New Suns in the Cosmos. IV. The Multifractal Nature of Stellar Magnetic Activity in Kepler Cool Stars

作者：de Freitas D B^*; Nepomuceno M M F; Gomes de Souza M; Leao I C; Das Chagas M L; Costa A D; Canto Martins B L; De Medeiros J R

来源：Astrophysical Journal, 2017, 843(2): 103.

DOI：10.3847/1538-4357/aa78aa

• methods: data analysis
• stars: activity
• stars: solar-type
• Sun: rotation

In the present study, we investigate the multifractal nature of a long-cadence time series observed by the Kepler mission for a sample of 34 M dwarf stars and the Sun in its active phase. Using the Multifractal Detrending Moving Average algorithm, 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 < S-ph > 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 < S-ph > 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 Skumanich's relationship. Finally, our approach suggests that the H index may be related to the evolution of stellar angular momentum and a star's magnetic properties.

• 出版日期2017-7-10