The differential rotation of <i>ε</i> Eri from MOST data

Fröhlich, H.‐E.

doi:10.1002/asna.200710876

Cited by 29 publications

(25 citation statements)

References 13 publications

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“…8 that shows that the distribution of ΔP does not especially depend on i, thus allowing a robust estimation of the relative amplitude of the DR as ΔP/P, where P = (P 1 + P 2 )/2. A similar conclusion has been reached by Fröhlich (2007), who also applied a Bayesian spot modelling with different priors.…”

Section: Mcmc Analysis For Eridani and Comparison With Previous Resultssupporting

confidence: 79%

“…Then we seek the interval(s) with the best fit in terms of spots with a fixed area and DR, i.e., during which the impact of starspot evolution is the weakest and a significant DR signal appears to be present. Even if this interval covers only one or two rotations of the star, previous studies by, say, Croll et al (2006), Croll (2006), andFröhlich (2007) have demonstrated that spot modelling can extract a meaningful signal of DR. This is possible thanks to the sensitivity of spot modelling to the drift in longitude of individual spots produced by a latitudinal shear.…”

Section: Light Curve Modelling and Analysismentioning

confidence: 99%

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Measuring stellar differential rotation with high-precision space-borne photometry

Lanza

Chagas

Medeiros

2014

A&A

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Context. Stellar differential rotation is important for understanding hydromagnetic stellar dynamos, instabilities, and transport processes in stellar interiors, as well as for a better treatment of tides in close binary and star-planet systems. Aims. We introduce a method of measuring a lower limit to the amplitude of surface differential rotation from high-precision, evenly sampled photometric time series, such as those obtained by space-borne telescopes. It is designed to be applied to main-sequence late-type stars whose optical flux modulation is dominated by starspots.Methods. An autocorrelation of the time series was used to select stars that allow an accurate determination of starspot rotation periods. A simple two-spot model was applied together with a Bayesian information criterion to preliminarily select intervals of the time series showing evidence of differential rotation with starspots of almost constant area. Finally, the significance of the differential rotation detection and a measurement of its amplitude and uncertainty were obtained by an a posteriori Bayesian analysis based on a Monte Carlo Markov chain approach. We applied our method to the Sun and eight other stars for which previous spot modelling had been performed to compare our results with previous ones. Results. We find that autocorrelation is a simple method for selecting stars with a coherent rotational signal that is a prerequisite for successfully measuring differential rotation through spot modelling. For a proper Monte Carlo Markov chain analysis, it is necessary to take the strong correlations among different parameters that exist in spot modelling into account. For the planet-hosting star Kepler-30, we derive a lower limit to the relative amplitude of the differential rotation of ΔP/P = 0.0523± 0.0016. We confirm that the Sun as a star in the optical passband is not suitable for measuring differential rotation owing to the rapid evolution of its photospheric active regions. In general, our method performs well in comparison to more sophisticated and time-consuming approaches.

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Section: Mcmc Analysis For Eridani and Comparison With Previous Resultssupporting

confidence: 79%

Section: Light Curve Modelling and Analysismentioning

confidence: 99%

Measuring stellar differential rotation with high-precision space-borne photometry

Lanza

Chagas

Medeiros

2014

A&A

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“…A weak differential rotation (dΩ between 0.017 and 0.056 rad d −1 ) was found for Eri by Fröhlich (2007) from a Bayesian reanalysis of the MOST light curve (Croll 2006;Croll et al 2006). This is a mildly active star with the same spectral type as KIC 8429280 (K2 V), but with a considerably longer rotational period (P rot 11.2 days).…”

Section: Differential Rotationmentioning

confidence: 96%

Magnetic activity and differential rotation in the very young star KIC 8429280

Frasca

Fröhlich

Bonanno

et al. 2011

A&A

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Aims. We present a spectroscopic and photometric analysis of the rapid rotator KIC 8429280, discovered by ourselves as a very young star and observed by the NASA Kepler mission, designed to determine its activity level, spot distribution, and differential rotation. Methods. We use ground-based data, such as high-resolution spectroscopy and multicolor broad-band photometry, to derive stellar parameters (v sin i, spectral type, T eff , log g, and [Fe/H]), and we adopt a spectral subtraction technique to highlight the strong chromospheric emission in the cores of hydrogen Hα and Ca ii H&K and infrared triplet (IRT) lines. We then fit a robust spot model to the high-precision Kepler photometry spanning 138 days. Model selection and parameter estimation is performed in a Bayesian manner using a Markov chain Monte Carlo method. Results. We find that KIC 8429280 is a cool (K2 V) star with an age of about 50 Myr, based on its lithium content, that has passed its T Tau phase and is spinning up approaching the ZAMS on its radiative track. Its high level of chromospheric activity is clearly indicated by the strong radiative losses in Ca ii H&K and IRT, Hα, and Hβ lines. Furthermore, its Balmer decrement and the flux ratio of Ca ii IRT lines imply that these lines are mainly formed in optically-thick regions similar to solar plages. The analysis of the Kepler data uncovers evidence of at least seven enduring spots. Since the star's inclination is rather high -nearly 70 • -the assignment of the spots to either the northern or southern hemisphere is not unambiguous. We find at least three solutions with nearly the same level of residuals. Even in the case of seven spots, the fit is far from being perfect. Owing to the exceptional precision of the Kepler photometry, it is not possible to reach the noise floor without strongly enhancing the degrees of freedom and, consequently, the non-uniqueness of the solution. The distribution of the active regions is such that the spots are located around three latitude belts, i.e. around the star's equator and around ±(50 • -60 • ), with the high-latitude spots rotating slower than the low-latitude ones. The equator-to-pole differential rotation dΩ 0.27 rad d −1 is at variance with some recent mean-field models of differential rotation in rapidly rotating main-sequence stars, which predict a much smaller latitudinal shear. Our results are consistent with the scenario of a higher differential rotation, which changes along the magnetic cycle, as proposed by other models.

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“…Although deriving a best set of parameter values is comparatively easy, assigning mean values and error bars is computationally demanding, but becomes quite feasible with the Markov-Chain Monte-Carlo (MCMC) technique (see Press et al 2007), which is related to the famous Metropolis formalism (Metropolis et al 1953). This method has already been successful in analyzing high-precision photometric data from the MOST space satellite (Croll 2006;Fröhlich 2007).…”

Section: Bayesian Spot Modelingmentioning

confidence: 99%

On the differential rotation of CoRoT-2a

Fröhlich

Küker

Hatzes

et al. 2009

A&A

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We apply a robust spot model to fit the light curve of CoRoT-2a. The spots are assumed to be long-living and each has its own rotation period. A model with three circular spots reproduces the basic features of the longitude-time spot coverage map. One of the spots exhibits a noticeably lower rotational frequency than the other two. From the rotational frequencies of the three dark features we infer a differential rotation of above 0.11 rad/d, in rough agreement with theoretical models. Mean field models of angular momentum transport by convection and meridional flow lead to an equatorial rotational frequency that exceeds that of the poles by 0.09 rad/d. The spot decay corresponds to a turbulent magnetic diffusivity of (1.2 ± 0.1) × 10 13 cm 2 /s.

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The differential rotation of ε Eri from MOST data

Cited by 29 publications

References 13 publications

Measuring stellar differential rotation with high-precision space-borne photometry

Measuring stellar differential rotation with high-precision space-borne photometry

Magnetic activity and differential rotation in the very young star KIC 8429280

On the differential rotation of CoRoT-2a

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