New Candidate Extreme T Subdwarfs from the Backyard Worlds: Planet 9 Citizen Science Project

Abstract

Schneider et al. presented the discovery of WISEA J041451.67−585456.7 and WISEA J181006.18−101000.5, which appear to be the first examples of extreme T-type subdwarfs (esdTs; metallicity −1 dex, T eff  1400 K). Here, we present new discoveries and follow-up of three T-type subdwarf candidates, with an eye toward expanding the sample of such objects with very low metallicity and extraordinarily high kinematics, properties that suggest membership in the Galactic halo. Keck/NIRES near-infrared spectroscopy of WISEA J155349.96 +693355.2, a fast-moving object discovered by the Backyard Worlds: Planet 9 citizen science project, confirms that it is a mid-T subdwarf. With H W2 = 22.3 mag, WISEA J155349.96+693355.2 has the largest W2 reduced proper motion among all spectroscopically confirmed L and T subdwarfs, suggesting that it may be kinematically extreme. Nevertheless, our modeling of the WISEA J155349.96+693355.2 near-infrared spectrum indicates that its metallicity is only mildly subsolar. In analyzing the J155349.96+693355.2 spectrum, we present a new grid of low-temperature, low-metallicity model atmosphere spectra. We also present the discoveries of two new esdT candidates, CWISE J073844.52−664334.6 and CWISE J221706.28−145437.6, based on their large motions and colors similar to those of the two known esdT objects. Finding more esdT examples is a critical step toward mapping out the spectral sequence and observational properties of this newly identified population.

Physical properties and trigonometric distance of the peculiar dwarf WISE J181005.5−101002.3

“…We compared the WISE1810 SED to the recent LOWZ models presented in Meisner et al (2021). The LOWZ models were built to be useful for studies of brown dwarfs and were computed at a low spectral resolution for a wide range of metallicities (between [Fe/H] = +1.0 and − 2.0 dex), effective temperatures (T eff between 500 and 1600 K), and surface gravities (log g between 3.5 and 5.5 in units of cm s −2 ) with steps of 0.5 dex, 50-100 K, and 0.5 dex, respectively (Meisner et al 2021). None of the LOWZ theoretical spectra is able to reproduce the entire observed SED of WISE1810.…”

“…Only a few metal-poor brown dwarf candidates have been announced so far in isolation (Burgasser et al 2003a;Murray et al 2011;Burningham et al 2014;Greco et al 2019;Meisner et al 2020bMeisner et al ,a, 2021Brooks et al 2022) or as companions (Burningham et al 2010;Scholz 2010;Pinfield et al 2012;Mace et al 2013) but all have metallicities likely higher than [Fe/H] = −0.7 dex. The first extremely metal-poor dwarf candidates ([Fe/H] ≤ −1.0 dex) located in the substellar transition zone have been recently identified in the Wide-Field Infrared Survey Explorer (WISE; Wright et al 2010) catalogue by Schneider et al (2020).…”

“…All of them, known as ultra-cool subdwarfs (Kirkpatrick et al 1997), enable the study of early star formation channels, nucleo-synthesis pathways, and Galactic assembly because they represent unique reservoirs of a pristine material that is somewhat processed, but with a far lower metallicity than the Sun, and produced by the Big Bang. They are also relevant objects to constrain atmospheric models at low temperatures and deficient chemical compositions (Rajpurohit et al 2014(Rajpurohit et al , 2016Zhang et al 2017bZhang et al , 2018Lodieu et al 2019b;Schneider et al 2020;Meisner et al 2020aMeisner et al , 2021Brooks et al 2022). Subd-warfs tend to exhibit thick disc or halo kinematics, high proper motions, and high heliocentric velocities (Gizis 1997), although it is not necessarily the case for all metal-poor ultra-cool dwarfs (e.g.…”

unTimely: a Full-sky, Time-domain unWISE Catalog

“…Because one of our primary science goals is to discover extremely faint and fast-moving objects, we ran the unTimely cataloging with lower than usual detection thresholds: S/N = 4.0 (2.5) in W1 (W2). We pushed the threshold so low in W2 because we suspect that the most extreme as-yet overlooked objects in the solar neighborhood are probably quite cold (e.g., Y dwarfs and late-T/Y subdwarfs; Burgasser et al 2005;Cushing et al 2011;Davy Kirkpatrick et al 2011;Luhman et al 2011;Zhang et al 2019;Schneider et al 2020;Meisner et al 2021c;Kirkpatrick et al 2021;Lodieu et al 2022) and hence would emit more flux at W2 than W1. Having chosen the S/N = 2.5 threshold for W2, we then selected a corresponding W1 threshold (S/N = 4.0) that yielded a source density roughly matching that obtained in W2 given its 2.5σ threshold.…”

Overview of the DESI Milky Way Survey

“…Our knowledge of the coldest (late T and Y type) brown dwarfs (350 T eff 500 K) is similarly restricted to this volume (Kirkpatrick et al 2019;Kota et al 2022). In addition to the Gaia 100 pc sample, a DESI secondary program will target fast-moving high proper motion identified using combinations of multi-epoch data from SDSS, the NOIRLab Source Catalog (Nidever et al 2021), and WISE data (e.g., Meisner et al 2021). These data will sample the faint end of the brown dwarf luminosity function, and provide better constraints on the low-mass end of the initial mass function.…”