TY - JOUR
T1 - Starspot rotation rates versus activity cycle phase
T2 - Butterfly diagrams of Kepler stars are unlike that of the Sun ?
AU - Nielsen, M. B.
AU - Gizon, L.
AU - Cameron, R. H.
AU - Miesch, M.
N1 - Funding Information:
Acknowledgements. The authors would like to thank Timo Reinhold, Alexander Shapiro, and Gibor Basri for useful discussions. MBN and LG acknowledge support from NYUAD Institute grant G1502. This paper includes data collected by the Kepler mission. Funding for the Kepler mission is provided by the NASA Science Mission directorate. Data presented in this paper were obtained from the Mikulski Archive for Space Telescopes (MAST). STScI is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. Support for MAST for non-HST data is provided by the NASA Office of Space Science via grant NNX09AF08G and by other grants and contracts.
Publisher Copyright:
© ESO 2019.
PY - 2019/2/1
Y1 - 2019/2/1
N2 - Context. During the solar magnetic activity cycle the emergence latitudes of sunspots change, leading to the well-known butterfly diagram. This phenomenon is poorly understood for other stars since starspot latitudes are generally unknown. The related changes in starspot rotation rates caused by latitudinal differential rotation can, however, be measured. Aims. Using the set of 3093 Kepler stars with measured activity cycles, we aim to study the temporal change in starspot rotation rates over magnetic activity cycles, and how this relates to the activity level, the mean rotation rate of the star, and its effective temperature. Methods. We measured the photometric variability as a proxy for the magnetic activity and the spot rotation rate in each quarter over the duration of the Kepler mission. We phase-folded these measurements with the cycle period. To reduce random errors, we performed averages over stars with comparable mean rotation rates and effective temperature at fixed activity-cycle phases. Results. We detect a clear correlation between the variation of activity level and the variation of the starspot rotation rate. The sign and amplitude of this correlation depends on the mean stellar rotation and - to a lesser extent - on the effective temperature. For slowly rotating stars (rotation periods between 15 - 28 days), the starspot rotation rates are clearly anti-correlated with the level of activity during the activity cycles. A transition is observed around rotation periods of 10 - 15 days, where stars with an effective temperature above 4200 K instead show positive correlation. Conclusions. Our measurements can be interpreted in terms of a stellar "butterfly diagram", but these appear different from that of the Sun since the starspot rotation rates are either in phase or anti-phase with the activity level. Alternatively, the activity cycle periods observed by Kepler are short (around 2.5 years) and may therefore be secondary cycles, perhaps analogous to the solar quasi-biennial oscillations.
AB - Context. During the solar magnetic activity cycle the emergence latitudes of sunspots change, leading to the well-known butterfly diagram. This phenomenon is poorly understood for other stars since starspot latitudes are generally unknown. The related changes in starspot rotation rates caused by latitudinal differential rotation can, however, be measured. Aims. Using the set of 3093 Kepler stars with measured activity cycles, we aim to study the temporal change in starspot rotation rates over magnetic activity cycles, and how this relates to the activity level, the mean rotation rate of the star, and its effective temperature. Methods. We measured the photometric variability as a proxy for the magnetic activity and the spot rotation rate in each quarter over the duration of the Kepler mission. We phase-folded these measurements with the cycle period. To reduce random errors, we performed averages over stars with comparable mean rotation rates and effective temperature at fixed activity-cycle phases. Results. We detect a clear correlation between the variation of activity level and the variation of the starspot rotation rate. The sign and amplitude of this correlation depends on the mean stellar rotation and - to a lesser extent - on the effective temperature. For slowly rotating stars (rotation periods between 15 - 28 days), the starspot rotation rates are clearly anti-correlated with the level of activity during the activity cycles. A transition is observed around rotation periods of 10 - 15 days, where stars with an effective temperature above 4200 K instead show positive correlation. Conclusions. Our measurements can be interpreted in terms of a stellar "butterfly diagram", but these appear different from that of the Sun since the starspot rotation rates are either in phase or anti-phase with the activity level. Alternatively, the activity cycle periods observed by Kepler are short (around 2.5 years) and may therefore be secondary cycles, perhaps analogous to the solar quasi-biennial oscillations.
KW - Methods: data analysis
KW - stars: activity
KW - stars: rotation
KW - starspots
KW - techniques: photometric
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U2 - 10.1051/0004-6361/201834373
DO - 10.1051/0004-6361/201834373
M3 - Article
AN - SCOPUS:85060990878
SN - 0004-6361
VL - 622
JO - Astronomy and Astrophysics
JF - Astronomy and Astrophysics
M1 - A85
ER -