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We present the discovery of WISEA J083011.95+283716.0, the first Y dwarf candidate identified through the Backyard Worlds: Planet 9 citizen science project. We identified this object as a red, fast-moving source with a faint \\(W2\\) detection in multi-epoch \\textit{AllWISE} and unWISE images. We have characterized this object with Spitzer Space Telescope and \\textit{Hubble Space Telescope} follow-up imaging. With mid-infrared detections in \\textit{Spitzer}'s \\emph{ch1} and \\emph{ch2} bands and flux upper limits in Hubble Space Telescope \\(F105W\\) and \\(F125W\\) filters, we find that this object is both very faint and has extremely red colors (\\(ch1-ch2 = 3.25\\pm0.23\\) mag, \\(F125W-ch2 \\geq 9.36\\) mag), consistent with a T\\(_{eff}\\sim300\\) K source, as estimated from the known Y dwarf population. A preliminary parallax provides a distance of \\(11.1^{+2.0}_{-1.5}\\) pc, leading to a slightly warmer temperature of \\(\\sim350\\) K. The extreme faintness and red Hubble Space Telescope and Spitzer Space Telescope colors of this object suggest it may be a link between the broader Y dwarf population and the coldest known brown dwarf WISE J0855\\(-\\)0714, and highlight our limited knowledge of the true spread of Y dwarf colors. We also present four additional Backyard Worlds: Planet 9 late-T brown dwarf discoveries within 30 pc.

We report the results of a spectroscopic survey of candidate T subdwarfs identified by the Backyard Worlds: Planet 9 program. Near-infrared spectra of 31 sources with red \\(J-W2\\) colors and large \\(J\\)-band reduced proper motions show varying signatures of subsolar metallicity, including strong collision-induced H\\(_2\\) absorption, obscured methane and water features, and weak K I absorption. These metallicity signatures are supported by spectral model fits and 3D velocities, indicating thick disk and halo population membership for several sources. We identify three new metal-poor T subdwarfs ([M/H] \\(\\lesssim\\) \\(-\\)0.5), CWISE J062316.19+071505.6, WISEA J152443.14\\(-\\)262001.8, and CWISE J211250.11-052925.2; and 19 new \"mild\" subdwarfs with modest metal deficiency ([M/H] \\(\\lesssim\\) \\(-\\)0.25). We also identify three metal-rich brown dwarfs with thick disk kinematics. We provide kinematic evidence that the extreme L subdwarf 2MASS J053253.46+824646.5 and the mild T subdwarf CWISE J113010.07+313944.7 may be part of the Thamnos population, while the T subdwarf CWISE J155349.96+693355.2 may be part of the Helmi stream. We define a metallicity classification system for T dwarfs that adds mild subdwarfs (d/sdT), subdwarfs (sdT), and extreme subdwarfs (esdT) to the existing dwarf sequence. We also define a metallicity spectral index that correlates with metallicities inferred from spectral model fits and iron abundances from stellar primaries of benchmark T dwarf companions. This expansion of the T dwarf classification system supports investigations of ancient, metal-poor brown dwarfs now being uncovered in deep imaging and spectroscopic surveys.

We present final Spitzer trigonometric parallaxes for 361 L, T, and Y dwarfs. We combine these with prior studies to build a list of 525 known L, T, and Y dwarfs within 20 pc of the Sun, 38 of which are presented here for the first time. Using published photometry and spectroscopy as well as our own follow-up, we present an array of color-magnitude and color-color diagrams to further characterize census members, and we provide polynomial fits to the bulk trends. Using these characterizations, we assign each object a \\(T_{\\rm eff}\\) value and judge sample completeness over bins of \\(T_{\\rm eff}\\) and spectral type. Except for types \\(\\ge\\) T8 and \\(T_{\\rm eff} <\\) 600K, our census is statistically complete to the 20-pc limit. We compare our measured space densities to simulated density distributions and find that the best fit is a power law (\\(dN/dM \\propto M^{-\\alpha}\\)) with \\(\\alpha = 0.6{\\pm}0.1\\). We find that the evolutionary models of Saumon & Marley correctly predict the observed magnitude of the space density spike seen at 1200K \\(< T_{\\rm eff} <\\) 1350K, believed to be caused by an increase in the cooling timescale across the L/T transition. Defining the low-mass terminus using this sample requires a more statistically robust and complete sample of dwarfs \\(\\ge\\)Y0.5 and with \\(T_{\\rm eff} <\\) 400K. We conclude that such frigid objects must exist in substantial numbers, despite the fact that few have so far been identified, and we discuss possible reasons why they have largely eluded detection.

