Empirical, Accurate Masses and Radii of Single Stars with TESS and Gaia

We present a methodology for the determination of empirical masses of single stars through the combination of three direct observables with Gaia and TESS: (i) the surface gravity via granulation-driven variations in the TESS light curve, (ii) the bolometric flux at Earth via the broadband spectral energy distribution, and (iii) the distance via the Gaia parallax. We demonstrate the method using 525 Kepler stars for which these measures are available in the literature, and show that the stellar masses can be measured with this method to a precision of $\sim$25\%, limited by the surface-gravity precision of the granulation "flicker" method ($\sim$0.1~dex) and by the parallax uncertainties ($\sim$10\% for the Kepler sample). We explore the impact of expected improvements in the surface gravity determinations---through the application of granulation background fitting and the use of recently published granulation-metallicity relations---and improvements in the parallaxes with the arrival of the Gaia second data release. We show that the application of this methodology to stars that will be observed by TESS should yield radii good to a few percent and masses good to $\approx$10\%. Importantly, the method does not require the presence of an orbiting, eclipsing, or transiting body, nor does it require spatial resolution of the stellar surface. Thus we can anticipate the determination of fundamental, accurate stellar radii and masses for hundreds of thousands of bright single stars---across the entire sky and spanning the Hertzsprung-Russell diagram---including those that will ultimately be found to host planets.