Lithium abundance in a sample of active stars: High-resolution spectrograph observation with the 1.8 m telescope

¹ Yunnan Astronomical Observatories, Chinese Academy of Sciences, Kunming 650216, PR China
e-mail: lfxing@ynao.ac.cn
² Key Laboratory for the Structure and Evolution of Celestial Objects, the Chinese Academy of Sciences, 650011 KunMing, PR China
³ University of Chinese Academy of Sciences, Beijing 100049, PR China

Received: 18 August 2020
Accepted: 8 July 2021

Abstract

Context. Observations of young, low-mass, main-sequence, and zero-age main-sequence stars show evidence of a correlation between lithium abundance and chromospheric activity, albeit with a very large scatter. Fast rotation stars (including T Tauri stars, RS CVn, and BY Dra systems) show the Li I doublet at 6707.8 Å in their spectra. The lithium depletion is probably related to the rotation (turbulent diffusion induced by rotation). Because the flare activity of stars increases with decreasing rotation period, a correlation can be expected between lithium abundance and chromospheric activity for active stars.

Aims. The aim of this paper is to investigate the relation between lithium abundance and the Ca II H and K emission index (R′_HK = L_HK/L_bol) for a sample of active stars.

Methods. Based on the high-resolution spectroscopic observations, we calculated lithium abundance for 14 chromospherically active late-type stars using the comparison of the measured Li I λ670.8 nm equivalent width with curve of growth calculations in non-local-thermodynamic-equilibrium conditions. We also searched the correlation between lithium abundance and the Ca II H & K emission index (log R′_HK) for the 14 chromospherically active late-type stars.

Results. The study of the relationship between lithium abundance and the Ca II H & K emission index (log R′_HK) found that the activity of sample stars increases with increasing lithium abundance. Next, the lithium abundance analogs progressively decrease as the rotation periods increase (rotation becomes slow) and the large values of the log R′_HK along with small values of Rossby numbers for the sample of chromospherically active stars.

Conclusions. The lithium abundance (log N(Li)) versus the chromospheric activity and log N(Li) against the rotation period both indicate that the lithium abundance analogs progressively increase as the chromospheric activity index increases and/or the rotation velocity increase (rotation period becomes small) for our sample of active stars. On the other hand, the log R′_HK against the Rossby number R_O shows that there is a clear trend of increasing activity with increasing rotation velocity for these active stars. Considering that the lithium abundance decreases with increasing stellar age in late-type stars, we can deduce that the chromospheric activity and the rotation velocity both decrease with the increase of stellar age for our sample active stars.