The Enigma of Li-Rich Giants and its Relation with Temporal Variations Observed in Radial Velocity and Stellar Activity Signals

Inês Rolo, Elisa Delgado Mena, Maria Tsantaki, João Gomes da Silva

Published 2024-06-18Version 1

Despite the large number of studies focused on the characterization of Li-rich stars and understanding of the mechanisms leading to such enrichment, their origin remains a mystery. Magnetic activity, in particular the phenomena usually associated with it, such as spots and plages, and the Li abundance (A(Li)) of stars, are in general thought to be connected. However, as of today it is unclear just how. In this work, we study a sample of young but evolved intermediate mass red giants, inhabitants of open clusters where planets have been searched. We aim at using radial velocity (RV) and stellar activity indicator signals to look for relations between Li abundances and stellar activity/variability. We explore how the standard deviation (STD), peak to peak amplitude (PTP), mean and median of typical stellar activity indicators (BIS, FWHM, $T_\textrm{eff}$, and H$\alpha$ index) change as a function of the Li content of 82 red giants. Furthermore, we compute weighted Pearson Correlation Coefficients ($\rho_w$) between time series of RV measurements and the stellar activity indicators for the stars in our sample. To aid our results, we also study Generalized Lomb-Scargle Periodograms (GLSP) to capture possible significant periodic temporal variations in our data. Our analysis indicates that the STD and PTP of BIS and FWHM, the mean and median of the H$\alpha$ index and $v\sin(i)$ increase exponentially with A(Li) in our sample of red giants. Significant temporal variations and correlations between RVs and activity indicators also tend to be found preferentially for stars where high A(Li) is observed. Most of the Li-rich stars in our sample either show strong correlations of RV with at least one of the stellar activity indicators or reveal significant periodic temporal variations in their stellar activity indicators GLSPs consistent with those found for RV.