The solar neighborhood. XX. Discovery and characterization of 21 new nearby white dwarf systems

We present medium-resolution spectroscopy and multi-epoch VRI photometry for 21 new nearby (< 50 pc) white dwarf (WD) systems brighter than V similar to 17. Of the new systems, ten are DA ( including a wide double-degenerate system with two DA components), eight are DC, two are DZ, and one is DB. In addition, we include multi-epoch VRI photometry for 11 known WD systems that do not have trigonometric parallax determinations. Using model atmospheres relevant for various types of WDs ( depending on spectral signatures), we perform spectral energy distribution modeling by combining the optical photometry with the near-infrared (near-IR) JHK S from the Two Micron All-Sky Survey to derive physical parameters (i.e., T-eff and distance estimates). We find that 12 new and six known WD systems are estimated to be within the NStars and Catalog of Nearby Stars (CNS) horizons of 25 pc. Coupled with identical analyses of the 56 WD systems presented in Paper XIX of this series, a total of 20 new WD systems and 18 known WD systems are estimated to be within 25 pc. These 38 systems of the 88 total studied represent a potential 34% increase in the 25 pc WD population ( currently known to consist of 110 systems with trigonometric parallaxes of varying qualities). We continue an ongoing effort via Cerro Tololo Inter-American Observatory Parllax Investigation to measure trigonometric parallaxes for the systems estimated to be within 25 pc to confirm proximity and to further fill the incompleteness gap in the local WD population. Another 38 systems ( both new and known) are estimated to be between 25 and 50 pc and are viable candidates for ground-based parallax efforts aiming to broaden the horizon of interest.