The inner structure of core helium burning (CHeB) stars remains uncertain due to the yet unknown nature of mixing at the boundary of their cores. Large convective cores beyond a bare Schwarzschild model are favoured both from theoretical arguments and from asteroseismological constraints. However, the exact nature of this extra mixing, and in particular the possible presence of semiconvective layers, is still debated. In this work, we approach this problem through a new avenue by performing the first fullsphere 3D hydrodynamics simulations of the interiors of CHeB stars. We use the ppmstar explicit gas dynamics code to simulate the inner 0.45$\, {\rm M}_{\odot }$ of a 3 M⊙ CHeB star. Simulations are performed using different Cartesian grid resolutions (7683, 11523, and 17283) and heating rates. We use two different initial states, one based on mesas's predictive mixing scheme (which significantly extends the core beyond the Schwarzschild boundary) and one based on the convective premixing approach (which exhibits a semiconvective interface). The general behaviour of the flow in the convective core and in the stable envelope (where internal gravity waves are observed) is consistent with our recent simulations of core convection in massive mainsequence stars, and so are the various luminosity scaling relations. The semiconvective layers are dominated by strong internal gravity waves that do not produce measurable species mixing, but overshooting motions from the convective core gradually homogenize the semiconvective interface. This process can possibly completely erase the semiconvective layers, which would imply that CHeB stars do not harbour a semiconvection zone.
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ABSTRACT 
ABSTRACT We present the first 3D hydrodynamics simulations of the excitation and propagation of internal gravity waves (IGWs) in the radiative interiors of lowmass stars on the red giant branch (RGB). We use the ppmstar explicit gas dynamics code to simulate a portion of the convective envelope and all the radiative zone down to the hydrogenburning shell of a $1.2\, {\rm M}_{\odot }$ upper RGB star. We perform simulations for different grid resolutions (7683, 15363, and 28803), a range of driving luminosities, and two different stratifications (corresponding to the bump luminosity and the tip of the RGB). Our RGB tip simulations can be directly performed at the nominal luminosity, circumventing the need for extrapolations to lower luminosities. A rich, continuous spectrum of IGWs is observed, with a significant amount of total power contained at high wavenumbers. By following the time evolution of a passive dye in the stable layers, we find that IGW mixing in our simulations is weaker than predicted by a simple analytical prescription based on shear mixing and not efficient enough to explain the missing RGB extra mixing. However, we may be underestimating the efficiency of IGW mixing given that our simulations include a limited portion of the convective envelope. Quadrupling its radial extent compared to our fiducial setup increases convective velocities by up to a factor 2 and IGW velocities by up to a factor 4. We also report the formation of a $\sim 0.2\, H_P$ penetration zone and evidence that IGWs are excited by plumes that overshoot into the stable layers.

ABSTRACT Supermassive stars are Population III stars with masses exceeding $10^4\, {\rm M}_{\odot }$ that could be the progenitors of the first supermassive black holes. Their interiors are in a regime where radiation pressure dominates the equation of state. In this work, we use the explicit gas dynamics code ppmstar to simulate the hydrogenburning core of a $10^4\, {\rm M}_{\odot }$ supermassive mainsequence star. These are the first threedimensional hydrodynamics simulations of core convection in supermassive stars. We perform a series of 10 simulations at different heating rates and on Cartesian grids with resolutions of 7683, 11523, and 17283. We examine different properties of the convective flow, including its largescale morphology, its velocity spectrum, and its mixing properties. We conclude that the radiation pressuredominated nature of the interior does not noticeably affect the behaviour of convection compared to the case of core convection in a massive mainsequence star where gas pressure dominates. Our simulations also offer support for the use of mixinglength theory in onedimensional models of supermassive stars.

