Thorough characterisation of the 16 Cygni system. Part II. Seismic inversions of the internal structure

Abstract

Context. The advent of space-based photometry observations provided high-quality asteroseismic data for a large number of stars. These observations enabled the adaptation of advanced analyses techniques, until then restricted to the field of helioseismology, to study the best asteroseismic targets. Amongst these, the 16Cyg binary system holds a special place, as they are the brightest solar twins observed by the Kepler mission. For this specific system, modellers have access to high-quality asteroseismic, spectroscopic and interferometric data, making it the perfect testbed for the limitations of stellar models. Aims. We aim to further constrain the internal structure and fundamental parameters of 16CygA&B using linear seismic inversion techniques of both global indicators and localised corrections of the hydrostatic structure. Methods. We start from the models defined by detailed asteroseismic modelling in our previous paper and extend our analysis by applying variational inversions to our evolutionary models. We carried out inversions of so-called seismic indicators and attempted to provide local corrections of the internal structure of the two stars. Results. Our results indicate that linear seismic inversions alone are not able to discriminate between standard and non-standard models for 16CygA&B. We confirm the results of our previous studies that used linear inversion techniques, but consider that the observed differences could be linked to small fundamental parameters variations rather than to a missing process in the models. Conclusions. We confirm the robustness and reliability of the results of the modelling we performed in our previous paper. We conclude that non-linear inversions are likely required to further investigate the properties of 16CygA&B from a seismic point of view, but that these inversions have to be coupled to analyses of the depletion of light elements such as lithium and beryllium to constrain the macroscopic transport of chemicals in these stars and also to constrain potential non-standard evolutionary paths.

Inversions of stellar structure from asteroseismic data

“…Applications of the SOLA method in its original form to actual asteroseismic data can be found as early as 2004 (di Mauro, 2004). More recent applications aiming at determining localized corrections to Kepler targets can be found in (Bellinger et al, 2017(Bellinger et al, , 2019Kosovichev and Kitiashvili, 2020;Bellinger et al, 2021;Buldgen et al, 2022). Such applications have been limited to the best Kepler targets, such as the 16Cyg binary system.…”

Probing stellar cores from inversions of frequency separation ratios

“…Indeed, the classical inversions for the Sun are local inversions that can scan through the entire structure, but they require a large set of frequencies to be efficient, which can only be achieved for the best asteroseismic targets (e.g. 16CygA and 16CygB, Bellinger et al 2017Bellinger et al , 2019Bellinger et al , 2021Buldgen et al 2022), and with the restriction that information can only be extracted for the deep layers. Therefore, inversions based on so-called indicators were developed (Reese et al 2012;Buldgen et al 2015bBuldgen et al ,a, 2018, and applied to various cases (e.g.…”

“…In addition, a very accurate characterisation of the mass and radius is also needed (Buldgen et al 2017b). Entropy inversions were conducted for the Sun (Buldgen et al 2017c), and for the 16Cyg binary system (Buldgen et al 2022) using the indicators defined in Buldgen et al (2018). However, even though the 16Cyg system is one of the best Kepler asteroseismic target, Buldgen et al (2022) found that no additional information about the central entropy could be extracted for 16CygB.…”

“…Entropy inversions were conducted for the Sun (Buldgen et al 2017c), and for the 16Cyg binary system (Buldgen et al 2022) using the indicators defined in Buldgen et al (2018). However, even though the 16Cyg system is one of the best Kepler asteroseismic target, Buldgen et al (2022) found that no additional information about the central entropy could be extracted for 16CygB. In addition, inversions based on individual frequencies are sensitive to the surface regions, which is a limiting factor as the nearsurface effects are poorly modelled.…”

“…In addition, inversions based on individual frequencies are sensitive to the surface regions, which is a limiting factor as the nearsurface effects are poorly modelled. In the case of 16CygA, the surface effects and the quality of the reference models prevented Buldgen et al (2022) to extract new constraints about the central entropy. Therefore, we chose to develop a new indicator based on frequency separation ratios instead of individual frequencies, aiming to carry out inversions of solar-like stars presenting convective cores.…”