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Register a new userNew Suns in the Cosmos V: Stellar rotation and multifractality in active textit{Kepler} stars
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Added by
Daniel Brito de Freitas
Publication date
2019
fields
Physics
and research's language is
English
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In the present study, high-precision time series photometry for the active emph{Kepler} stars is described in the language of multifractals. We explore the potential of using the rescaled range analysis ($R/S$) and multifractal detrended moving average analysis (MFDMA) methods to characterize the multiscale structure of the observed time series from a sample of $sim$40 000 active stars. Among these stars, 6486 have surface differential rotation measurement, whereas 1846 have no signature of differential rotation. As a result, the Hurst exponent ($H$) derived from both methods shows a strong correlation with the period derived from rotational modulation. In addition, the variability range $R_{var}$ reveals how this correlation follows a high activity ``line. We also verify that the $H$-index is an able parameter for distinguishing the different signs of stellar rotation that can exist between the stars with and without differential rotation. In summary, the results indicate that the Hurst exponent is a promising index for estimating photometric magnetic activity.
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In the present study, we investigate the multifractal nature of a long-cadence time series observed by the textit{Kepler} mission for a sample of 34 M dwarf stars and the Sun in its active phase. Using the Multifractal Detrending Moving Average algorithm (MFDMA), which enables the detection of multifractality in nonstationary time series, we define a set of multifractal indices based on the multifractal spectrum profile as a measure of the level of stellar magnetic activity. This set of indices is given by the ($A$,$Delta alpha$,$C$,$H$)-quartet, where $A$, $Delta alpha$ and $C$ are related to geometric features from the multifractal spectrum and the global Hurst exponent $H$ describes the global structure and memorability of time series dynamics. As a test, we measure these indices and compare them with a magnetic index defined as $S_{ph}$ and verify the degree of correlation among them. First, we apply the Poincare plot method and find a strong correlation between the $leftlangle S_{ph}rightrangle$ index and one of the descriptors that emerges from this method. As a result, we find that this index is strongly correlated with long-term features of the signal. From the multifractal perspective, the $leftlangle S_{ph}rightrangle$ index is also strongly linked to the geometric properties of the multifractal spectrum except for the $H$ index. Furthermore, our results emphasize that the rotation period of stars is scaled by the $H$ index, which is consistent with Skumanichs relationship. Finally, our approach suggests that the $H$ index may be related to the evolution of stellar angular momentum and a stars magnetic properties.
The present study reports the discovery of Sun-like stars, namely main-sequence stars with $T_{rm eff}$, $log g$ and rotation periods $P_{rot}$ similar to solar values, presenting evidence of surface differential rotation. An autocorrelation of the time series was used to select stars presenting photometric signal stability from a sample of 881 stars with light curves collected by the $Kepler$ space-borne telescope, in which we have identified 17 stars with stable signals. A simple two-spot model together with a Bayesian information criterion were applied to these stars in the search for indications of differential rotation; in addition, for all 17 stars, it was possible to compute the spot rotation period $P$, the mean values of the individual spot rotation periods and their respective colatitudes, and the relative amplitude of the differential rotation.
The present work reports on the discovery of three stars that we have identified to be rotating Sun-like stars, based on rotational modulation signatures inferred from light curves from the CoRoT missions Public Archives. In our analysis, we performed an initial selection based on rotation period and position in the Period--$T_{rm eff}$ diagram. This revealed that the stars CoRoT IDs 100746852, 102709980, and 105693572 provide potentially good matches to the Sun with similar rotation period. To refine our analysis, we applied a novel procedure, taking into account the fluctuations of the features associated to photometric modulation at different time intervals and the fractality traces that are present in the light curves of the Sun and of these New Sun candidates alike. In this sense, we computed the so-called Hurst exponent for the referred stars, for a sample of fourteen CoRoT stars with sub- and super-solar rotational periods, and for the Sun, itself, in its active and quiet phases. We found that the Hurst exponent can provide a strong discriminant of Sun-like behavior, going beyond what can be achieved with solely the rotation period itself. In particular, we find that CoRoT ID 105693572 is the star that most closely matches the solar rotation properties, as far as the latters imprints on light curve behavior is concerned. The stars CoRoT IDs 100746852 and 102709980 have significant smaller Hurst exponents than the Sun, notwithstanding their similarity in rotation periods.
In present paper, we investigate the multifractality signatures in hourly time series extracted from CoRoT spacecraft database. Our analysis is intended to highlight the possibility that astrophysical time series can be members of a particular class of complex and dynamic processes which require several photometric variability diagnostics to characterize their structural and topological properties. To achieve this goal, we search for contributions due to nonlinear temporal correlation and effects caused by heavier tails than the Gaussian distribution, using a detrending moving average algorithm for one-dimensional multifractal signals (MFDMA). We observe that the correlation structure is the main source of multifractality, while heavy-tailed distribution plays a minor role in generating the multifractal effects. Our work also reveals that rotation period of stars is inherently scaled by degree of multifractality. As a result, analyzing the multifractal degree of referred series, we uncover an evolution of multifractality from shorter to larger periods.
We present rotation periods for 71 single dwarf members of the open cluster NGC6811 determined using photometry from NASAs Kepler Mission. The results are the first from The Kepler Cluster Study which combine Keplers photometry with ground-based spectroscopy for cluster membership and binarity. The rotation periods delineate a tight sequence in the NGC6811 color-period diagram from ~1 day at mid-F to ~11 days at early-K spectral type. This result extends to ~1 Gyr similar prior results in the ~600 Myr Hyades and Praesepe clusters, suggesting that rotation periods for cool dwarf stars delineate a well-defined surface in the 3-dimensional space of color (mass), rotation, and age. It implies that reliable ages can be derived for field dwarf stars with measured colors and rotation periods, and it promises to enable further understanding of various aspects of stellar rotation and activity for cool stars.
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