Probing tiny convective cores with the acoustic modes of lowest degree

TL;DR

This study investigates how rapid sound speed variations at the edges of convective cores in solar-like stars affect low-degree p-mode oscillation frequencies, providing methods to infer core properties from seismic data.

Contribution

It introduces a refined model for sound speed variation at the core edge and analyzes its impact on low-degree mode frequency combinations, enhancing core diagnostic techniques.

Findings

Frequency combinations involving low-degree modes are significantly affected by core edge variations.

The frequency derivative of certain diagnostic tools can directly estimate the amplitude of sound speed variations.

High-frequency frequency derivatives are notably altered by core edge structures, depending on stellar age.

Abstract

Solar-like oscillations are expected to be excited in stars of up to about 1.6 solar masses. Most of these stars will have convective cores during their Main-sequence evolution. At the edges of these convective cores there is a rapid variation in the sound speed which influences the frequencies of acoustic oscillations. In this paper we build on earlier work by Cunha and Metcalfe, to investigate further the impact that these rapid structural variations have on different p-mode frequency combinations, involving modes of low degree. In particular, we adopt a different expression to describe the sound speed variation at the edge of the core, which we show to reproduce more closely the profiles derived from the equilibrium models. We analyse the impact of this change on the frequency perturbation derived for radial modes. Moreover, we consider three different small frequency separations involving, respectively, modes of degree l = 0, 1, 2, 3; l = 0, 1; and l = 0, 2, and show that they are all significantly affected by the sharp sound speed variation at the edge of the core. In particular, we confirm that the frequency derivative of the diagnostic tool that combines modes of degree up to 3 can potentially be used to infer directly the amplitude of the relative sound speed variation at the edge of the core. Concerning the other two diagnostic tools, we show that at high frequencies they can be up to a few microhertzs smaller than what would be expected in the absence of the rapid structural variation at the edge of the core. Also, we show that the absolute values of their frequency derivatives are significantly increased, in a manner that is strongly dependent on stellar age.