ASASSN-21ay: Discovery of an RGB Star Undergoing an Unusual, Long-Term Variation
ATel #14379; Z. Way, D. Desai, K. Z. Stanek, C. S. Kochanek, T. Jayasinghe, 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 8 Feb 2021; 19:50 UT
Credential Certification: Krzysztof Stanek (stanek.32@osu.edu)
Subjects: Optical, Variables
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 ``Brutus'', ``Cassius'', ``Payne-Gaposchkin'', ``Leavitt'', and ``Paczynski'' telescopes, we identified a stellar source undergoing an unusual, long-term dimming event. The source ASASSN-21ay (Gaia DR2: RA=16 50 33.71, DEC=+31 24 59.1, source_id=1313137361141189760, d=1600 pc, L=25.6 L_Sun, T_eff=4940 K) was first observed by ASAS-SN on UT 2013-12-31 at V~12.8 mag. Presently, ASASSN-21ay has ~2450 epochs of ASAS-SN data.
The ASAS-SN light curve shows the source slowly rising, reaching a broad peak of V~12.6 and g~13.0 mag in September 2018. Since then, ASASSN-21ay has steadily faded to g~13.6 on UT 2021-02-05. There are no matches in Vizier or the AAVSO catalogs to a known variable star. The star also showed long-term variability in the CRTS V-band data (Drake et al. 2009, ApJ, 696, 870) where it was initially observed at V~12.6 mag on UT 2005-7-6. It then brightened to have a broad peak in August 2009 and then returned to its original brightness by the time of the final CRTS epoch on UT 2013-10-22. The overall amplitude of ~0.15 mag was significantly smaller than the ~0.6 mag amplitude of the more recent changes.
The ASAS-SN data also show short term variations. After fitting and subtracting a quadratic polynomial to remove the long term trends, there is a strong signal at P~46 days in both the V and g-band data. We see no 46 day period in the CRTS data using the same procedure.
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 grants AST-1515927 and 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).
