Seismic analysis of the double-mode radial pulsator SX Phoenicis

TL;DR

This paper conducts a detailed seismic analysis of SX Phoenicis, fitting observed frequencies and amplitudes to models, revealing insights into its evolutionary stage, internal structure, and convective properties.

Contribution

It presents the first comprehensive seismic modeling of SX Phoenicis using multiple opacity tables and constrains the star's convective efficiency and atmospheric microturbulence.

Findings

Seismic models match observed frequencies across different opacity data.

Star is in post-main sequence, hydrogen shell-burning phase.

Convective efficiency is low to moderate, with specific microturbulent velocities.

Abstract

We present the results of complex seismic analysis of the prototype star SX Phoenicis. This analysis consists of a simultaneous fitting of the two radial-mode frequencies, the corresponding values of the bolometric flux amplitude (the parameter $f$) and of the intrinsic mode amplitude $\varepsilon$. The effects of various parameters as well as the opacity data are examined. With each opacity table it is possible to find seismic models that reproduce the two observed frequencies with masses allowed by evolutionary models appropriate for the observed values of the effective temperature and luminosity. All seismic models are in the post-main sequence phase. The OPAL and OP seismic models are in hydrogen shell-burning phase and the OPLIB seismic model has just finished an overall contraction and starts to burn hydrogen in a shell. The OP and OPLIB models are less likely due to the requirement of high initial hydrogen abundance ($X_0=0.75)$ and too high metallicity ($Z\approx 0.004$) as for a Population II star. The fitting of the parameter $f$, whose empirical values are derived from multi-colour photometric observations, provides constraints on the efficiency of convective transport in the outer layers of the star and on the microturbulent velocity in the atmosphere. Our complex seismic analysis with each opacity data indicates low to moderately efficient convection in the star's envelope, described by the mixing length parameter of $\alpha_{\rm MLT}\in (0.0,~0.7)$, and the microturbulent velocity in the atmosphere of about $\xi_{\rm t}\in(4,~8)~\kms$.