Extra modes in helium-core-burning stars probing an infra core cavity

TL;DR

This work addresses unexplained extra peaks in the oscillation spectra of helium-core-burning stars by applying a three-cavity gravito-acoustic asymptotic formalism. By deriving and fitting the resonance condition for two inner g-cavities coupled to an outer p-cavity, the authors extract two radiative-scale spacings, $ΔΠ_i$ and $ΔΠ_o$, and compute the total radiative spacing $ΔΠ_{rad}$, constraining the full radiative core and pointing to core overshoot or mixing as a key physical process. The results indicate the radiative core is effectively split into two regions, with $ΔΠ_i$ in the range $[900,2000]$ s and small coupling $q_g$, while $ΔΠ_{rad}$ is explained by the reciprocal-sum relation $1/ΔΠ_{rad} ≈ 1/ΔΠ_i + 1/ΔΠ_o$, suggesting a structural discontinuity likely tied to overshoot. This method provides new seismic constraints on core mixing in HeCB stars and links extra peaks to a global three-cavity pattern, offering a route to better understand stellar core physics and evolutionary states.

Abstract

Dipole mixed modes observed in the oscillation pattern of red giant stars probe the radiative regions in the stellar core. Oscillation spectra of helium-core-burning stars sometimes show extra peaks that remain unexplained by the dipole mixed-mode pattern expected from the coupling of a radiative cavity in the stellar core and a pressure cavity in the stellar envelope. We use the asymptotic expansion developed for a multi-cavity star in order to characterize these extra peaks. The analytical resonance condition of the multi-cavity gravito-acoustic modes, with two inner gravity cavities and an outer pressure cavity, helps us explain that the apparent extra peaks are dipole mixed modes that follow the 3-cavity oscillation pattern. The derivation of the two asymptotic period spacings associated with the two distinct regions in the radiative core provides an estimate of the full radiative cavity. Our results provide new constraints for analysing the overshoot or mixing in the core of helium-core-burning stars. An important structure discontinuity inside the radiative core may explain the larger than expected observed period spacings.

Figures (4)

• Figure 1: Échelle spectrum of the star KIC 2575024. Radial modes, in red, and quadrupole modes, in green, appear at $x\simeq 0$ and $x\simeq -0.16$, respectively, where $x = \nu/\Delta\nu - ({n_{\mathrm{p}}}+\varepsilon)$. Most of the dipole mixed modes can be fit with the two-cavity asymptotic expansion ($m=\pm1$ doublets, identified as blue and purple peaks). Extra peaks indicated by green diamonds are observed at different frequency ranges. The dashed line represents a threshold of eight times the background.
• Figure 2: Period spacings of the inner gravity cavity, $\Delta\Pi_{\mathrm{i}}$ (diamonds, with a red color modulated according to $q_{\mathrm{g}}$), outer gravity cavity, $\Delta\Pi_{\mathrm{o}}$ (blue squares), and associated radiative period spacings, $\Delta\Pi_{\mathrm{rad}}$ (purple triangles), as a function of the large separation. Asymptotic period spacings derived in the two-cavity case are also plotted (light gray crosses).
• Figure 3: Period spacings in the oscillation spectrum of two HeCB stars, in the red clump (KIC 9815488, top) or the secondary clump (KIC 2583884, bottom). The gray curve indicates the expected period spacings for usual two-cavity mixed modes; the light blue curve corresponds to the three-cavity case. The dark blue curve is derived from the fitted peaks, which are identified with orange ticks. The light gray regions of the oscillation spectra indicate the ranges where radial and quadrupole modes hamper the detection of extra peaks.
• Figure 4: Period spacings in the oscillation spectrum of the stars KIC 3544063 and KIC 9332840 showing evidence of structural discontinuities in the inner core 2022NatCo..13.7553V. Same styles and symbols as in Fig. \ref{['fig-dP3cav']}. For both stars, the glitch signature induces shorter period spacings in the frequency range around 35.5 $\mu$Hz.