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Abstract We conducted a search for new ultracool companions to nearby white dwarfs using multiple methods, including the analysis of colors and examination of images in both the optical and the infrared. Through this process, we identified 51 previously unrecognized systems with candidate ultracool companions. 31 of these systems are resolved in at least one catalog, and all but six are confirmed as comoving companions via common proper motion and consistent parallax measurements (when available). We have followed up four comoving companions with near-infrared spectroscopy and confirm their ultracool nature. The remaining twenty candidates are unresolved, but show clear signs of infrared excess which is most likely due to the presence of a cold, low-mass companion or a dusty circumstellar disk. Three of these unresolved systems have existing optical spectra that clearly show the presence of a cool stellar companion to the white dwarf primary via spectral decomposition. These new discoveries, along with our age estimates for the primary white dwarfs, will serve as valuable benchmark systems for future characterization of ultracool dwarfs. 

Abstract A complete accounting of nearby objects—from the highest-mass white dwarf progenitors down to low-mass brown dwarfs—is now possible, thanks to an almost complete set of trigonometric parallax determinations from Gaia, ground-based surveys, and Spitzer follow-up. We create a census of objects within a Sun-centered sphere of 20 pc radius and check published literature to decompose each binary or higher-order system into its separate components. The result is a volume-limited census of ∼3600individualstar formation products useful in measuring the initial mass function across the stellar (<8M_⊙) and substellar (≳5M_Jup) regimes. Comparing our resulting initial mass function to previous measurements shows good agreement above 0.8M_⊙and a divergence at lower masses. Our 20 pc space densities are best fit with a quadripartite power law, $\xi \left(M\right)=\mathit{dN}/\mathit{dM}\propto M^{-\alpha }$, with long-established values ofα= 2.3 at high masses (0.55 <M< 8.00M_⊙), andα= 1.3 at intermediate masses (0.22 <M< 0.55M_⊙), but at lower masses, we findα= 0.25 for 0.05 <M< 0.22M_⊙, andα= 0.6 for 0.01 <M< 0.05M_⊙. This implies that the rate of production as a function of decreasing mass diminishes in the low-mass star/high-mass brown dwarf regime before increasing again in the low-mass brown dwarf regime. Correcting for completeness, we find a star to brown dwarf number ratio of, currently, 4:1, and an average mass per object of 0.41M_⊙.