Frasca, Antonio ; Zhang, Jiayi ; Alonso-Santiago, Javier ; Fu, Jian-Ning ; Molenda-Żakowicz, Joanna ; De Cat, Peter ; Catanzaro, Giovanni

submitted to Astronomy & Astrophysics (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 have used the code ROTFIT to determine the atmospheric parameters (Teff, log g, and [Fe/H]), radial velocity (RV), and projected rotation velocity (vsini). Moreover, for late-type stars (Teff ≤ 6500 K), we also calculated the Hα and LiI λ6708 net equivalent width by means of the subtraction of inactive photospheric templates. For our study, we have also used rotation periods available in the literature and we have purposely determined them for 89 stars by analyzing the available TESS photometry. RESULTS. We have 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. From the study of the lithium depletion boundary for late-type members (Teff ≤ 6500 K) we obtain a reliable age for the Pleiades of 107±7 Myr, fully compatible with the current canonical value. The activity indicators based on the net Hα emission, namely the line flux (FHα ) and the ratio of line to bolometric luminosity (RHα), when plotted as a function of Teff , show the typical behavior expected for a 100-Myr old cluster, with the hottest stars less active, on average, than the coldest ones. When plotted against the rotation period or the Rossby number RO, our RHα values display the typical activity-rotation trend with a steep decay for RO greater than a given threshold (≈ 0.2) and a nearly flat (saturated) activity level for smaller values of RO . However, we still see a slight dependence on RO in the saturated regime which is well fitted by a power law with a slope of −0.16, in agreement with some previous work in the literature. For three sources with multi-epoch data we have found LAMOST spectra acquired during flares, which are characterized by strong and broad Hα profiles and the presence of the He i λ6678Å emission line. Among our targets we identify 39 possible SB1 and ten SB2. As concerns the study of spectroscopic binaries. we have also shown the potential of the LAMOST-MRS spectra, which allowed us to refine the orbital solution of some binary and to discover one of the investigated targets, QQ Tau, to be a double-lined binary for which we provide, for the first time, a preliminary solution.

Keyword(s): Stars: fundamental parameters ; Stars: activity ; Stars: flare ; Stars: binaries: spectroscopic ; Stars: abundances ; Ppen clusters and associations: individual: Pleiades