Magnetic activity and differential rotation in the young Sun-like stars KIC 7985370 and KIC 7765135



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Fröhlich, H. E. and Frasca, A. and Catanzaro, G. and Bonanno, A. and Corsaro, E. and Molenda-Żakowicz, J. and Klutsch, A. and Montes Gutiérrez, David (2012) Magnetic activity and differential rotation in the young Sun-like stars KIC 7985370 and KIC 7765135. Astronomy & Astrophysics, 543 . ISSN 0004-6361

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Aims. We present a detailed study of the two Sun-like stars KIC 7985370 and KIC 7765135, to determine their activity level, spot distribution, and differential rotation. Both stars were previously discovered by us to be young stars and were observed by the NASA Kepler mission.
Methods. The fundamental stellar parameters (vsini, spectral type, T_eff, log g, and [Fe/H]) were derived from optical spectroscopy by comparison with both standard-star and synthetic spectra. The spectra of the targets allowed us to study the chromospheric activity based on the emission in the core of hydrogen Hα and Ca ii infrared triplet (IRT) lines, which was revealed by the subtraction of inactive templates. The high-precision Kepler photometric data spanning over 229 days were then fitted with a robust spot model. Model selection and parameter estimation were performed in a Bayesian manner, using a Markov chain Monte Carlo method.
Results. We find that both stars are Sun-like (of G1.5 V spectral type) and have an age of about 100–200 Myr, based on their lithium content and kinematics. Their youth is confirmed by their high level of chromospheric activity, which is comparable to that displayed by the early G-type stars in the Pleiades cluster. The Balmer decrement and flux ratio of their Ca ii-IRT lines suggest that the formation of the core of these lines occurs mainly in optically thick regions that are analogous to solar plages. The spot model applied to the Kepler photometry requires at least seven persistent spots in the case of KIC 7985370 and nine spots in the case of KIC 7765135 to provide a satisfactory fit to the data. The assumption of the longevity of the star spots, whose area is allowed to evolve with time, is at the heart of our spot-modelling approach. On both stars, the surface differential rotation is Sun-like, with the high-latitude spots rotating slower than the low-latitude ones. We found, for both stars, a rather high value of the equator-to-pole differential rotation (dΩ ≈ 0.18 rad d^-1), which disagrees with the predictions of some mean-field models of differential rotation for rapidly rotating stars. Our results agree instead with previous works on solar-type stars and other models that predict a higher latitudinal shear, increasing with equatorial angular velocity, that can vary during the magnetic cycle.

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© ESO, 2012.
We would like to thank the Kepler project and the team who created the MAST Kepler web site and search interfaces. We also thank an anonymous referee for helpful and constructive comments. This work has been partly supported by the Italian Ministero dell'Istruzione, Universita e Ricerca (MIUR), which is gratefully acknowledged. JMZ acknowledges the Polish Ministry grant No. NN203 405139. D.M. and A.K. acknowledge the Universidad Complutense de Madrid (UCM), the Spanish Ministerio de Ciencia e Innovación (MCINN) under grants AYA2008-000695 and AYA2011-30147-C03-02, and the Comunidad de Madrid under PRICIT project S2009/ESP-1496 (AstroMadrid). This research made use of SIMBAD and VIZIER databases, operated at the CDS, Strasbourg, France. This publication makes use of data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation.

Uncontrolled Keywords:Pre-main-sequence; H-alpha emission; Chromospheric activity; V889 Herculis; G-dwarfs; Spectroscopic survey; Solar neighborhood; Standard stars; Dynamo models; Debris disks
Subjects:Sciences > Physics > Astrophysics
Sciences > Physics > Astronomy
ID Code:37779
Deposited On:20 May 2016 15:39
Last Modified:10 Dec 2018 15:05

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