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ASAS-SN V-band Light Curve of Multi-Planet M-dwarf Host YZ Cet Reveals a Rotation Period of 68 Days

ATel #10643; T. Jayasinghe, K. Z. Stanek, C. S. Kochanek, T. W.-S. Holoien, J. V. Shields, T. A. Thompson (OSU), B. J. Shappee (Hubble Fellow, Carnegie Observatories), J. L. Prieto (Diego Portales; MAS), Subo Dong (KIAA-PKU)
on 15 Aug 2017; 16:02 UT
Credential Certification: Krzysztof Stanek (stanek.32@osu.edu)

Subjects: Optical, Planet, Variables

Referred to by ATel #: 10678

The ASAS-SN project (Shappee et al. 2014; Kochanek et al. 2017) provides all-sky, high-cadence variability data for objects with V-band magnitudes between ~8 and ~17. In a recent paper, Astudillo-Defru et al. (2017) studied a nearby (3.6pc) M-dwarf YZ Cet, which contains a system of (at least) three Earth-mass planets, and reported a very long rotation period for the planet host (~83 days). Data from the ASAS project (Pojmanski 1997) acquired between 2000 to 2009 were used in their analysis. Here we provide a contemporary, well sampled ASAS-SN V-band light curve for that star.

YZ Cet has a high RA/Dec proper motion of ~1.4 arcsec/yr (van Leeuwen 2007), and therefore moves considerably even in large-pixel ASAS-SN images (coordinates of YZ-Cet changed by ~7" over a 4-year period in our data). We retrieved an ASAS-SN light curve, allowing for that large proper motion, yielding this light curve of 854 V-band photometric measurements obtained between August 2013 and August 2017. A periodic signal, most likely due to rotational modulation, is clearly present in 366 measurements made after May 2016 (middle panel). An analysis of variance (AOV) periodogram shows a stellar rotation period of ~68.3 days, significantly shorter than ~83 days reported by Astudillo-Defru et al. (2017). There is no evidence for the 83 days period in ASAS-SN data. We also derive the best AoV period using the entire ASAS-SN light curve, which yields the same value of P=68.3 days. The phased light curve is shown in the bottom panel of this figure. ASAS-SN light curve of YZ Cet is available upon request.

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-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