ASAS-SN Discovery of a Likely Extreme Spotted Variable Star

ATel #11034; T. Jayasinghe, K. Z. Stanek, C. S. Kochanek, J. V. Shields, T. A. Thompson (OSU), G. Pojmanski (Warsaw Univ.), B. J. Shappee (Univ. of Hawaii), T. W.-S. Holoien (Carnegie Observatories), J. L. Prieto (Diego Portales; MAS), Subo Dong (KIAA-PKU)
on 6 Dec 2017; 19:47 UT
Credential Certification: Krzysztof Stanek (stanek.32@osu.edu)

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As part of an ongoing effort by the ASAS-SN project (Shappee et al. 2014; Kochanek et al. 2017) to characterize and catalog all bright variable stars, we report the discovery of a very unusual variable, most likely an extreme RS CVn-type spotted star. The source ASASSN-V J211256.97+313724.6 (position from Gaia, g~13.2, l=77.8149, b=-11.5179) was first observed by ASAS-SN on UT 2013-10-30. Presently, ASASSN-V J211256.97+313724.6 has more than ~1300 V-band data points.

ASAS-SN light curve of ASASSN-V J211256.97+313724.6 is clearly periodic, with a period of P=5.412 (2014 data). While the period and phase of maximum light appear not to change from 2014 through 2016, the amplitude of variability decays rapidly from Delta_V~0.58 in 2014 to Delta_V~0.24 in 2016. The amplitude of this source further drops to Delta_V~0.16 in 2017, with the fluctuations in brightness becoming less regular. This is shown in the ASAS-SN light curve , where data collected from Jan-June (July-Dec) during each year are shown in blue (red).

The star is red (J-H=0.609, H-K=0.146) and it is likely an RS CVn variable with a very large, dominant spot prior to 2017. Iwanek et al. (2017) report that chromospherically active stars in the Galactic bulge with periods near ~5 days show the largest variability amplitudes, although none of the stars in the Iwanek sample have I-band amplitudes greater than the V-band amplitude of ASASSN-V J211256.97+313724.6 in 2014.

We derive a similar period using ASAS data (Pojmanski et al. 2000) from 2006-2014. ASAS data, while likely somewhat blended with nearby stars due to larger pixel scale, also show evidence of high amplitude (Delta_V ~0.56 mag) variability, consistent with our results. This implies an extreme degree of stellar spots, persisting for a ~decade.

We would like to thank I. Soszynski, R. Beaton, M. Laszlo and J. Strader for useful discussions on this topic. We also 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-1515927, 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).