New Candidate Extreme T Subdwarfs from the Backyard Worlds: Planet 9 Citizen Science Project
astro-ph.SR
/ Authors
Aaron M. Meisner, Adam C. Schneider, Adam J. Burgasser, Federico Marocco, Michael R. Line, Jacqueline K. Faherty, J. Davy Kirkpatrick, Dan Caselden, Marc J. Kuchner, Christopher R. Gelino
and 22 more authors
Jonathan Gagne, Christopher Theissen, Roman Gerasimov, Christian Aganze, Chih-Chun Hsu, John P. Wisniewski, Sarah L. Casewell, Daniella C. Bardalez Gagliuffi, Sarah E. Logsdon, Peter R. M. Eisenhardt, Katelyn Allers, John H. Debes, Michaela B. Allen, Nikolaj Stevnbak Andersen, Sam Goodman, Leopold Gramaize, David W. Martin, Arttu Sainio, Michael C. Cushing, The Backyard Worlds, :, Planet 9 Collaboration
/ Abstract
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.