VVV J165507.19-421755.5: A Nearby T Dwarf Hidden in the Galactic Plane

© 2022. The Author(s). Published by the American Astronomical Society. This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY), https://creativecommons.org/licenses/by/4.0/We present the discovery of VVV J165507.19−421755.5, a mid-T dwarf found through ongoing unWISE-based proper motion searches. A near-infrared spectrum of this object obtained with the NIRES instrument on the Keck II telescope indicates a spectral classification of T5. Using data from the VISTA Variables in the Via Lactea (VVV) catalog with a 9 year baseline, we measure a proper motion of ( μαcos(δ) , μ δ ) = (−631.0 ± 1.3, −315.0 ± 1.4) mas yr−1 and a trigonometric parallax of π abs = 66.0 ± 4.8 mas, corresponding to a distance of 15.2 ± 1.1 pc. The trigonometric parallax agrees well with our photometric distance estimate (16.1 −3.9+5.1 pc) assuming that VVV J165507.19−421755.5 is a single T5 dwarf. VVV J165507.19−421755.5 is a new member of the 20 parsec census.Peer reviewe

Discovery of a Mid-L Dwarf Companion to the L 262-74 System

We present the discovery of CWISE J151044.74$-$524923.5, a wide low-mass companion to the nearby ($\sim$24.7 pc) system L 262-74, which was identified through the Backyard Worlds: Planet 9 citizen science project. We detail the properties of the system, and we assess that this companion is a mid-L dwarf, which will need to be verified spectroscopically. With an angular separation of 74\farcs3, we estimate a projected physical separation of $\sim$1837 au from the central system

WISEA J041451.67–585456.7 and WISEA J181006.18–101000.5: The First Extreme T-type Subdwarfs?

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 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 among 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] ⩽ −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

A Wide Planetary Mass Companion Discovered through the Citizen Science Project Backyard Worlds: Planet 9

Through the Backyard Worlds: Planet 9 citizen science project we discovered a late-type L dwarf co-moving with the young K0 star BD+60 1417 at a projected separation of 37″ or 1662 au. The secondary - CWISER J124332.12+600126.2 (W1243) - is detected in both the CatWISE2020 and 2MASS reject tables. The photometric distance and CatWISE proper motion both match that of the primary within ∼1σ and our estimates for a chance alignment yield a zero probability. Follow-up near-infrared spectroscopy reveals W1243 to be a very red 2MASS (J-K s = 2.72), low surface gravity source that we classify as L6-L8γ. Its spectral morphology strongly resembles that of confirmed late-type L dwarfs in 10-150 Myr moving groups as well as that of planetary mass companions. The position on near- and mid-infrared color-magnitude diagrams indicates the source is redder and fainter than the field sequence, a telltale sign of an object with thick clouds and a complex atmosphere. For the primary we obtained new optical spectroscopy and analyzed all available literature information for youth indicators. We conclude that the Li i abundance, its loci on color-magnitude and color-color diagrams, and the rotation rate revealed in multiple TESS sectors are all consistent with an age of 50-150 Myr. Using our re-evaluated age of the primary and the Gaia parallax, along with the photometry and spectrum for W1243, we find T eff = 1303 ± 31 K, log g = 4.3 ± 0.17 cm s-2, and a mass of 15 ± 5 M Jup. We find a physical separation of ∼1662 au and a mass ratio of ∼0.01 for this system. Placing it in the context of the diverse collection of binary stars, brown dwarfs, and planetary companions, the BD+60 1417 system falls in a sparsely sampled area where the formation pathway is difficult to assess

Ross 19B: An Extremely Cold Companion Discovered via the Backyard Worlds: Planet 9 Citizen Science Project

Through the Backyard Worlds: Planet 9 citizen science project, we have identified a wide-separation (∼10', ∼9900 au projected) substellar companion to the nearby (∼17.5 pc), mid-M dwarf Ross 19. We have developed a new formalism for determining chance alignment probabilities based on the BANYAN Σ 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 ± 0.12) with an age of ${7.2}_{-3.6}^{+3.8}$ 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 ${500}_{-100}^{+115}$ K, and a mass in the range 15–40 MJup. 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

Redder than Red: Discovery of an Exceptionally Red L/T Transition Dwarf

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 ) _MKO color of 2.97 ± 0.03 mag and a J _MKO − W2 color of 4.93 ± 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 γ –T0 γ . We tentatively find that CWISE J0506+0738 is variable at 3–5 μ m based on multiepoch 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 β Pic moving group. A future parallax measurement will help to establish a more definitive moving group membership for this unusual object