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

July 2021 • 2021ApJ...915..120M

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

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, Teff ≲ 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 HW2 = 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.


IPAC Authors

Chris Gelino

Associate Scientist


Davy Kirkpatrick

Senior Scientist

Federico Marocco

Assistant Scientist