Asymptotic Giant Branch (AGB) stars play a key role in the chemical evolution of galaxies. These stars are the fundamental stellar site for the production of light elements such as C, N and F, and half of the elements heavier than Fe via the slow neutron capture process (sprocess). Hence, detailed computational models of AGB stars’ evolution and nucleosynthesis are essential for galactic chemical evolution. In this work, we discuss the progress in updating the NuGrid data set of AGB stellar models and abundance yields. All stellar models have been computed using the MESA stellar evolution code, coupled with the postprocessing mppnp code to calculate the full nucleosynthesis. The final data set will include the initial masses Mini/M⊙ = 1, 1.65, 2, 3, 4, 5, 6 and 7 for initial metallicities Z = 0.0001, 0.001, 0.006, 0.01, 0.02 and 0.03. Observed sprocess abundances on the surfaces of evolved stars as well as the typical light elements in the composition of Hdeficient postAGB stars are reproduced. A key shortterm goal is to complete and expand the AGB stars data set for the full metallicity range. Chemical yield tables are provided for the available models.more » « less

Abstract We update the capabilities of the openknowledge software instrument Modules for Experiments in Stellar Astrophysics (
MESA ). The newauto _diff module implements automatic differentiation inMESA , an enabling capability that alleviates the need for hardcoded analytic expressions or finitedifference approximations. We significantly enhance the treatment of the growth and decay of convection inMESA with a new model for timedependent convection, which is particularly important during latestage nuclear burning in massive stars and electrondegenerate ignition events. We strengthenMESA ’s implementation of the equation of state, and we quantify continued improvements to energy accounting and solver accuracy through a discussion of different energy equation features and enhancements. To improve the modeling of stars inMESA , we describe key updates to the treatment of stellar atmospheres, molecular opacities, Compton opacities, conductive opacities, element diffusion coefficients, and nuclear reaction rates. We introduce treatments of starspots, an important consideration for lowmass stars, and modifications for superadiabatic convection in radiationdominated regions. We describe new approaches for increasing the efficiency of calculating monochromatic opacities and radiative levitation, and for increasing the efficiency of evolving the late stages of massive stars with a new operatorsplit nuclear burning mode. We close by discussing major updates toMESA ’s software infrastructure that enhance source code development and community engagement. 
null (Ed.)ABSTRACT The abundances of neutron (n)capture elements in the carbonenhanced metalpoor (CEMP)r/s stars agree with predictions of intermediate ndensity nucleosynthesis, at Nn ∼ 1013–1015 cm−3, in rapidly accreting white dwarfs (RAWDs). We have performed Monte Carlo simulations of this intermediateprocess (iprocess) nucleosynthesis to determine the impact of (n,γ) reaction rate uncertainties of 164 unstable isotopes, from 131I to 189Hf, on the predicted abundances of 18 elements from Ba to W. The impact study is based on two representative onezone models with constant values of Nn = 3.16 × 1014 and 3.16 × 1013 cm−3 and on a multizone model based on a realistic stellar evolution simulation of Heshell convection entraining H in a RAWD model with [Fe/H] = −2.6. For each of the selected elements, we have identified up to two (n,γ) reactions having the strongest correlations between their rate variations constrained by Hauser–Feshbach computations and the predicted abundances, with the Pearson product–moment correlation coefficients rP > 0.15. We find that the discrepancies between the predicted and observed abundances of Ba and Pr in the CEMPi star CS 31062−050 are significantly diminished if the rate of 137Cs(n,γ)138Cs is reduced and the rates of 141Ba(n,γ)142Ba or 141La(n,γ)142La increased. The uncertainties of temperaturedependent βdecay rates of the same unstable isotopes have a negligible effect on the predicted abundances. Onezone Monte Carlo simulations can be used instead of computationally timeconsuming multizone Monte Carlo simulations in reaction rate uncertainty studies if they use comparable values of Nn. We discuss the key challenges that RAWD simulations of i process for CEMPi stars meet by contrasting them with recently published lowZ asymptotic giant branch (AGB) i process.more » « less

