We present the discovery of CWISE J050626.96\(+\)073842.4 (CWISE J0506\(+\)0738), an L/T transition dwarf with extremely red near-infrared colors discovered through the Backyard Worlds: Planet 9 ...citizen science project. Photometry from UKIRT and CatWISE give a \((J-K)_{\rm MKO}\) color of 2.97\(\pm\)0.03 mag and a \(J_{\rm MKO}-\)W2 color of 4.93\(\pm\)0.02 mag, making CWISE J0506\(+\)0738 the reddest known free-floating L/T dwarf in both colors. We confirm the extremely red nature of CWISE J0506\(+\)0738 using Keck/NIRES near-infrared spectroscopy and establish that it is a low-gravity late-type L/T transition dwarf. The spectrum of CWISE J0506\(+\)0738 shows possible signatures of CH\(_4\) absorption in its atmosphere, suggesting a colder effective temperature than other known, young, red L dwarfs. We assign a preliminary spectral type for this source of L8\(\gamma\)-T0\(\gamma\). We tentatively find that CWISE J0506\(+\)0738 is variable at 3-5 \(\mu\)m based on multi-epoch WISE photometry. Proper motions derived from follow-up UKIRT observations combined with a radial velocity from our Keck/NIRES spectrum and a photometric distance estimate indicate a strong membership probability in the \(\beta\) Pic moving group. A future parallax measurement will help to establish a more definitive moving group membership for this unusual object.
Through the Backyard Worlds: Planet 9 citizen science project, we have identified a wide-separation (\(\sim\)10', \(\sim\)9900 au projected) substellar companion to the nearby (\(\sim\)17.5 pc), ...mid-M dwarf Ross 19. We have developed a new formalism for determining chance alignment probabilities based on the BANYAN \(\Sigma\) tool, and find a 100% probability that this is a physically associated pair. Through a detailed examination of Ross 19A, we find that the system is metal-poor (Fe/H=\(-\)0.40\(\pm\)0.12) with an age of 7.2\(^{+3.8}_{-3.6}\) Gyr. Combining new and existing photometry and astrometry, we find that Ross 19B is one of the coldest known wide-separation companions, with a spectral type on the T/Y boundary, an effective temperature of 500\(^{+115}_{-100}\) K, and a mass in the range 15-40 \(M_{\rm Jup}\). This new, extremely cold benchmark companion is a compelling target for detailed characterization with future spectroscopic observations using facilities such as the Hubble Space Telescope or James Webb Space Telescope.
We present the discoveries of WISEA J041451.67-585456.7 and WISEA J181006.18-101000.5, two low-temperature (1200\(-\)1400 K), high proper motion T-type subdwarfs. Both objects were discovered via ...their high proper motion (\(>\)0.5 arcsec yr\(^{-1}\)); WISEA J181006.18-101000.5 as part of the NEOWISE proper motion survey and WISEA J041451.67-585456.7 as part of the citizen science project Backyard Worlds; Planet 9. We have confirmed both as brown dwarfs with follow-up near-infrared spectroscopy. Their spectra and near-infrared colors are unique amongst known brown dwarfs, with some colors consistent with L-type brown dwarfs and other colors resembling those of the latest-type T dwarfs. While no forward model consistently reproduces the features seen in their near-infrared spectra, the closest matches suggest very low metallicities (Fe/H \(\leq\) -1), making these objects likely the first examples of extreme subdwarfs of the T spectral class (esdT). WISEA J041451.67-585456.7 and WISEA J181006.18-101000.5 are found to be part of a small population of objects that occupy the "substellar transition zone," and have the lowest masses and effective temperatures of all objects in this group.
