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

Red giant stars, in their late evolutionary stages, exhaust the hydrogen in their core and begin burning hydrogen in a surrounding shell. The subsequent helium core fusion marks the transition to the clump phase, a distinct feature in color-magnitude diagrams. Clump stars, with their shared observational characteristics, serve as valuable tools in astrophysical studies for determining distance, galactic extinction, galaxy density, and stellar chemical evolution. Asteroseismology, the study of stellar oscillations, offers a powerful means to probe the internal structure of stars, especially with the high-precision data from missions like CoRoT and Kepler. These data allow differentiation between hydrogen and helium core-burning stars. Oscillations in red giants, primarily mixed modes resulting from the interaction of acoustic waves in the envelope and gravity waves in the core, provide crucial information about core properties. The high precision of Kepler data allows detailed characterization of oscillation spectra, revealing fine details of internal structure. Sharp variations in the stellar interior, or glitches, significantly impact mode frequencies, especially when their scale is comparable to or smaller than the mode's wavelength. Theoretical work has shown that core glitches in red giants induce cyclical modulation in mixed-mode frequencies, with modulation scale and amplitude determined by the glitch's location and amplitude. A previous study identified this signature in the oscillation spectrum of KIC 9332840. This research expands upon this finding through a systematic search for core glitches in a larger sample of red-giant clump stars, aiming to determine the prevalence and nature of these structural discontinuities.

Previous research established the importance of asteroseismology in understanding stellar interiors, particularly with the advent of high-precision data from space-based missions like CoRoT and Kepler. Studies have highlighted the use of red giant clump stars as standard candles for various astrophysical measurements. Theoretical models have explored the impact of structural discontinuities (glitches) on oscillation modes, suggesting that these glitches induce cyclical modulations in the frequencies of mixed modes. A key paper demonstrated this effect theoretically and identified such a modulation in the oscillation spectrum of one specific red giant star. However, a systematic search for such phenomena across a broader sample of stars had not yet been undertaken.

This study systematically searches for core glitches in a sample of 359 low-mass red-giant clump stars from the Kepler mission. The stars were selected based on their mass (below 1.9 solar masses) to ensure similar evolutionary tracks during the clump phase, and a high signal-to-noise ratio in their light curves. The researchers measured global asteroseismic properties and individual mode frequencies for each star. They assessed the presence of core glitches by identifying significant deviations from the typical mixed-mode pattern. The position and amplitude of the glitches were inferred using theoretical models that describe how core glitches induce modulations in mixed-mode frequencies. These models are based on the concept that a discontinuity in the stellar structure creates a change in the phase of the wave propagation. This change in phase introduces a cyclic modulation into the frequencies of the mixed modes, thereby providing a distinct observational signature. The glitch signature is analyzed in the stretched period (r), obtained by a transformation of the frequencies which removes the main pressure-mode pattern. A step-function model was used to represent the structural discontinuity, characterized by three parameters: glitch amplitude, position within the radiative cavity, and phase. The observed mixed-mode frequencies were fitted to this model using a Bayesian approach implemented in the python package emcee. The parameters were estimated using a Markov Chain Monte Carlo method (MCMC). To interpret the findings, the authors compared their observations with predictions from stellar evolutionary models generated using the MESA code. Nine evolutionary sequences of stellar models were considered, varying in mass and metallicity. The models included a step-function overshoot to simulate the mixing beyond the convective core, with a radiative temperature gradient in the overshoot region. The authors computed the glitch position and amplitude from these models to compare with the observations. They also examined the period spacing of gravity modes (ΔΠ₁) to infer timescales needed for chemical discontinuity formation and analyzed other stellar properties (mass, metallicity, large frequency separation (Δν)) of stars with and without detected glitches to investigate potential correlations.

The study found clear evidence of core glitch signatures, indicative of structural discontinuities, in 24 out of 359 stars (6.7%). The glitch positions and amplitudes showed considerable dispersion, possibly due to the range of metallicities in the sample. The characteristics of the frequency modulations suggest strong discontinuities near the edges of the gravity-wave resonant cavity. The authors considered several potential causes for the discontinuities, such as the hydrogen-burning shell, the chemical discontinuity from the first dredge-up, and the helium flash. However, they concluded that the most likely explanation is the chemical discontinuity produced by mixing beyond the convective core boundary. A comparison of the observed glitch parameters with those from stellar models supported this interpretation. Models employing radiative temperature gradients in the extra-mixing region accurately predicted the observed glitch characteristics, while models with adiabatic stratifications did not. The distribution of period spacings (ΔΠ₁) for stars with and without glitches was similar, suggesting the glitch's existence is not purely dependent on evolutionary stage. The lack of differences in the distributions of mass, metallicity, and seismic parameters between stars with and without glitches indicates that no specific stellar properties are uniquely correlated with glitch occurrence. This suggests that either glitches are present in all stars but often below the detection threshold or they are intermittently smoothed out by an unknown physical mechanism. The authors presented the parameters of the glitches detected in the sample of stars. Histograms of the glitch position and amplitude were presented, and their distribution was compared to predictions from the stellar models.

The detection of core glitches in a substantial fraction of red-giant clump stars supports models that assume radiative thermal stratification in layers adjacent to the convective core. It also rules out models with chemical mixing prescriptions that do not allow the formation of significant structural variations. The fact that glitches are found in only about 12.5% of the stars studied suggests that the formation of these glitches is not a continuous process but rather an intermittent phenomenon. The similarity of other stellar properties (mass, metallicity, etc.) between stars with and without glitches further strengthens this conclusion. The authors propose two scenarios to explain this: either the glitches are always present but their amplitudes are often too small to be detected, or they are temporarily smoothed out by an unknown mechanism. The former seems unlikely based on their models. The latter would require considering new physical processes not currently incorporated into stellar models. This research significantly impacts state-of-the-art stellar models by providing crucial constraints on the description of core mixing processes in red-giant stars. Future improvements to stellar models, incorporating the findings of this study, will enhance the accuracy of clump-star characterization, benefiting related research areas such as galactic archaeology.

This study provides strong observational evidence for the presence of structural discontinuities, or glitches, in the cores of a subset of red-giant clump stars. The findings support models incorporating radiative temperature gradients in the regions beyond the convective core. The intermittent nature of glitch formation suggests the need for revisiting current stellar evolution models to incorporate additional physical processes. These results have significant implications for improving our understanding of stellar evolution and will enhance the accuracy of clump-star applications in diverse astrophysical research areas.

The study's sample size, while relatively large compared to previous work, might not fully represent the diversity of red-giant clump stars. The reliance on Kepler data limits the analysis to stars observed by that mission. The identification and characterization of glitches were done using theoretical models, which are subject to limitations and simplifications. The model that the researchers used to fit their data is a simplified representation of the reality and could be improved.