STEPAR: an automatic code to infer stellar atmospheric parameters

Context. STEPAR is an automatic code written in Python 3.X designed to compute the stellar atmospheric parameters T_eff, log g, [Fe/H], and ξ of FGK-type stars by means of the equivalent width (EW) method. This code has already been extensively tested in different spectroscopic studies of FGK-type stars with several spectrographs and against thousands of Gaia-ESO Survey UVES U580 spectra of late-type, low-mass stars as one of its 13 pipelines.
Aims: We describe the code that we tested against a library of well characterised Gaia benchmark stars. We also release the code to the community and provide the link for download.
Methods: We carried out the required EW determination of Fe I and Fe II spectral lines using the automatic tool TAME. STEPAR implements a grid of MARCS model atmospheres and the MOOG radiative transfer code to compute stellar atmospheric parameters by means of a Downhill Simplex minimisation algorithm.
Results: We show the results of the benchmark star test and also discuss the limitations of the EW method, and hence the code. In addition, we find a small internal scatter for the benchmark stars of 9 ± 32 K in T_eff, 0.00 ± 0.07 dex in log g, and 0.00 ± 0.03 dex in [Fe/H]. Finally, we advise against using STEPAR on double-lined spectroscopic binaries or spectra with R < 30 000, S/N < 20, or v sin i > 15 km s^-1, and on stars later than K4 or earlier than F6.