We present the discovery of 13 new widely separated T dwarf companions to M dwarf primaries, identified using WISE/NEOWISE data by the CatWISE and Backyard Worlds: Planet 9 projects. This sample represents a \\(\\sim\\)60% increase in the number of known M+T systems, and allows us to probe the most extreme products of binary/planetary system formation, a discovery space made available by the CatWISE2020 catalog and the Backyard Worlds: Planet 9 effort. Highlights among the sample are WISEP J075108.79-763449.6, a previously known T9 thought to be old due to its SED, which we now find is part of a common-proper-motion pair with L 34-26 A, a well studied young M3 V star within 10 pc of the Sun; CWISE J054129.32-745021.5 B and 2MASS J05581644-4501559 B, two T8 dwarfs possibly associated with the very fast-rotating M4 V stars CWISE J054129.32-745021.5 A and 2MASS J05581644-4501559 A; and UCAC3 52-1038 B, which is among the widest late T companions to main sequence stars, with a projected separation of \\(\\sim\\)7100 au. The new benchmarks presented here are prime \\(JWST\\) targets, and can help us place strong constraints on formation and evolution theory of substellar objects as well as on atmospheric models for these cold exoplanet analogs.

We present Spitzer follow-up imaging of 95 candidate extremely cold brown dwarfs discovered by the Backyard Worlds: Planet 9 citizen science project, which uses visually perceived motion in multi-epoch WISE images to identify previously unrecognized substellar neighbors to the Sun. We measure Spitzer [3.6]-[4.5] color to phototype our brown dwarf candidates, with an emphasis on pinpointing the coldest and closest Y dwarfs within our sample. The combination of WISE and Spitzer astrometry provides quantitative confirmation of the transverse motion of 75 of our discoveries. Nine of our motion-confirmed objects have best-fit linear motions larger than 1\"/yr; our fastest-moving discovery is WISEA J155349.96+693355.2 (total motion ~2.15\"/yr), a possible T type subdwarf. We also report a newly discovered wide-separation (~400 AU) T8 comoving companion to the white dwarf LSPM J0055+5948 (the fourth such system to be found), plus a candidate late T companion to the white dwarf LSR J0002+6357 at 5.5' projected separation (~8,700 AU if associated). Among our motion-confirmed targets, five have Spitzer colors most consistent with spectral type Y. Four of these five have exceptionally red Spitzer colors suggesting types of Y1 or later, adding considerably to the small sample of known objects in this especially valuable low-temperature regime. Our Y dwarf candidates begin bridging the gap between the bulk of the Y dwarf population and the coldest known brown dwarf.

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 \\(\\sim\\)3,600 individual star formation products useful in measuring the initial mass function across the stellar (\\(<8 M_\\odot\\)) and substellar (\\(\\gtrsim 5 M_{Jup}\\)) regimes. Comparing our resulting initial mass function to previous measurements shows good agreement above 0.8\\(M_\\odot\\) and a divergence at lower masses. Our 20-pc space densities are best fit with a quadripartite power law, \\(\\xi(M) = dN/dM \\propto M^{-\\alpha}\\) with long-established values of \\(\\alpha = 2.3\\) at high masses (\\(0.55 < M < 8.00 M_\\odot\\)) and \\(\\alpha = 1.3\\) at intermediate masses (\\(0.22 < M < 0.55 M_\\odot\\)), but at lower masses we find \\(\\alpha = 0.25\\) for \\(0.05 < M <0.22 M_\\odot\\) and \\(\\alpha = 0.6\\) for \\(0.01 < M < 0.05 M_\\odot\\). 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 0.41 \\(M_\\odot\\).

Chemically mediated plant–herbivore interactions contribute to the diversity of terrestrial communities and the diversification of plants and insects. While our understanding of the processes affecting community structure and evolutionary diversification has grown, few studies have investigated how trait variation shapes genetic and species diversity simultaneously in a tropical ecosystem. We investigated secondary metabolite variation among subpopulations of a single plant species, Piper kelleyi (Piperaceae), using high-performance liquid chromatography (HPLC), to understand associations between plant phytochemistry and host-specialized caterpillars in the genus Eois (Geometridae: Larentiinae) and associated parasitoid wasps and flies. In addition, we used a genotyping-by-sequencing approach to examine the genetic structure of one abundant caterpillar species, Eois encina, in relation to host phytochemical variation. We found substantive concentration differences among three major secondary metabolites, and these differences in chemistry predicted caterpillar and parasitoid community structure among host plant populations. Furthermore, E. encina populations located at high elevations were genetically different from other populations. They fed on plants containing high concentrations of prenylated benzoic acid. Thus, phytochemistry potentially shapes caterpillar and wasp community composition and geographic variation in species interactions, both of which can contribute to diversification of plants and insects.