Uncertainties on near-core mixing in red-clump stars: effects on the period spacing and on the luminosity of the AGB bump

Abstract

Low-mass stars in the He-core-burning phase (HeCB) play a major role in stellar, galactic, and extragalactic astrophysics. The ability to predict accurately the properties of these stars, however, depends on our understanding of convection, which remains one of the key open questions in stellar modelling. We argue that the combination of the luminosity of the AGB bump (AGBb) and the period spacing of gravity modes (∆Π 1 ) during the HeCB phase, provides us with a decisive test to discriminate between competing models of these stars. We use the MESA, BaSTI, and PARSEC stellar evolution codes to model a typical giant star observed by Kepler. We explore how various near-core-mixing scenarios affect the predictions of the above-mentioned constraints, and we find that ∆Π 1 depends strongly on the prescription adopted. Moreover we show that the detailed behaviour of ∆Π 1 shows the signature of sharp variations in the Brunt-Väisälä frequency, which could potentially give additional information about near-core features. We find evidence for the AGBb among Kepler targets, and a first comparison with observations shows that, even if standard models are able to reproduce the luminosity distribution, no standard model can account for satisfactorily the period spacing of HeCB stars. Our analysis allows us to outline a candidate model to describe simultaneously the two observed distributions: a model with a moderate overshooting region characterized by an adiabatic thermal stratification. This prescription will be tested in the future on cluster stars, to limit possible observational biases.

The treatment of mixing in core helium-burning models – III. Suppressing core breathing pulses with a new constraint on overshoot

“…Our models with a solar-calibrated MLT parameter αMLT = 1.60 automatically match ∆ν during the CHeB phase. However, prior to the cessation of core convection they rapidly cross the ∆ν − ∆P1 diagram (which is also seen for the various CHeB models in Figure 3 in Bossini et al 2015) and therefore predict the existence of more stars around ∆ν ≈ 3 µHz and ∆P1 ≈ 300 s than are observed. It is possible that a selection effect is responsible for the absence of observed stars in this part of the diagram.…”

“…Interestingly, this approach, in particular for models with 'penetrative convection' where the adiabatic temperature gradient is imposed in the overshooting region, tends to generate a structure very similar to the maximal overshoot models in Papers I and II (see e.g. Figure 2 and 4 in Bossini et al 2015).…”

Impact of convective boundary mixing on the TP-AGB

“…Asteroseismic studies of Core Helium Burning (CHeB) stars support the existence of the processes mentioned above (Charpinet et al 2011;Constantino et al 2015), and might even help to select a preferred algorithm (Bossini et al 2015). The evidence from star counts (Constantino et al 2016) also seems to suggest that the naive implementation of the Schwarzschild Criterion in CHeB wrongly estimates the size of He-burning cores (as pointed out by Gabriel et al 2014).…”

“…2 of De Gerónimo et al 2019) that, as long as f CHeB is included and kept within a moderate range (0 < f 0.035) the evolution of all relevant quantities remains almost unchanged. Due to the lack of very tight constraints and the fact that under moderate assumptions for CBM in the He-burning core all algorithms lead to similar sizes in the convective cores (Bossini et al 2015;Constantino et al 2015Constantino et al , 2017De Gerónimo et al 2019), it is common practice to take the same calibration as in the core hydrogen burning phase and to apply it to the core helium burning as well (e.g. Weiss & Ferguson 2009;Miller Bertolami 2016;Jones et al 2016;Ritter et al 2018).…”

Modelling of Red Giant Stars: The state-of-the-art

“…Recent advances in asteroseismic observations and techniques ( [59,60]) are starting to add very direct observational constraints to the core mixing process during the central He-burning phase, that coupled to theoretical inferences and indications from star counts in GGCs, make a strong case for the core mixed region to be extended beyond the Schwarzschild border.…”