ASAS-SN Discovery of a Spotted Giant Star with Extreme Variability
ATel #13593; T. Jayasinghe, K. Z. Stanek, C. S. Kochanek, Z. Way, P. Vallely, C. Basinger, 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)
on 26 Mar 2020; 16:03 UT
Credential Certification: Tharindu Jayasinghe (jayasinghearachchilage.1@osu.edu)
Subjects: Optical, Star, 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 "Cassius", "Payne-Gaposchkin", and "Paczynski" telescopes, we identified a giant star with extreme rotational variability. The ASAS-SN variable source ASASSN-V J180944.67-445127.5 (position from Gaia DR2, source_id=6720745308452211712, d~8.3 kpc) was first observed by ASAS-SN on UT 2015-07-19. The line of sight extinction to ASASSN-V J180944.67-445127.5 is A_V~0.47 mag (A_g~0.57 mag) (Schlafly & Finkbeiner, 2011). Assuming a distance of d~8.3 kpc (Bailer-Jones et al., 2018), this source has M_V~-3.4 mag (M_g~-3.2 mag) and a luminosity of ~1400 L_sun. Presently, ASASSN-V J180944.67-445127.5 has more than ~1520 data points.
The ASASSN-V J180944.67-445127.5 has a quiescent mean magnitude of g~12 (V~11.7). The ASAS-SN photometry for ASASSN-V J180944.67-445127.5 shows strong variability in the g-band data, with a maximum amplitude of A(g)~0.73 mag (diamond points). This degree of variability is highly unusual for spotted giant stars, and we do not find any rotational variables (classification probabilities > 0.95) with this amplitude in the ASAS-SN variable stars database (Jayasinghe et al., 2018). In the V-band data, ASASSN-V J180944.67-445127.5 varies less, with a maximum amplitude of A(V)~0.38 mag (square points). The ASAS-SN g-band light curve suggests a clear change in the variability amplitude after UT 2019-02-11, which is indicative of changes in the spot coverage on the giant. ASASSN-V J180944.67-445127.5 is a periodic, rotational variable , with a period of P=37.79 days based on the ASAS-SN g-band data.
We thank Dr. Marc Pinsonneault for useful discussions on this object. 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 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). For more information about the ASAS-SN project, see the ASAS-SN Homepage and the list of all ASAS-SN transients.
