Starspot simulations forKepler
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Abstract:
The NASA Kepler mission yields an unprecedented amount of data in the form of photometric light curves. Apart from valuable information on exoplanets and stellar pulsations, the light curves contain rotation signals from starspots crossing the stellar disk. These modulations of the light curves are modeled and 105 simulations are carried out to analyze and understand a similar analysis of the Kepler light curves. The periodogram is calculated for each light curve. Under the assumption that the main source of variability at periods >1 day is due to spots, the simulations show that the rotation period can be easily determined for spot lifetimes of 30–60 days, but becomes more unreliable for spots lifetimes of 10–20 days. The amplitude of the periodogram peaks appear to be only weakly dependent on changes in the size of the spots, while the width of the peaks shows no clear change with increasing spot lifetimes (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)Keywords:
Starspot
Stellar rotation
Periodogram
Rotation period
Solar rotation
Kepler-30 is a unique target to study stellar activity and rotation in a young solar-like star accompanied by a compact planetary system. We use about 4 years of high-precision photometry collected by the Kepler mission to investigate the fluctuations caused by photospheric convection, stellar rotation, and starspot evolution as a function of the timescale. Our main goal is to apply methods for the analysis of timeseries to find the timescales of the phenomena that affect the light variations. We correlate those timescales with periodicities in the star as well as in the planetary system. We model the flux rotational modulation induced by active regions using spot modelling and apply the MFDMA in standard and multiscale versions for analysing the behaviour of variability and light fluctuations that can be associated with stellar convection and the evolution of magnetic fields on timescales ranging from less than 1 day up to about 35 days. The light fluctuations produced by stellar activity can be described by the multifractal Hurst index that provides a measure of their persistence. The spot modeling indicates a lower limit to the relative surface differential rotation of $ΔΩ/Ω\sim 0.02\pm 0.01$ and suggests a short-term cyclic variation in the starspot area with a period of $\sim 34$ days, virtually close to the synodic period of 35.2 days of the planet Kepler-30b. By subtracting the two timeseries of the SAP and PDC Kepler pipelines, we reduce the rotational modulation and find a 23.1-day period close to the synodic period of Kepler-30c. This period also appears in the multifractal analysis as a crossover of the fluctuation functions associated with the characteristic evolutionary timescales of the active regions in Kepler-30 as confirmed by spot modelling. These procedures and methods may be greatly useful for analysing current TESS and future PLATO data.
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Rotation period
Stellar rotation
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Abstract Stellar rotation is crucial for studying stellar evolution, since it provides information about age, angular momentum transfer, and magnetic fields of stars. In the case of the Sun, due to its proximity, detailed observation of sunspots at various latitudes and longitudes allows a precise estimation of the solar rotation period and its differential rotation. Here, we present for the first time an analysis of stellar differential rotation using starspot transit mapping as a means of detecting differential shear in solar-type and M stars. The aim of this study is to investigate the relationship between rotational shear, ΔΩ, and both the star's effective temperature ( T eff ) and its average rotation period ( P ¯ ). We present differential rotation profiles derived from previously collected spot transit mapping data for 13 slowly rotating stars ( P rot ≥ 4.5 days), with spectral types ranging from M to F, which were observed by the Kepler and CoRoT satellites. Our findings reveal a significant negative correlation between rotational shear and the mean period of stellar rotation (correlation coefficient of −0.77), which may be an indicator of stellar age. On the other hand, a weak correlation was observed between differential rotation and the effective temperature of the stars. Overall, the study provides valuable insights into the complex relationship between stellar parameters and differential rotation, which may enhance our understanding of stellar evolution and magnetic dynamos.
