First long-term activity study of AU Microscopii: a possible chromospheric cycle

Abstract

M stars are ideal targets to search for Earth-like planets. However, they usually have high levels of magnetic activity, which could affect their habitability and make difficult the detection of exoplanets orbiting around them. Unfortunately, long-term variability of dM stars has not been extensively studied, due to their low intrinsic brightness. For this reason, in 1999 we started the HKα project, which systematically observes the spectra of a large number of stars, in particular dM stars, at the Complejo Astronómico El Leoncito (CASLEO). In this work, we study the long-term activity of the young active dM1 star AU Microscopii. We analyse the Mount Wilson index S derived from CASLEO spectra obtained between 2004 and 2016, which we complement with the S-index derived from HARPS, FEROS, and UVES public spectra. We also analyse the simultaneous photometric counterpart provided by the ASAS public data base for this star between 2000 and 2009, and our own photometry. In both totally independent time series, we detect a possible activity cycle of period ∼5 yr. We also derived a precise rotation period for this star Prot = 4.85 d, consistent with the literature. This activity cycle reflects that an αΩ dynamo could be operating in this star.