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

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 Δν) and the pattern of gravity modes (through the determination of their asymptotic period spacing ΔΠ1). It has been shown that red giant branch (RGB) stars regroup on a well-defined sequence in the Δν − ΔΠ1 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 Δν − ΔΠ1 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 they 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 ≲ 2 M⊙) 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, which is in agreement with the observations.

A hybrid GNA instability

“…Thus, it provides a strong support and reliability to the presented GNA modal analysis. The famous space missions, CoRoT and Kepler 51 , have found mixed p-g modes in the red giants and revealed their deep internal structure. The p- mode depends on the properties on the envelope surrounding the core (outer) and the g- mode depends on the properties of the core structure (inner).…”

“…The p- mode depends on the properties on the envelope surrounding the core (outer) and the g- mode depends on the properties of the core structure (inner). It is believed that such space missions can detect the presence of p- mode, and hence, the GNA mode, in the neutron star “inner crust” region, subject to the achievement of required ultracam detection resolutions and refinements 51 . It may be conjectured that the semi-analytic model formalism presented here could enable us to identify and characterize the diversified stabilization/destabilization factors, significantly regulating the interior crustal behaviours of neutron stars and other compact astroobjects in a novel superfluidic instability perspective.…”

“…43) excited in the inner crust region of neutron stars are basically dictated by the multiparametric dispersion windows featuring the interior of neutron stars (described judiciously by Eqs. [44][45][46][47][48][49][50][51][52][53][54][55][56]. ( 42)…”

Inversions of Stellar Structure From Asteroseismic Data

“…. for which pulsational properties may significantly differ from what their standard evolutionary counterparts would predict (See e.g., Deheuvels et al, 2022).…”

Inversions of stellar structure from asteroseismic data

“…As such, static modelling may also prove useful for modelling non-standard evolutionary products such as results of mergers, stripped cores, ... for which pulsational properties may significantly differ from what their standard evolutionary counterparts would predict (See e.g. Deheuvels et al, 2022).…”