Starspot rotation rates versus activity cycle phase

Butterfly diagrams of Kepler stars are unlike that of the Sun ?

Martin Bo Nielsen, Laurent Gizon, R. H. Cameron, M. Miesch

    Research output: Contribution to journalArticle

    Abstract

    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.

    Original languageEnglish (US)
    Article numberA85
    JournalAstronomy and Astrophysics
    Volume622
    DOIs
    StatePublished - Feb 1 2019

    Fingerprint

    starspots
    butterfly
    sun
    diagram
    diagrams
    stars
    rate
    temperature
    Kepler mission
    quasi-biennial oscillation
    solar oscillations
    stellar rotation
    cycles
    random errors
    sunspots
    sunspot

    Keywords

    • Methods: data analysis
    • stars: activity
    • stars: rotation
    • starspots
    • techniques: photometric

    ASJC Scopus subject areas

    • Astronomy and Astrophysics
    • Space and Planetary Science

    Cite this

    Starspot rotation rates versus activity cycle phase : Butterfly diagrams of Kepler stars are unlike that of the Sun ? / Nielsen, Martin Bo; Gizon, Laurent; Cameron, R. H.; Miesch, M.

    In: Astronomy and Astrophysics, Vol. 622, A85, 01.02.2019.

    Research output: Contribution to journalArticle

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    title = "Starspot rotation rates versus activity cycle phase: Butterfly diagrams of Kepler stars are unlike that of the Sun ?",
    abstract = "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.",
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    T1 - Starspot rotation rates versus activity cycle phase

    T2 - Butterfly diagrams of Kepler stars are unlike that of the Sun ?

    AU - Nielsen, Martin Bo

    AU - Gizon, Laurent

    AU - Cameron, R. H.

    AU - Miesch, M.

    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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