Evidence of structural discontinuities in the inner core of red-giant stars

Abstract

Red giants are stars in the late stages of stellar evolution. Because they have exhausted the supply of hydrogen in their core, they burn the hydrogen in the surrounding shell . Once the helium in the core starts fusing, the star enters the clump phase, which is identified as a striking feature in the color-magnitude diagram. Since clump stars share similar observational properties, they are heavily used in astrophysical studies, as probes of distance, extinction through the galaxy, galaxy density, and stellar chemical evolution. In this work, we perform the detailed observational characterization of the deepest layers of clump stars using asteroseismic data from Kepler. We find evidence for large core structural discontinuities in about 6.7% of the stars in our sample, implying that the region of mixing beyond the convective core boundary has a radiative thermal stratification. These stars are otherwise similar to the remaining stars in our sample, which may indicate that the building of the discontinuities is an intermittent phenomenon.

Fig. 1. Glitch positions and amplitudes.

Histograms showing the distributions of the dimensionless glitch position ($x^{*}={\tilde{\omega }}_g^{*}/{\omega }_g$; a) and dimensionless amplitude (A; b) for the subsample of stars exhibiting glitch signatures. The blue shaded regions indicate the ranges for the corresponding parameters in our sequences of models (see Stellar models description subsection). The latter are also shown, as a function of the core-helium mass fraction (He_c) in c, d. The vertical orange line in b indicates the lowest glitch amplitude that was detected for this sample of stars. Source data are provided as a Source Data file.

Fig. 2. Brunt-Vaisälä frequency as a function of the relative buoyancy radius (x=ω~gr/ωg) for one of the computed models (in violet).

The origin of the abscissa marks the inner edge of the g-mode cavity. a shows the complete radiative cavity and b, c, d highlight specific parts. The black dotted line labeled 1 (in a, b) corresponds to the glitch position ($x^{*}={\tilde{\omega }}_g^{*}/{\omega }_g$) due to the chemical discontinuity caused by matter penetration in the radiative cavity and the dashed green line displays the jump in the Brunt-Vaisälä frequency at the discontinuity position. The black dotted lines labeled 2 to 5 (in a, c, d) correspond to the glitch position caused by the different helium subflashes. Source data are provided as a Source Data file.

Fig. 3. Properties of stars with and without detected glitches.

Histograms representing the number of stars as a function of the period spacing ΔΠ₁  (a), mass (b) and metallicity (c) present in the full sample (in blue) and the stars for which a discontinuity was found (in red). Period spacing as a function of the large frequency separation (d) and stellar mass as a function of metallicity (e) for the total analyzed sample (blue circles) and the stars for which a discontinuity was found (red crosses). Plots requiring metallicity data show only the subset of stars in the APOKASC catalog. The black squares in the top left corner of d and e correspond to the parameters median uncertainties which were computed with the fitted techniques referred in method subsection Target selection and data preparation. Source data are provided as a Source Data file.

Fig. 4. Échelle diagrams for three stars in our sample.

Observed mixed mode periods (green dots) as a function of the stretched periods (τ) modulo ΔΠ₁ in an échelle diagram for the stars KIC3544063 (a), KIC9332840 (b) and KIC1995859 (c). The uncertainties were computed following the fitting technique described in method subsection Core glitches identification and characterization. The blue line on a and b represents the best fitted model to the observed mixed mode pattern, the orange lines mark intervals of the inferred signature period (τ_sig) given in Table 1 and the dashed-black lines show the frequency positions of the radial (ℓ = 0) modes. The star KIC1995859 (c) shows no evidence of a glitch. Source data are provided as a Source Data file.