Seismic signature of electron degeneracy in the core of red giants: hints for mass transfer between close red-giant companions

TL;DR

This paper uses seismic data from space missions to link the internal core properties of red giants, especially electron degeneracy, to their position in characteristic frequency diagrams, revealing insights into stellar evolution and mass transfer effects.

Contribution

It provides a theoretical explanation for the seismic patterns of red giants and introduces the role of electron degeneracy in their core as a key factor, also exploring effects of binary interactions.

Findings

Red giants align on a sequence in the $\Delta u$-$\Delta\Pi_1$ plane when core electron degeneracy is significant.

The position in the seismic diagram can estimate core density and core overshooting.

Peculiar red giants below the sequence are explained by binary mass transfer involving degenerate cores.

Abstract

The detection of mixed modes in red giants with space missions CoRoT and Kepler has revealed their deep internal structure. These modes allow us to characterize the pattern of pressure modes (through the measurement of their asymptotic frequency separation $\Delta\nu$) and the pattern of gravity modes (through the determination of their asymptotic period spacing $\Delta\Pi_1$). It has been shown that red giant branch (RGB) stars regroup on a well-defined sequence in the $\Delta\nu$-$\Delta\Pi_1$ plane. Our first goal is to theoretically explain the features of this sequence and understand how it can be used to probe the interiors of red giants. Using a grid of red giant models computed with MESA, we demonstrate that red giants join the $\Delta\nu$-$\Delta\Pi_1$ sequence whenever electron degeneracy becomes strong in the core. We argue that this can be used to estimate the central densities of these stars, and potentially to measure the amount of core overshooting during the main sequence part of the evolution. We also investigate a puzzling subsample of red giants that are located below the RGB sequence, in contradiction with stellar evolution models. After checking the measurements of the asymptotic period spacing for these stars, we show that they are mainly intermediate-mass red giants. This is doubly peculiar because these stars should have nondegenerate cores and are expected to be located well above the RGB sequence. We show that these peculiarities are well accounted for if these stars result from the interaction between two low-mass ($M\lesssim2\,M_\odot$) close companions during the red giant branch phase. If the secondary component has already developed a degenerate core before mass transfer begins, it becomes an intermediate-mass giant with a degenerate core. The secondary star is then located below the degenerate sequence, in agreement with the observations.