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Solar rotation
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Phased flaring, or the periodic occurrence of stellar flares, may probe electromagnetic star-planet interaction (SPI), binary interaction, or magnetic conditions in spots. For the first time, we explore flare periodograms for a large sample of flare stars to identify periodicity due to magnetic interactions with orbiting companions, magnetic reservoirs, or rotational phase. Previous large surveys have explored periodicity at the stellar rotation period, but we do not assume periods must correspond with rotation in this work. Two min TESS light curves of 284 cool stars are searched for periods from 1-10 d using two newly-developed periodograms. Because flares are discrete events in noisy and incomplete data, typical periodograms are not well-suited to detect phased flaring. We construct and test a new Bayesian likelihood periodogram and a modified Lomb-Scargle periodogram. We find 6 candidates with a false-alarm probability below 1%. Three targets are >3-sigma detections of flare periodicity; the others are plausible candidates which cannot be individually confirmed. Periods range from 1.35 to 6.7 d and some, but not all, correlate with the stellar rotation period or its 1/2 alias. Periodicity from 2 targets may persist from TESS Cycle 1 into Cycle 3. The periodicity does not appear to persist for the others. Long-term changes in periodicity may result from the spot evolution observed from each candidate, which suggests magnetic conditions play an important role in sustaining periodicity.
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Starspot
Stellar rotation
Rotation period
Flare star
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Brightness variations due to dark spots on the stellar surface encode information about stellar surface rotation and magnetic activity. In this work, we analyze the Kepler long-cadence data of 26,521 main-sequence stars of spectral types M and K in order to measure their surface rotation and photometric activity level. Rotation-period estimates are obtained by the combination of a wavelet analysis and autocorrelation function of the light curves. Reliable rotation estimates are determined by comparing the results from the different rotation diagnostics and four data sets. We also measure the photometric activity proxy using the amplitude of the flux variations on an appropriate timescale. We report rotation periods and photometric activity proxies for about 60% of the sample, including 4431 targets for which McQuillan et al. did not report a rotation period. For the common targets with rotation estimates in this study and in McQuillan et al., our rotation periods agree within 99%. In this work, we also identify potential polluters, such as misclassified red giants and classical pulsator candidates. Within the parameter range we study, there is a mild tendency for hotter stars to have shorter rotation periods. The photometric activity proxy spans a wider range of values with increasing effective temperature. The rotation period and photometric activity proxy are also related, with being larger for fast rotators. Similar to McQuillan et al., we find a bimodal distribution of rotation periods.
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Starspot
Stellar rotation
Hertzsprung–Russell diagram
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Abstract Magnetic activity on stars manifests itself in the form of dark spots on the stellar surface, which cause modulations of a few percent in the light curve of the star as it rotates. When a planet eclipses its host star, it might cross in front of one of these spots, creating a “bump” in the transit light curve. By modeling these spot signatures, it is possible to determine the physical properties of the spots such as size, temperature, and location. In turn, monitoring of the spots’ longitude provides estimates of the stellar rotation and differential rotation. This technique was applied to the star Kepler-17, a solar–type star orbited by a hot Jupiter. The model yields the following spot characteristics: average radius of 49 ± 10 Mm, temperatures of 5100 ± 300 K, and surface area coverage of 6 ± 4%. The rotation period at the transit latitude, , occulted by the planet was found to be 11.92 ± 0.05 day, slightly smaller than the out-of-transit average period of 12.4 ± 0.1 day. Adopting a solar-like differential rotation, we estimated the differential rotation of Kepler-17 to be rd day −1 , which is close to the solar value of 0.050 rd day −1 , and a relative differential rotation of . Because Kepler-17 is much more active than our Sun, it appears that, for this star, larger rotation rate is more effective in the generation of magnetic fields than shear.
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Starspot
Stellar rotation
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Star (game theory)
Spots
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We present measurements of the rotation rates of individual starspots on the rapidly rotating young K0 dwarf AB Doradus, at six epochs between 1988 December and 1996 December. The equatorial rotation period of the star decreased from 0.5137 to 0.5129 days between 1988 December and 1992 January. It then increased steadily, attaining a value of 0.5133 days by 1996 December. The latitude dependence of the rotation rate mirrored the changes in the equatorial rotation rate. The beat period between the equatorial and polar rotation periods dropped from 140 days to 70 days initially, then rose steadily. The most rigid rotation, in 1988 December, occurred when the starspot coverage was at a maximum. The time-dependent part of the differential rotation is found to have Delta Omega / Omega ~ 0.004, which should alter the oblateness of the star enough to explain the period changes observed in several close binaries via the mechanism of Applegate (1992).
