LAMOST medium-resolution observations of the Pleiades

¹ INAF – Osservatorio Astrofisico di Catania, via S. Sofia, 78, 95123 Catania, Italy
² Institute for Frontiers in Astronomy and Astrophysics, Beijing Normal University, Beijing 102206, PR China
³ School of Physics and Astronomy, Beijing Normal University, Beijing 100875, PR China
⁴ University of Wrocław, Faculty of Physics and Astronomy, Astronomical Institute, ul. Kopernika 11, 51-622 Wrocław, Poland
⁵ Royal observatory of Belgium, Ringlaan 3, 1180 Brussel, Belgium

^★ Corresponding author: antonio.frasca@inaf.it

Received: 3 January 2025
Accepted: 7 April 2025

Abstract

Aims. In this work, we present the results of our analysis of medium-resolution LAMOST spectra of late-type candidate members of the Pleiades with the aim of determining the stellar parameters, activity level, and lithium abundance.

Methods. We used the ROTFIT code to determine the atmospheric parameters (T_eff, log g, and [Fe/H]), along with the radial velocity (RV) and projected rotation velocity (v sin i). Moreover, for late-type stars (T_eff ≤ 6500 K), we also calculated the Hα and Li Iλ6708 net equivalent width by means of the subtraction of inactive photospheric templates. We also used the rotation periods from the literature and we purposely determined them for 89 stars by analyzing the available Transiting Exoplanet Survey Satellite (TESS) photometry.

Results. We derived the RV, v sin i, and atmospheric parameters for 1581 spectra of 283 stars. Literature data were used to assess the accuracy of the derived parameters. The RV distribution of the cluster members peaks at 5.0 km s⁻¹ with a dispersion of 1.4 km s⁻¹, while the average metallicity is [Fe/H]=−0.03±0.06, in line with previous determinations. Fitting empirical isochrones of Li depletion to EW measures of stars with T_eff ≤ 6500 K, we obtained a reliable age for the Pleiades of 118±6 Myr, in agreement with the recent literature. The activity indicators Hα line flux (F_Hα) and luminosity ratio (R^′_Hα) show the hottest stars to be less active (on average) than the coldest ones, as expected for a 100-Myr old cluster. When plotted against the Rossby number, R_O, our R^′_Hα values display a typical activity-rotation trend, with a steep decay for R_O ≥ 0.2 and a nearly flat (saturated) activity level for smaller values. However, we still see a slight dependence on R_O in the saturated regime, which is well fitted by a power law with a slope of −1.18 ± 0.02; this is in agreement with a number of previous works. For three sources with multi-epoch data, we had access to LAMOST spectra acquired during flares, which are characterized by strong and broad Hα profiles and the presence of the He I λ6678 Å emission line. Among our targets, we identified 39 possible SB1 and ten SB2 systems. We have also shown the potential of the LAMOST-MRS spectra in allowing us to refine the orbital solution of a number of binaries and to discover a new double-lined binary as well.