A grid of MARCS model atmospheres for S stars

TL;DR

This paper presents a comprehensive grid of model atmospheres for S-type stars, analyzing their spectral features and photometry to accurately determine their physical parameters and avoid errors from using inappropriate models.

Contribution

It introduces a new grid of S-star model atmospheres covering key parameters and a tool for best-model identification, improving parameter estimation accuracy.

Findings

Applying M-star models to S stars can cause Teff errors up to 400K.

The developed tool effectively constrains Teff, C/O, and [s/Fe] for S stars.

The parameter space of S stars is mapped using the new models and indices.

Abstract

S-type stars are late-type giants whose atmosphere is enriched in carbon and s-process elements because of either extrinsic pollution by a binary companion or intrinsic nucleosynthesis and dredge-up on the thermally-pulsing AGB. A large grid of S-star model atmospheres has been computed covering the range 2700 < Teff < 4000 K with 0.5 < C/O < 0.99. ZrO and TiO band strength indices as well as VJHKL photometry are needed to disentangle Teff, C/O and [s/Fe]. A "best-model finding tool" was developed using a set of well-chosen indices and checked against photometry as well as low- and high-resolution spectroscopy. It is found that applying M-star model atmospheres (i.e., with a solar C/O ratio) to S stars can lead to errors on Teff up to 400K. We constrain the parameter space occupied by S stars of the vast sample of Henize stars in terms of Teff, [C/O] and [s/Fe].