ASASSN-21nn: An Unusual Dimming Event in a Red Giant Star

ATel #14803; M. Rizzo Smith, T. Jayasinghe, K. Z. Stanek, C. S. Kochanek, D. Rowan, C. Christy, D. Desai, Z. Way, 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 21 Jul 2021; 16:37 UT
Distributed as an Instant Email Notice Transients
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-21nn (RA=18:49:15.54, DEC=-47:32:50.7, d~6.2 kpc, Teff=4075 K, BP-RP=1.73 mag, G=12.04, EDR3 source_id=6704449549694340352) 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 2014-04-29. Presently, ASASSN-21nn has more than ~2800 data points.

ASASSN-21nn has a quiescent mean magnitude of g~13.3 (V~12.6). The latest ASAS-SN photometry shows that ASASSN-21nn gradually faded from g~13.3 on UT 2021-07-05, to g~13.6 on UT 2021-07-15 and is still in its dimmed state as of UT 2021-07-19. Over the previous ~2800 days, the source had no significant drops in its ASAS-SN g or V-band brightness.

We detect a periodic signal in the ASAS-SN V and g-band data at ~241 days and ~233 days and peak-to-peak amplitudes of ~0.30 mags and ~0.25 mags respectively. There are no matches in Vizier or AAVSO to a previously known variable star.

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)