ASASSN-21co: A Candidate for a Giant Eclipsing Binary with a 12 Year Period

ATel #14436; Z. Way, K. Z. Stanek, C. S. Kochanek, T. Jayasinghe, D. Desai, T. A. Thompson (OSU), B. J. Shappee (Univ. of Hawaii), T. W.-S. Holoien (Carnegie Observatories), J. L. Prieto (Diego Portales; MAS), Subo Dong (KIAA-PKU), M. Stritzinger (Aarhus)
on 3 Mar 2021; 17:33 UT
Credential Certification: Krzysztof Stanek (stanek.32@osu.edu)

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During the ongoing All-Sky Automated Survey for SuperNovae (ASAS-SN, Shappee et al. 2014, Kochanek et al. 2017), using data from the quadruple 14-cm ``Cassius'', ``Payne-Gaposchkin'', and ``Paczynski'' telescopes, we identified a stellar source undergoing a deep dimming episode. The source ASASSN-21co (RA=18:17:51.52, DEC=-58:07:49.31, d=2800 pc, Teff=3725 K, BP-RP=2.22 mag, G=10.86, DR2 source_id=6647970630972147840) is bright and red in Gaia DR2 (Gaia Collaboration et al., 2018, A&A, 616, A1) leading it to be classified as a red giant in the TESS Input Catalog (Stassun, K. G., et al., 2019, AJ, 158, 138). It was first observed by ASAS-SN on UT 2014-05-17. Presently, ASASSN-21co has more than ~2400 data points.

ASASSN-21co has a quiescent mean magnitude of g~12.5 (V~11.7). The latest ASAS-SN photometry shows that ASASSN-21co faded from g~12.5 on UT 2021-02-08, to g~13.1 on UT 2021-03-01 and is currently in its dimmed state as of UT 2021-03-03. Over the previous ~2480 days, the source has no previous dimming in its ASAS-SN g or V-band brightness. However, there is a similar, probable eclipse in the ASAS public data (Pojmanski, G., 1997, Acta Astronomica, 47, 467) occurring around UT 2009-04-23 that lasted ~80 days. This would indicate an eclipse period of ~11.9 years.

We also see a possible detection of a low-amplitude periodic signal in the ASAS-SN g-band data at ~9.7 days. There are no matches in Vizier or AAVSO to a previously known variable star and the star is not variable in CRTS data (Drake et al. 2009, ApJ, 696, 870).

Follow-up observations of this unusual object are strongly encouraged.

We thank Las Cumbres Observatory and its staff for their continued support of ASAS-SN. ASAS-SN is funded in part by the Gordon and Betty Moore Foundation through grant GBMF5490 to the Ohio State University, NSF grant AST-1908570, the Mt. Cuba Astronomical Foundation, the Center for Cosmology and AstroParticle Physics (CCAPP) at OSU, and the Chinese Academy of Sciences South America Center for Astronomy (CASSACA).