2017, Kiefer, René, Schad, Ariane, Davies, Guy, Roth, Markus

Context. The Sun and solar-like stars undergo activity cycles for which the underlying mechanisms are not well understood. The oscillations of the Sun are known to vary with its activity cycle and these changes provide diagnostics on the conditions below the photosphere. Kepler has detected solar-like oscillations in hundreds of stars but as of yet, no widespread detection of signatures of magnetic activity cycles in the oscillation parameters of these stars have been reported. Aims. We analysed the photometric short cadence Kepler time series of a set of 24 solar-like stars, which were observed for at least 960 d each, with the aim to find signatures of stellar magnetic activity in the oscillation parameters. Methods. We analyse the temporal evolution of oscillation parameters by measuring mode frequency shifts, changes in the height of the p-mode envelope, as well as granulation timescales. Results. For 23 of the 24 investigated stars, we find significant frequency shifts in time. We present evidence for magnetic activity in six of these stars. We find that the amplitude of the frequency shifts decreases with stellar age and rotation period. For KIC 8006161 (the most prominent example), we find that frequency shifts are smallest for the lowest and largest for the highest p-mode frequencies, as they are for the Sun. Conclusions. These findings show that magnetic activity can be routinely observed in the oscillation parameters for solar-like stars, which opens up the possibility of placing the solar activity cycle in the context of other stars by asteroseismology.

2022, Strassmeier, Klaus G., Steffen, Matthias

A spectroscopic investigation of the lithium resonance doublet in ξ Boo A and ξ Boo B in terms of both abundance and isotopic ratio is presented. We obtained new R = 130,000 spectra with a signal-to-noise ratio (S/N) per pixel of up to 3200 using the 11.8 m LBT and PEPSI. From fits with synthetic line profiles based on 1D-LTE MARCS model atmospheres and 3D-NLTE corrections, we determine the abundances of both isotopes. For ξ Boo A, we find A(Li) = 2.40 ± 0.03 dex and 6Li/7Li <1.5 ± 1.0% in 1D-LTE, which increases to ≈2.45 for the 3D-NLTE case. For ξ Boo B we obtain A(Li) = 0.37 ± 0.09 dex in 1D-LTE with an unspecified 6Li/7Li level. Therefore, no 6Li is seen on any of the two stars. We consider a spot model for the Li fit for ξ Boo B and find A(Li) = 0.45 ± 0.09 dex. The 7Li abundance is 23 times higher for ξ Boo A than the Sun's, but three times lower than the Sun's for ξ Boo B while both fit the trend of single stars in the similar-aged M35 open cluster. Effective temperatures are redetermined from the TiO band head strength. We note that the best-fit global metallicities are −0.13 ± 0.01 dex for ξ Boo A but +0.13 ± 0.02 dex for ξ Boo B. Lithium abundance for the K5V benchmark star 61 Cyg A was obtained to A(Li) ≈ 0.53 dex when including a spot model but to ≈0.15 dex without a spot model.