Paper

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

Goodman, Sam, Line, Michael R, Hsu, Chih-Chun, Caselden, Dan, Faherty, Jacqueline K, Allers, Katelyn, Burgasser, Adam J, Casewell, Sarah L, Allen, Michaela B, Gagne, Jonathan, Planet 9 Collaboration, Cushing, Michael C, Meisner, Aaron M, Theissen, Christopher, Martin, David W, Daniella C Bardalez Gagliuffi, Aganze, Christian, Debes, John H, Logsdon, Sarah E, Eisenhardt, Peter R M, Gramaize, Leopold, Sainio, Arttu, Wisniewski, John P, Schneider, Adam C, The Backyard Worlds, Gelino, Christopher R, Kuchner, Marc J, Gerasimov, Roman, Marocco, Federico, J Davy Kirkpatrick, Nikolaj Stevnbak Andersen

2021

Overview

Schneider et al. (2020) 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.

• Publisher: Cornell University Library, arXiv.org
• Subject: Atmospheric models / Extrasolar planets / Galactic halos / Infrared analysis / Infrared spectra / Infrared spectroscopy / Kinematics / Low temperature / Metallicity / Near infrared radiation / Spectrum analysis