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 (firstname.lastname@example.org)
Subjects: Optical, Variables
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