Spitzer Follow-up of Extremely Cold Brown Dwarfs Discovered by the Backyard Worlds: Planet 9 Citizen Science Project

Abstract

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 multiepoch Wide-field Infrared Survey Explorer (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 −1 ; our fastest-moving discovery is WISEA J155349.96+693355.2 (μ ≈ 2 15 yr −1), 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 (∼8700 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.

The Field Substellar Mass Function Based on the Full-sky 20 pc Census of 525 L, T, and Y Dwarfs

“…New members of the 20 pc census have been announced not only by brown dwarf researchers specifically looking for examples (e.g., Kendall et al 2004) but also by researchers in unassociated fields who have serendipitously found others (e.g., Hall 2002;Thorstensen & Kirkpatrick 2003). New additions to the sample have been published as single-object papers (e.g., Ruiz et al 1997;Folkes et al 2007); as part of large photometric (e.g., Delfosse et al 1997;Lucas et al 2010), spectroscopic (e.g., Schmidt et al 2010), proper motion (e.g., Smith et al 2014;Meisner et al 2020aMeisner et al , 2020b, or parallax surveys (e.g., from Gaia: Reylé 2018; Scholz 2020); or as the result of dedicated searches for companions around higher mass stars (e.g., Freed et al 2003;Thalmann et al 2009) or around other brown dwarfs (e.g., Volk et al 2003;Gelino et al 2011). Construction of the census of the closest L, T, and Y dwarfs has been the effort of many dozens of lead authors presenting results in hundreds of publications.…”

“…Both the Backyard Worlds (Kuchner et al 2017) and CatWISE (Eisenhardt et al 2020) teams were continuing to identify new candidate late-T and Y dwarfs from WISE data as Spitzer hurtled toward its assigned decommissioning date in 2020 late-January. As chronicled in Meisner et al (2020aMeisner et al ( , 2020b, candidates lacking extant Spitzer photometry were added to Spitzer photometric programs 14034 (Meisner, PI), 14076 (Faherty, PI), and 14299 (Faherty, PI). As these new IRAC data became available, we used the new Spitzer photometry to predict a distance to each candidate using the M ch2 versus ch1−ch2 color 41 relation of Kirkpatrick et al (2019a).…”

“…Not all of the late-type candidates were included in programs 14034, 14076, or 14299, however, either because ch1−ch2 data already existed in the Spitzer Heritage Archive, mainly from our own, earlier programs (70062, 80109, or 11059), or because their discoveries occurred after the end of the Spitzer mission. These objects, which were selected by the community scientists of Backyard Worlds, team members of CatWISE, or both, were uncovered via the same selection criteria discussed in Meisner et al (2020aMeisner et al ( , 2020b and are listed in Table 1. Also included in this table are additional late-T and Y dwarf candidates, observed as part of Spitzer photometric program 14329 (Marocco, PI), that were discovered as part of the CatWISE2020 effort (Marocco et al 2020b) and have not previously been published.…”

“…Four objects were observed over the nights of 2018 September 1, October 27, and November 17, and 2019 October 28 and December 19 (UT) using the Near-Infrared Echellette Spectrometer (NIRES; see, e.g., Wilson et al 2004) at the W. M. Keck II telescope on Maunakea, Hawaii. Setup and reductions were identical to those described in Meisner et al (2020b) and covered a spectral range of 0.94-2.45 μm at a resolving power of ∼2700. Note that the spectra for Gaia 0412 −0734 were combined across nights.…”

“…Observations were done in high-throughput prism mode with the 0 6 slit, which gives a resolving power of R ≈ 450 covering 0.8-2.45 μm. Reductions followed the steps described in Meisner et al (2020b).…”