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Rotation period
Stellar rotation
Solar rotation
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Using a model based on the rotational modulation of the visibility of active regions, we analyse the high-accuracy CoRoT lightcurve of the active young star CoRoT102899501. Spectroscopic follow-up observations are used to derive its fundamental parameters. We compare its chromospheric activity level with a model of chrosmospheric activity evolution established by combining relationships between the R'HK index and the Rossby number with a recent model of stellar rotation evolution on the main sequence. We measure the spot coverage of the stellar surface as a function of time, and find evidence for a tentative increase from 5-14% at the beginning of the observing run to 13-29% 35 days later. A high level of magnetic activity on CoRoT102899501 is corroborated by a strong emission in the Balmer and Ca II HK lines (logR'HK ~ -4). The starspots used as tracers of the star rotation constrain the rotation period to 1.625+/-0.002 days and do not show evidence for differential rotation. The effective temperature (Teff=5180+/-80 K), surface gravity (logg=4.35+/-0.1), and metallicity ([M/H]=0.05+/-0.07 dex) indicate that the object is located near the evolutionary track of a 1.09+/-0.12 M_Sun pre-main sequence star at an age of 23+/-10 Myrs. This value is consistent with the "gyro-age" of about 8-25 Myrs, inferred using a parameterization of the stellar rotation period as a function of colour index and time established for the I-sequence of stars in stellar clusters. We conclude that the high magnetic activity level and fast rotation of CoRoT102899501 are manifestations of its stellar youth consistent with its estimated evolutionary status and with the detection of a strong Li I 6707.8 A absorption line in its spectrum. We argue that a magnetic activity level comparable to that observed on CoRoT102899501 could have been present on the Sun at the time of planet formation.
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Rotation period
Stellar rotation
Effective temperature
Surface gravity
Rossby number
Balmer series
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Stellar rotation is crucial for studying stellar evolution since it provides information about age, angular momentum transfer, and magnetic fields of stars. In the case of the Sun, due to its proximity, detailed observation of sunspots at various latitudes and longitudes allows the precise estimate of the solar rotation period and its differential rotation. Here, we present for the first time an analysis of stellar differential rotation using starspot transit mapping as a means of detecting differential shear in solar-type and M stars. The aim of this study is to investigate the relationship between rotational shear, $\Delta\Omega$, with both the star's effective temperature ($T_{\text{eff}}$) and average rotation period ($P_{\text{r}}$). We present differential rotation profiles derived from previously collected spot transit mapping data for 13 slowly rotating stars ($P_{\text{rot}} \geq 4.5$ days), with spectral types ranging from M to F, which were observed by the Kepler and CoRoT satellites. Our findings reveal a significant negative correlation between rotational shear and the mean period of stellar rotation (correlation coefficient of -0.77), which may be an indicator of stellar age. On the other hand, a weak correlation was observed between differential rotation and the effective temperature of the stars. Overall, the study provides valuable insights into the complex relationship between stellar parameters and differential rotation, which may enhance our understanding of stellar evolution and magnetic dynamos.
Starspot
Stellar rotation
Rotation period
Solar rotation
Sunspot
Stellar kinematics
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In this paper we present the results of a different approach in the study of the so-called rotation-activity connection, which is a well established correlation between rotation and magnetic activity at chromospheric and outer atmospheric levels. The present study concerns the photospheric level and was carried out by using V-band photometric light curve amplitudes as indicators of starspot coverage and of magnetic activity. A high degree of correlation between the envelope of maximum V-band light curve amplitudes and the rotation period is found for the active star members of young open clusters (IC 2602, IC 2391, Alpha Persei, Pleiades and Hyades), as well as for active field stars. This correlation shows a diffe rent behaviour in two different rotation period ranges. Moreover, some evidence of a possible activity "saturation"is found among the most rapidly rotating stars of the sample. Additional correlations between photospheric and other magnetic activity indicators in the chromosphere, transition region and corona are also investigated. The results presented here can be considered as an extension of the well established rotation-activity connection valid from the corona, transition region and chromosphere, down to the photosphere.
Starspot
Rotation period
Chromosphere
Stellar rotation
Photosphere
Pleiades
Corona (planetary geology)
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