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

July 2020 • 2020ApJ...898...77S

Authors • Schneider, Adam C. • Burgasser, Adam J. • Gerasimov, Roman • Marocco, Federico • Gagné, Jonathan • Goodman, Sam • Beaulieu, Paul • Pendrill, William • Rothermich, Austin • Sainio, Arttu • Kuchner, Marc J. • Caselden, Dan • Meisner, Aaron M. • Faherty, Jacqueline K. • Mamajek, Eric E. • Hsu, Chih-Chun • Greco, Jennifer J. • Cushing, Michael C. • Kirkpatrick, J. Davy • Bardalez-Gagliuffi, Daniella • Logsdon, Sarah E. • Allers, Katelyn • Debes, John H. • Backyard Worlds: Planet 9 Collaboration

Abstract • 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] $\leqslant $ -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.

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Federico Marocco

Assistant Scientist