SPECIES II. Stellar parameters of the EXPRESS program giant star sample

TL;DR

This study estimates stellar parameters for giant stars in the EXPRESS project, improving parameter accuracy and enabling better understanding of planet occurrence around evolved, higher-mass stars.

Contribution

It introduces an updated method for deriving stellar parameters from spectra, accounting for post-MS phases, and validates results against interferometry and asteroseismology.

Findings

Most stars are in the RGB phase.

Derived stellar masses are 1.41 and 1.87 Msun for RGB and HB.

Parameter estimates show high agreement with independent methods.

Abstract

As part of the search for planets around evolved stars, we can understand planet populations around significantly higher-mass stars than the Sun on the main sequence. This population is difficult to study any other way, particularly with radial-velocities since these stars are too hot and rotate too fast to measure precise velocities. Here we estimate stellar parameters for all of the giant stars from the EXPRESS project, which aims to detect planets orbiting evolved stars, and study their occurrence rate as a function of stellar mass. We analyse high resolution echelle spectra of these stars, and compute the atmospheric parameters by measuring the equivalent widths for a set of iron lines, using an updated method implemented during this work. Physical parameters are computed by interpolating through a grid of stellar evolutionary models, following a procedure that carefully takes into account the post-MS evolutionary phases. Probabilities of the star being in the red giant branch (RBG) or the horizontal branch (HB) are estimated from the derived distributions. Results: We find that, out of 166 evolved stars, 101 of them are most likely in the RGB phase, while 65 of them are in the HB phase. The mean derived mass is 1.41 and 1.87 Msun for RGB and HB stars, respectively. To validate our method, we compared our results with interferometry and asteroseismology studies. We find a difference in the radius with interferometry of 1.7%. With asteroseismology, we find 2.4% difference in logg, 1.5% in radius, 6.2% in mass, and 11.9% in age. Compared with previous spectroscopic studies, and find a 0.5% difference in Teff, 1% in logg, and 2% in [Fe/H]. We also find a mean mass difference with respect to the EXPRESS original catalogue of 16%. We show that the method presented here can greatly improve the estimates of the stellar parameters for giant stars compared to what was presented previously.