Characterization of Kepler targets based on medium-resolution LAMOST spectra analyzed with ROTFIT^★,★★

¹ INAF - Osservatorio Astrofisico di Catania, via S. Sofia, 78, 95123 Catania, Italy
e-mail: afrasca@oact.inaf.it
² University of Wrocław, Faculty of Physics and Astronomy, Astronomical Institute, ul. Kopernika 11, 51-622 Wrocław, Poland
³ Royal observatory of Belgium, Ringlaan 3, 1180 Brussel, Belgium
⁴ Department of Astronomy, Beijing Normal University, Beijing, 100875 PR China
⁵ Key Lab for Optical Astronomy, National Astronomical Observatories, Chinese Academy of Sciences, Beijing, 100101 PR China

Received: 4 February 2022
Accepted: 24 April 2022

Abstract

Aims. In this work we present the results of our analysis of 16 300 medium-resolution LAMOST spectra of late-type stars in the Kepler field with the aim of determining the stellar parameters, activity level, lithium atmospheric content, and binarity.

Methods. We have used a version of the code rotfit specifically developed for the lamost medium-resolution spectra to determine stellar parameters via the adoption of a grid of spectra of real stars. We provide a catalog with the atmospheric parameters (T_eff, log g, and [Fe/H]), radial velocity (RV), and projected rotation velocity (v sin i). For cool stars (T_eff ≤ 6500 K), we also calculated the Ha and Li iλ6708 equivalent width, which are important indicators of chromospheric activity and evolutionary stage, respectively.

Results. From the sample of 16 300 spectra, we have derived the RV and atmospheric parameters for 14 300 spectra of 7443 stars. Literature data (mainly from high- or medium-resolution spectra) were used for a quality control of the results and to assess the accuracy of the derived parameters. The T_eff and log g values are in good agreement with the literature, although their distribution displays some clustering effects, which may be the result of the nonuniform distribution of the templates in the parameter space. The most relevant differences are found for [Fe/H], which appears to be overestimated for metal-poor stars; this overestimation is also likely due to the template grid. We propose a relation to correct the [Fe/H] values derived with rotfit. We were able to identify interesting objects, such as double-lined binaries, stars with variable RVs, lithium-rich giants, and emission-line objects. Based on the Hα flux, we found 327 active stars. We were able to detect the Li iλ6708 line and measure its equivalent width for 1657 stars, both giants and stars on the main sequence. Regarding the latter, we performed a discrete age classification based on the atmospheric lithium abundance and the upper envelopes of a few open clusters. Among the giants, we found 195 Li-rich stars, 161 of which are reported here for the first time. No relationship is found between stellar rotation and lithium abundance, which allows us to rule out merger scenarios as the predominant explanation of the enrichment of Li in our sample. The fraction of Li-rich giants, ≈ 4%, is higher than expected.