Context. Grids of stellar models, computed with the same physical ingredients, allow one to study the impact of a given physics on a broad range of initial conditions and they are a key ingredient for modeling the evolution of galaxies. Aims. We present here a grid of single star models for masses between 0.8 and 120 M⊙, with and without rotation for a mass fraction of heavy element Z = 0.006, representative of the Large Magellanic Cloud (LMC). Methods. We used the GENeva stellar Evolution Code. The evolution was computed until the end of the central carbon-burning phase, the early asymptotic giant branch phase, or the core helium-flash for massive, intermediate, and low mass stars, respectively. Results. The outputs of the present stellar models are well framed by the outputs of the two grids obtained by our group for metallicities above and below the one considered here. The models of the present work provide a good fit to the nitrogen surface enrichments observed during the main sequence for stars in the LMC with initial masses around 15 M⊙. They also reproduce the slope of the luminosity function of red supergiants of the LMC well, which is a feature that is sensitive to the time-averaged mass loss rate over the red supergiant phase. The most massive black hole that can be formed from the present models at Z = 0.006 is around 55 M⊙. No model in the range of mass considered will enter into the pair-instability supernova regime, while the minimal mass to enter the region of pair pulsation instability is around 60 M⊙ for the rotating models and 85 M⊙ for the nonrotating ones. Conclusions. The present models are of particular interest for comparisons with observations in the LMC and also in the outer regions of the Milky Way. We provide public access to numerical tables that can be used for computing interpolated tracks and for population synthesis studies.
Understanding the nature of the first stars is key to understanding the early Universe. With new facilities such as James Webb Space Telescope (JWST) we may soon have the first observations of the earliest stellar populations, but to understand these observations we require detailed theoretical models. Here we compute a grid of stellar evolution models using the Geneva code with the aim to improve our understanding of the evolution of zero-metallicity stars, with particular interest in how rotation affects surface properties, interior structure, and metal enrichment. We produce a range of models of initial masses (Mini) from $1.7$ to $120\, \mathrm{M}_{\odot }$, focusing on massive models of $9 \le M_{\rm ini}\le 120\, \mathrm{M}_{\odot }$. Our grid includes models with and without rotation, with rotating models having an initial velocity of 40 per cent of the critical velocity. We find that rotation strongly impacts the evolution of the first stars, mainly through increased core size and stronger H-burning shells during core He-burning. Without radiative mass loss, angular momentum builds at the surface in rotating models, thus models of initial masses $M_{\rm ini}\ge 60 \, \mathrm{M}_{\odot }$ reach critical rotation on the main sequence and experience mass loss. We find that rotational mixing strongly affects metal enrichment, but does not always increase metal production as we see at higher metallicities. This is because rotation leads to an earlier CNO boost to the H shell during He-burning, which may hinder metal enrichment depending on initial mass and rotational velocity. Electronic tables of this new grid of Population III models are publicly available.
Context. Being part of the brightest solar-like stars, and close solar analogues, the 16 Cygni system is of great interest to the scientific community and may provide insight into the past and future evolution of our Sun. It has been observed thoroughly by the Kepler satellite, which provided us with data of an unprecedented quality. Aims. This paper is the first of a series aiming to extensively characterise the system. We test several choices of micro- and macro-physics to highlight their effects on optimal stellar parameters and provide realistic stellar parameter ranges. Methods. We used a recently developed method, WhoSGlAd, that takes the utmost advantage of the whole oscillation spectrum of solar-like stars by simultaneously adjusting the acoustic glitches and the smoothly varying trend. For each choice of input physics, we computed models which account, at best, for a set of seismic indicators that are representative of the stellar structure and are as uncorrelated as possible. The search for optimal models was carried out through a Levenberg-Marquardt minimisation. First, we found individual optimal models for both stars. We then selected the best candidates to fit both stars while imposing a common age and composition. Results. We computed realistic ranges of stellar parameters for individual stars. We also provide two models of the system regarded as a whole. We were not able to build binary models with the whole set of choices of input physics considered for individual stars as our constraints seem too stringent. We may need to include additional parameters to the optimal model search or invoke non-standard physical processes.
