Ultracool dwarf benchmarks with \emph{Gaia} primaries

We explore the potential of \emph{Gaia} for the field of benchmark ultracool/brown dwarf companions, and present the results of an initial search for metal-rich/metal-poor systems. A simulated population of resolved ultracool dwarf companions to \emph{Gaia} primary stars is generated and assessed. Of order $\sim$24,000 companions should be identifiable outside of the Galactic plane ($|b| > 10\,$deg) with large-scale ground- and space-based surveys including late M, L, T, and Y types. Our simulated companion parameter space covers $0.02 \le M/M_{\odot} \le 0.1$, $0.1 \le {\rm age/Gyr} \le 14$, and $-2.5 \le {\rm [Fe/H]} \le 0.5$, with systems required to have a false alarm probability $<10^{-4}$, based on projected separation and expected constraints on common-distance, common-proper motion, and/or common-radial velocity. Within this bulk population we identify smaller target subsets of rarer systems whose collective properties still span the full parameter space of the population, as well as systems containing primary stars that are good age calibrators. Our simulation analysis leads to a series of recommendations for candidate selection and observational follow-up that could identify $\sim$500 diverse \emph{Gaia} benchmarks. As a test of the veracity of our methodology and simulations, our initial search uses UKIDSS and SDSS to select secondaries, with the parameters of primaries taken from Tycho-2, RAVE, LAMOST and TGAS. We identify and follow-up 13 new benchmarks. These include M8-L2 companions, with metallicity constraints ranging in quality, but robust in the range $-0.39 \le {\rm [Fe/H]} \le +0.36$, and with projected physical separation in the range $0.6\,<\,s/{\rm kau}\,<76$. Going forward, \emph{Gaia} offers a very high yield of benchmark systems, from which diverse sub-samples may be able to calibrate a range of foundational ultracool/sub-stellar theory and observation.