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Thanks to its exceptional near-infrared photometry, JWST can effectively contribute to the discovery, characterisation, and understanding of multiple stellar populations in globular clusters, especially at low masses where theHubbleSpace Telescope (HST) faces limitations. This paper continues the efforts of the JWST GO-1979 programme in exploring the faintest members of the globular cluster NGC 6397. In this work, we show that the combination of HST and JWST data allows us to identify two groups of MS stars: MSa, the first-generation group, and MSb, the second-generation group. We measured the ratio between the two groups and combined it with measurements from the literature focused on more central fields and more massive stars compared to our study. Our findings suggest that the MSa and MSb stars are present in a ≈30−70 ratio regardless of the distance from the centre of the cluster and the mass of the stars used so far. However, considering the limited areal coverage of our study, a more comprehensive spatial analysis is necessary to definitively confirm complete spatial mixing. 

ABSTRACT In the fourth paper of this series, we present – and publicly release – the state-of-the-art catalogue and atlases for the two remaining parallel fields observed with the Hubble Space Telescope for the large programme on ω Centauri. These two fields are located at ∼12 arcmin from the centre of the globular cluster (in the west and south-west directions) and were imaged in filters from the ultraviolet to the infrared. Both fields were observed at two epochs separated by about 2 yr that were used to derive proper motions and to compute membership probabilities. 

Context. With the advent of space-based asteroseismology, determining accurate properties of red-giant stars using their observed oscillations has become the focus of many investigations due to their implications in a variety of fields in astrophysics. Stellar models are fundamental in predicting quantities such as stellar age, and their reliability critically depends on the numerical implementation of the physics at play in this evolutionary phase. Aims. We introduce the Aarhus red giants challenge, a series of detailed comparisons between widely used stellar evolution and oscillation codes that aim to establish the minimum level of uncertainties in properties of red giants arising solely from numerical implementations. We present the first set of results focusing on stellar evolution tracks and structures in the red-giant-branch (RGB) phase. Methods. Using nine state-of-the-art stellar evolution codes, we defined a set of input physics and physical constants for our calculations and calibrated the convective efficiency to a specific point on the main sequence. We produced evolutionary tracks and stellar structure models at a fixed radius along the red-giant branch for masses of 1.0  M ⊙ , 1.5  M ⊙ , 2.0  M ⊙ , and 2.5  M ⊙ , and compared the predicted stellar properties. Results. Once models have been calibrated on the main sequence, we find a residual spread in the predicted effective temperatures across all codes of ∼20 K at solar radius and ∼30–40 K in the RGB regardless of the considered stellar mass. The predicted ages show variations of 2–5% (increasing with stellar mass), which we attribute to differences in the numerical implementation of energy generation. The luminosity of the RGB-bump shows a spread of about 10% for the considered codes, which translates into magnitude differences of ∼0.1 mag in the optical V -band. We also compare the predicted [C/N] abundance ratio and find a spread of 0.1 dex or more for all considered masses. Conclusions. Our comparisons show that differences at the level of a few percent still remain in evolutionary calculations of red giants branch stars despite the use of the same input physics. These are mostly due to differences in the energy generation routines and interpolation across opacities, and they call for further investigation on these matters in the context of using properties of red giants as benchmarks for astrophysical studies. 

Contact. The large quantity of high-quality asteroseismic data that have been obtained from space-based photometric missions and the accuracy of the resulting frequencies motivate a careful consideration of the accuracy of computed oscillation frequencies of stellar models, when applied as diagnostics of the model properties. Aims. Based on models of red-giant stars that have been independently calculated using different stellar evolution codes, we investigate the extent to which the differences in the model calculation affect the model oscillation frequencies and other asteroseismic diagnostics. Methods. For each of the models, which cover four different masses and different evolution stages on the red-giant branch, we computed full sets of low-degree oscillation frequencies using a single pulsation code and, from these frequencies, typical asteroseismic diagnostics. In addition, we carried out preliminary analyses to relate differences in the oscillation properties to the corresponding model differences. Results. In general, the differences in asteroseismic properties between the different models greatly exceed the observational precision of these properties. This is particularly true for the nonradial modes whose mixed acoustic and gravity-wave character makes them sensitive to the structure of the deep stellar interior and, hence, to details of their evolution. In some cases, identifying these differences led to improvements in the final models presented here and in Paper I; here we illustrate particular examples of this. Conclusions. Further improvements in stellar modelling are required in order fully to utilise the observational accuracy to probe intrinsic limitations in the modelling and improve our understanding of stellar internal physics. However, our analysis of the frequency differences and their relation to stellar internal properties provides a striking illustration of the potential, in particular, of the mixed modes of red-giant stars for the diagnostics of stellar interiors.