Sunspot group braking and photometric period variations in spotted stars

Lanza, A. F.; Rodonò, M.; Zappalà, R. A.

doi:10.1007/3-540-55310-x_176

Cited by 23 publications

(26 citation statements)

References 8 publications

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“…Even drifts as small as 20 • −30 • per rotation period can be significantly detected (e.g., Lanza et al 2007;Silva-Valio & Lanza 2011). On the other hand, techniques based on periodogram analysis need larger phase shifts, up to 180 • , to resolve the peaks corresponding to the rotation frequencies of individual spots (Lanza et al 1993(Lanza et al , 1994. Therefore, they are prone to severe problems owing to the intrinsic spot evolution because such large shifts are produced only after a timescale comparable to 1/ΔΩ, where ΔΩ is the amplitude of DR.…”

Section: Light Curve Modelling and Analysismentioning

confidence: 99%

“…This limits the precision in the determination of the rotation period attainable with periodogram techniques, even in the case of a uniformly sampled time series (Lanza et al 1993(Lanza et al , 1994. On the other hand, if starspot intrinsic evolution is negligible, periodogram techniques coupled with a pre-whitening approach can be successful for estimating DR in solar-like stars by pinpointing the rotation frequencies of spots at different latitudes .…”

Section: Introductionmentioning

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: Light Curve Modelling and Analysismentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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

Lanza

Chagas

Medeiros

2014

A&A

Self Cite

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“…The 6174 data points are taken from 2009 May 2 to The double-peaked periodogram is a clear fingerprint of differential rotation, as shown, e.g., by Lanza et al (1994) in their simulation of light curves of differentially rotating spotted stars. They show that a Fourier analysis of uninterrupted time series of photometry with high precision ( ΔF F = 10 −5 −10 −6 , which is comparable with the precision of the Kepler data) may detect a Sun-like latitudinal differential rotation.…”

Section: Photometric Datamentioning

confidence: 99%

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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“…The appearance of two high-amplitude, closely-spaced frequencies is a natural signature of differentially rotating spots (Lanza et al 1993), especially if their second harmonics are also prominent, but higher harmonics are not (Clarke 2003). The frequency splitting Δ f ≈ 0.05 d −1 of HD 174648 suggests that the periods of rotation at the spot latitudes differ by ∼1%.…”

Section: Spots and Differential Rotation 331 Observational Constramentioning

confidence: 99%

CoRoT’s view on variable B8/9 stars: spots versus pulsations

Degroote

Acke

Samadi

et al. 2011

A&A

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Context. There exist few variability studies of stars in the region in the Hertzsprung-Russell diagram between the A and B-star pulsational instability strips. With the aid of the high precision continuous measurements of the CoRoT space satellite, low amplitudes are more easily detected, making a study of this neglected region worthwhile. Aims. We collected a small sample of B stars observed by CoRoT to determine the origin of the different types of variability observed. Methods. We combine literature photometry and spectroscopy to measure the fundamental parameters of the stars in the sample, and compare asteroseismic modelling of the light curves with (differentially rotating) spotted star models. Results. We found strong evidence for the existence of spots and differential rotation in HD 174648, and formulated hypotheses for their origin. We show that the distinction between pulsations and rotational modulation is difficult to make solely based on the light curve, especially in slowly rotating stars.

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Sunspot group braking and photometric period variations in spotted stars

Cited by 23 publications

References 8 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

CoRoT’s view on variable B8/9 stars: spots versus pulsations

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