LOTUS: A (non-)LTE Optimization Tool for Uniform derivation of Stellar atmospheric parameters

Abstract

Precise fundamental atmospheric stellar parameters and abundance determination of individual elements in stars are important for all stellar population studies. Non-Local Thermodynamic Equilibrium (Non-LTE; hereafter NLTE) models are often important for such high precision, however, can be computationally complex and expensive, which renders the models less utilized in spectroscopic analyses. To alleviate the computational burden of such models, we developed a robust 1D, LTE and NLTE fundamental atmospheric stellar parameter derivation tool, LOTUS, to determine the effective temperature T eff , surface gravity log g, metallicity [Fe/H] and microturbulent velocity v mic for FGK type stars, from equivalent width (EW) measurements of Fe i and Fe ii lines. We utilize a generalized curve of growth method to take into account the EW dependencies of each Fe i and Fe ii line on the corresponding atmospheric stellar parameters. A global differential evolution optimization algorithm is then used to derive the optimized fundamental parameters. Additionally, LOTUS can determine precise uncertainties for each stellar parameter using a Markov Chain Monte Carlo (MCMC) algorithm. We test and apply LOTUS on a sample of benchmark stars, as well as stars with available asteroseismic surface gravities from the K2 survey, and metal-poor stars from R-process Alliance (RPA) survey. We find very good agreement between our NLTE-derived parameters in LOTUS to non-spectroscopic values within T eff = ±30 K and log g = ±0.20 dex for benchmark stars. We provide open access of our code, as well as of the interpolated pre-computed NLTE EW grids available on Github , and documentation with working examples on Readthedocs .

3D non-LTE iron abundances in FG-type dwarfs

“…It is also a precise and illuminating tracer of the evolution of galaxies, being formed copiously by both core-collapse supernovae on short timescales, and Type Ia supernovae at later cosmic epochs (Maoz & Graur 2017). In spectroscopic surveys, including extremely large ones such as APOGEE (Ahumada et al 2020) and GALAH (Buder et al 2021), iron is often used to infer other fundamental stellar parameters such as the effective temperature (T eff ) and surface gravity (log g), via excitation and ionisation equilibria of Fe I and Fe II lines (Frebel et al 2013;Ruchti et al 2013;Tsantaki et al 2013;Ezzeddine et al 2017;Li & Ezzeddine 2022).…”