Long-term Spot Activity Variations in Chromospherically Active Stars

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Scientific paper

A detailed investigation of the long-term photometric variations in a sample of 18 chromospherically active stars has been carried out to explore the possibility of detecting activity cycle akin to the sunspot cycle. A traditional starspot model was used to model photometric data mostly drawn from archives to extract the spot parameters. We used integrated magnitude, amplitude, minimum, maximum and mean magnitude, total spot area and polar spot area as different photometric diagnostics to examine any periodicity in the photometric V band data for program stars. Our results are suggestive of evidences for the existence of long-term activity cycles in all the program stars with periods ranging from about 4 years to more than 50 years. Secular variations in photometric light curves of active stars are governed by a change of their overall spot coverage. Spot migration does not affect the shape or the length of the observed cycles. Integrated amplitude correlates tightly with the other photometric activity tracers suggesting that loss of light is due to cool spots on the stellar surface. Correlations between different tracers of stellar activity indicate that the integrated amplitude is perhaps the best tracer among the ones used in the analysis. Most of our program stars also display the existence of active and/or preferred longitudes. A correlation between cycle length and stellar rotational period appears evident in the sense that cycle lengths were generally longer for stars with longer rotation periods. Existence of strong correlation between activity cycle period (P_cyc) and stellar rotation period (P_rot) suggests that stellar activity is driven by magnetic field which in turn is controlled by the dynamo action.

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