HD 38858: a solar-type star with an activity cycle of ∼10.8 yr

Abstract

Context.The detection of chromospheric activity cycles in solar-analogue and twin stars can be used to place the solar cycle in a wider context. However, relatively few of these stars with activity cycles have been detected. It is well known that the cores of the Ca IIH&K lines are modulated by stellar activity. The behaviour of the Balmer and other optical lines with stellar activity is not yet completely understood.Aims.We search for variations in the Ca IIH&K, Balmer, and Fe IIlines modulated by stellar activity. In particular, we apply a novel strategy to detect possible shape variations in the Hαline.Methods.We analysed activity signatures in HD 38858 using HARPS and CASLEO spectra obtained between 2003 and 2017. We calculated the Mount Wilson index (SMW), log(R′HK), and the statistical moments of the Ca IIH&K, Balmer, and other optical lines. We searched for periodicities using the generalized Lomb-Scargle periodogram.Results.We detect a long-term activity cycle of 10.8 yr in Ca IIH&K and Hαin the solar-analogue star HD 38858. In contrast, this cycle is marginally detected in the Fe IIlines. We also detect a noticeable variation in radial velocity that seems to be produced by stellar activity.Conclusions.HD 38858 is the second solar-analogue star where we find a clear activity cycle that is replicated in the Balmer lines. Spectral indexes based on the shape of Hαline seem to be more reliable than the fluxes in the same line for detecting activity variations. The cyclic modulation we detected gives place to a variation in radial velocity that previously has been associated with a super-Earth planet. Finally, due to the similarity of HD 38858 with the Sun, we recommend to continue monitoring this star.