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Register a new userConvective boundary mixing in low- and intermediate-mass stars I. Core properties from pressure-mode asteroseismology
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Convective boundary mixing (CBM) is ubiquitous in stellar evolution. It is a necessary ingredient in the models in order to match observational constraints from clusters, binaries and single stars alike. We compute `effective overshoot measures that reflect the extent of mixing and which can differ significantly from the input overshoot values set in the stellar evolution codes. We use constraints from pressure modes to infer the CBM properties of Kepler and CoRoT main-sequence and subgiant oscillators, as well as in two radial velocity targets (Procyon A and $alpha$ Cen A). Collectively these targets allow us to identify how measurement precision, stellar spectral type, and overshoot implementation impact the asteroseismic solution. With these new measures we find that the `effective overshoot for most stars is in line with physical expectations and calibrations from binaries and clusters. However, two F-stars in the CoRoT field (HD 49933 and HD 181906) still necessitate high overshoot in the models. Due to short mode lifetimes, mode identification can be difficult in these stars. We demonstrate that an incongruence between the radial and non-radial modes drives the asteroseismic solution to extreme structures with highly-efficient CBM as an inevitable outcome. Understanding the cause of seemingly anomalous physics for such stars is vital for inferring accurate stellar parameters from TESS data with comparable timeseries length.
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The Gaia M-dwarf gap is a significant under-density of stars observed near $M_G = 10.2$ in a color-magnitude diagram for stars within 200 pc of the Sun. It has been proposed that the gap is the manifestation of structural instabilities within stellar interiors due to non-equilibrium $^{3}$He fusion prior to some stars becoming fully convective. To test this hypothesis, we use Dartmouth stellar evolution models, MARCS model atmospheres, and simple stellar population synthesis to create synthetic $M_G$-($G_{rm BP} - G_{rm RP})$ color-magnitude diagrams. We confirm that the proposed $^{3}$He instability is responsible for the appearance of the M-dwarf gap. Our synthetic gap shows qualitatively similar features to the observed gap including: its vertical extent in $M_G$, its slope in the color-magnitude diagram, and its relative prominence at bluer colors as compared to redder colors. Furthermore, corresponding over-densities of stars above the gap are reproduced by the models. While qualitatively similar, the synthetic gap is approximately 0.2 magnitudes bluer and, accounting for this color offset, 0.16 magnitudes brighter than the observed gap. Our results reveal that the Gaia M dwarf gap is sensitive to conditions within cores of M dwarf stars, making the gap a powerful tool for testing the physics of M dwarf stars and potentially using M dwarfs to understand the local star formation history.
Convective boundary mixing is one of the major uncertainties in stellar evolution. In order to study its dependence on boundary properties and turbulence strength in a controlled way, we computed a series of 3D hydrodynamical simulations of stellar convection during carbon burning with a varying boosting factor of the driving luminosity. Our 3D implicit large eddy simulations were computed with the PROMPI code. We performed a mean field analysis of the simulations within the Reynolds-averaged Navier-Stokes framework. Both the vertical RMS velocity within the convective region and the bulk Richardson number of the boundaries are found to scale with the driving luminosity as expected from theory. The positions of the convective boundaries were estimated through the composition profiles across them, and the strength of convective boundary mixing was determined by analysing the boundaries within the framework of the entrainment law. We find that the entrainment is approximately inversely proportional to the bulk Richardson number. Although the entrainment law does not encompass all the processes occurring at boundaries, our results support the use of the entrainment law to describe convective boundary mixing in 1D models, at least for the advanced phases. The next steps and challenges ahead are also discussed.
Aims: We investigate from a theoretical perspective if space asteroseismology can be used to distinguish between different thermal structures and shapes of the near-core mixing profiles for different types of coherent oscillation modes in massive stars with convective cores, and if this capacity depends on the evolutionary stage of the models along the main sequence. Methods: We compute 1D stellar structure and evolution models for four different prescriptions of the mixing and temperature gradient in the near-core region. Their effect on the frequencies of dipole prograde gravity modes in both Slowly Pulsating B and $beta$ Cep stars is investigated, as well as for pressure modes in $beta$ Cep stars. Results: A comparison between the mode frequencies of the different models at various stages during the main sequence evolution reveals that they are more sensitive to a change in temperature gradient than to the exact shape of the mixing profile in the near-core region. Depending on the duration of the observed light curve, one can distinguish between either just the temperature gradient, or also between the shapes of the mixing coefficient. The relative frequency differences are in general larger for more evolved models, and are largest for the higher-frequency pressure modes in $beta$ Cep stars. Conclusions:In order to unravel the core boundary mixing and thermal structure of the near-core region, one must have asteroseismic masses and radii with $sim 1%$ relative precision for hundreds of stars.
Spectroscopic studies of Galactic O and B stars show that many stars with masses above 8 M$_{odot}$ are observed in the HR diagram just beyond the Main-Sequence (MS) band predicted by stellar models computed with a moderate overshooting. This may be an indication that the convective core sizes in stars in the upper part of the HR diagram are larger than predicted by these models. Combining stellar evolution models and spectroscopic parameters derived for a large sample of Galactic O and B stars, including brand new information about their projected rotational velocities, we reexamine the question of the convective core size in MS massive stars. We confirm that for stars more massive than about 8 M$_{odot}$, the convective core size at the end of the MS phase increases more rapidly with the mass than in models computed with a constant step overshoot chosen to reproduce the main sequence width in the low mass range (around 2 M$_{odot}$). This conclusion is valid for both the cases of non-rotating models and rotating models either with a moderate or a strong angular momentum transport. The increase of the convective core mass with the mass obtained from the TAMS position is, however, larger than the one deduced from the surface velocity drop for masses above about 15 M$_{odot}$. Although observations available at the moment cannot decide what is the best choice between the core sizes given by the TAMS and the velocity drop, we discuss different methods to get out of this dilemma. At the moment, comparisons with eclipsing binaries seem to favor the solution given by the velocity drop. While we confirm the need for larger convective cores at higher masses, we find tensions in-between different methods for stars more massive than 15 M$_{odot}$. The use of single-aged stellar populations (non-interacting binaries or stellar clusters) would be a great asset to resolve this tension.
The semi-empirical initial-final mass relation (IFMR) connects spectroscopically analyzed white dwarfs in star clusters to the initial masses of the stars that formed them. Most current stellar evolution models, however, predict that stars will evolve to white dwarfs $sim$0.1 M$_odot$ less massive than that found in the IFMR. We first look at how varying theoretical mass-loss rates, third dredge-up efficiencies, and convective-core overshoot may help explain the differences between models and observations. These parameters play an important role at the lowest masses (M$_{rm initial}$ $<$ 3 M$_odot$). At higher masses, only convective-core overshoot meaningfully affects white dwarf mass, but alone it likely cannot explain the observed white dwarf masses nor why the IFMR scatter is larger than observational errors predict. These higher masses, however, are also where rotational mixing in main sequence stars begins to create more massive cores, and hence more massive white dwarfs. This rotational mixing also extends a stars lifetime, making faster rotating progenitors appear like less massive stars in their semi-empirical age analysis. Applying the observed range of young B-dwarf rotations to the MIST or SYCLIST rotational models demonstrates a marked improvement in reproducing both the observed IFMR data and its scatter. The incorporation of both rotation and efficient convective-core overshoot significantly improves the match with observations. This work shows that the IFMR provides a valuable observational constraint on how rotation and convective-core overshoot affect the core evolution of a star.
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