SPECIES

II. Stellar parameters of the EXPRESS giant star sample^⋆

¹ School of Physics and Astronomy, Queen Mary University London, 327 Mile End Road, London E1 4NS, UK
e-mail: m.soto@qmul.ac.uk
² European Southern Observatory, Alonso de Córdova 3107, Vitacura, Casilla, 19001 Santiago, Chile
³ Instituto de Astronomía, Universidad Católica del Norte, Angamos 0610, 1270709 Antofagasta, Chile
⁴ Departamento de Astronomía, Universidad de Chile, Camino El Observatorio 1515, Las Condes, Santiago, Chile

Received: 7 September 2020
Accepted: 4 January 2021

Abstract

Context. 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, such as using radial-velocities to measure planet masses and orbital mechanics, since the stars are too hot and rotate too fast to present the quantity of narrow stellar spectral lines that is necessary for measuring velocities at the level of a few m s⁻¹.

Aims. Our goal is to 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.

Methods. We analysed the high-resolution echelle spectra of these stars and computed their atmospheric parameters by measuring the equivalent widths for a set of iron lines, using an updated method implemented during this work. Physical parameters, such as mass and radius, were computed by interpolating through a grid of stellar evolutionary models, following a procedure that carefully takes into account the post-main sequence evolutionary phases. The atmospheric parameters, as well as the photometric and parallax data, are used as constraints during the interpolation process. The probabilities of the star being in the red giant branch (RGB) or the horizontal branch (HB) are estimated from the derived distributions.

Results. We obtained atmospheric and physical stellar parameters for the whole EXPRESS sample, which comprises a total of 166 evolved stars. We find that 101 of them are most likely first ascending the RGB phase, while 65 of them have already reached the HB phase. The mean derived mass is 1.41 ± 0.46 M_⊙ and 1.87 ± 0.53 M_⊙ for RGB and HB stars, respectively. To validate our method, we compared our derived physical parameters with data from interferometry and asteroseismology studies. In particular, when comparing to stellar radii derived from interferometric angular diameters, we find: ΔR_inter = −0.11 R_⊙, which corresponds to a 1.7% difference. Similarly, when comparing with asteroseismology, we obtain the following results: Δ log g = 0.07 cgs (2.4%), ΔR = −0.12 R_⊙ (1.5%), ΔM = 0.08 M_⊙ (6.2%), and Δage = −0.55 Gyr (11.9%). Additionally, we compared our derived atmospheric parameters with previous spectroscopic studies. We find the following results: ΔT_eff = 22 K (0.5%), Δ log g = −0.03 (1.0%) and Δ[Fe/H] = −0.04 dex (2%). We also find a mean systematic difference in the mass with respect to those presented in the EXPRESS original catalogue of ΔM = −0.28 ± 0.27 M_⊙, corresponding to a systematic mean difference of 16%. For the rest of the atmospheric and physical parameters we find a good agreement between the original catalogue and the results presented here. Finally, we find excellent agreement between the spectroscopic and trigonometric log g values, showing the internal consistency and robustness of our method.

Conclusions. We show that our method, which includes a re-selection of iron lines and changes in the interpolation of evolutionary models, as well as Gaia parallaxes and newer extinction maps, can greatly improve the estimates of stellar parameters for giant stars compared to those presented in our previous work. This method also results in smaller mass estimates, an issue that has been described in results for giant stars from spectroscopy studies in the literature. The results provided here will improve the physical parameter estimates of planetary companions found orbiting these stars and give us insights into their formation and the effect of stellar evolution on their survival.