane can estimate the metallicity with a typical standard deviation of 0.1 dex. This precision is comparable to that from NIR spectroscopic methods that have been recently developed. Overlap between the upcoming Large Synoptic Survey Telescope (LSST) sources and the VISTA Hemisphere Survey sources will provide a sample of M dwarfs with estimated metallicities that can in turn be used as a chemical probe of the Milky Way Galaxy.