Context. The CoRoT and Kepler missions have paved the way for synergies between exoplanetology and asteroseismology. The use of seismic data helps providing stringent constraints on the stellar properties which directly impact the results of planetary studies. Amongst the most interesting planetary systems discovered by Kepler, Kepler-444 is unique by the quality of its seismic and classical stellar constraints. Its magnitude, age and the presence of 5 small-sized planets orbiting this target makes it an exceptional testbed for exoplanetology. Aims. We aim at providing a detailed characterization of Kepler-444, focusing on the dependency of the results on variations of key ingredients of the theoretical stellar models. This thorough study will serve as a basis for future investigations of the planetary evolution of the system orbiting Kepler-444. Methods. We use local and global minimization techniques to study the internal structure of the exoplanet-host star Kepler-444. We combine seismic observations from the Kepler mission, Gaia DR2 data, and revised spectroscopic parameters to precisely constrain its internal structure and evolution. Results. We provide updated robust and precise determinations of the fundamental parameters of Kepler-444 and demonstrate that this low-mass star bore a convective core during a significant portion of its life on the main sequence. Using seismic data, we are able to estimate the lifetime of the convective core to approximately 8 Gyr out of the 11 Gyr of the evolution of Kepler-444. The revised stellar parameters found by our thorough study are M = 0.754 ± 0.03 M⊙, R = 0.753 ± 0.01 R⊙, and Age = 11 ± 1 Gyr.
Context. The advent of space-based photometry missions such as CoRoT, Kepler and TESS has sparkled the rapid development of asteroseismology and its synergies with exoplanetology. In the near future, the advent of PLATO will further strengthen such multi-disciplinary studies. In that respect, testing asteroseismic modelling strategies and their importance for our understanding of planetary systems is crucial. Aims. We carried out a detailed modelling of Kepler-93, an exoplanet host star observed by the Kepler satellite for which high-quality seismic data are available. This star is particularly interesting because it is a solar-like star very similar to the PLATO benchmark target (G spectral type, ∼6000 K, ∼1 M⊙ and ∼1 R⊙) and provides a real-life testbed for potential procedures to be used in the PLATO mission. Methods. We used global and local minimisation techniques to carry out the seismic modelling of Kepler-93, for which we varied the physical ingredients of the given theoretical stellar models. We supplemented this step by seismic inversion techniques of the mean density. We then used these revised stellar parameters to provide new planetary parameters and to simulate the orbital evolution of the system under the effects of tides and atmospheric evaporation. Results. We provide the following fundamental parameters for Kepler-93: ρ̄⋆ = 1.654 ± 0.004 g cm−3, M⋆ = 0.907 ± 0.023 M⊙, R⋆ = 0.918 ± 0.008 R⊙, and Age = 6.78 ± 0.32 Gyr. The uncertainties we report for this benchmark star are well within the requirements of the PLATO mission and give confidence in the ability of providing precise and accurate stellar parameters for solar-like exoplanet-host stars. For the exoplanet Kepler-93b, we find Mp = 4.01 ± 0.67 M⊕, Rp = 1.478 ± 0.014 R⊕, and a semi-major axis a = 0.0533 ± 0.0005 AU. According to our simulations of the orbital evolution of the system, it seems unlikely that Kepler-93b formed with a mass high enough (Mp, initial > 100 M⊕) to be impacted on its orbit by stellar tides. Conclusions. For the benchmark case of a solar twin of the PLATO mission, detailed asteroseismic modelling procedures will be able to provide fundamental stellar parameters within the requirements of the PLATO mission. We also illustrate the synergies that can be achieved regarding the orbital evolution and atmospheric evaporation of exoplanets when these parameters are obtained. We also note the importance of the high-quality radial velocity follow-up, which here is a limiting factor, for providing precise planetary masses and mean densities to constrain the formation scenarii of exoplanets.
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