null (Ed.)ABSTRACT We present two mixing models for postprocessing of 3D hydrodynamic simulations applied to convective–reactive iprocess nucleosynthesis in a rapidly accreting white dwarf (RAWD) with [Fe/H] = −2.6, in which H is ingested into a convective He shell. A 1D advective twostream model adopts physically motivated radial and horizontal mixing coefficients constrained by 3D hydrodynamic simulations. A simpler approach uses diffusion coefficients calculated from the same simulations. All 3D simulations include the energy feedback of the 12C(p, γ)13N reaction from the H entrainment. Global oscillations of shell H ingestion in two of the RAWD simulations cause bursts of entrainment of H and nonradial hydrodynamic feedback. With the same nuclear network as in the 3D simulations, the 1D advective twostream model reproduces the rate and location of the H burning within the He shell closely matching the 3D simulation predictions, as well as qualitatively displaying the asymmetry of the XH profiles between the upstream and downstream. With a full iprocess network the advective mixing model captures the difference in the ncapture nucleosynthesis in the upstream and downstream. For example, 89Kr and 90Kr with halflives of $3.18\,\,\mathrm{\mathrm{min}}$ and $32.3\,\,\mathrm{\mathrm{s}}$ differ by a factor 2–10 in the two streams. In this particular application the diffusion approach provides globally the same abundance distribution as the advective twostream mixing model. The resulting iprocess yields are in excellent agreement with observations of the exemplary CEMPr/s star CS31062050.more » « less

null (Ed.)The slow neutroncapture process (sprocess) efficiency in lowmass AGB stars (1.5 < M/M⊙ < 3) critically depends on how mixing processes in stellar interiors are handled, which is still affected by considerable uncertainties. In this work, we compute the evolution and nucleosynthesis of lowmass AGB stars at low metallicities using the MESA stellar evolution code. The combined data set includes models with initial masses Mini/M⊙=2 and 3 for initial metallicities Z=0.001 and 0.002. The nucleosynthesis was calculated for all relevant isotopes by postprocessing with the NuGrid mppnp code. Using these models, we show the impact of the uncertainties affecting the main mixing processes on heavy element nucleosynthesis, such as convection and mixing at convective boundaries. We finally compare our theoretical predictions with observed surface abundances on lowmetallicity stars. We find that mixing at the interface between the Heintershell and the COcore has a critical impact on the sprocess at low metallicities, and its importance is comparable to convective boundary mixing processes under the convective envelope, which determine the formation and size of the 13Cpocket. Additionally, our results indicate that models with very low to no mixing below the Heintershell during thermal pulses, and with a 13Cpocket size of at least ∼3 × 10−4 M⊙, are strongly favored in reproducing observations. Online access to complete yield data tables is also provided.more » « less

ABSTRACT We have modelled the multicycle evolution of rapidly accreting CO white dwarfs (RAWDs) with stable H burning intermittent with strong Heshell flashes on their surfaces for 0.7 ≤ MRAWD/M⊙ ≤ 0.75 and [Fe/H] ranging from 0 to −2.6. We have also computed the iprocess nucleosynthesis yields for these models. The i process occurs when convection driven by the Heshell flash ingests protons from the accreted Hrich surface layer, which results in maximum neutron densities Nn, max ≈ 1013–1015 cm−3. The Hingestion rate and the convective boundary mixing (CBM) parameter ftop adopted in the onedimensional nucleosynthesis and stellar evolution models are constrained through threedimensional (3D) hydrodynamic simulations. The mass ingestion rate and, for the first time, the scaling laws for the CBM parameter ftop have been determined from 3D hydrodynamic simulations. We confirm our previous result that the highmetallicity RAWDs have a low mass retention efficiency ($\eta \lesssim 10{{\ \rm per\ cent}}$). A new result is that RAWDs with [Fe/H] $\lesssim 2$ have $\eta \gtrsim 20{{\ \rm per\ cent}}$; therefore, their masses may reach the Chandrasekhar limit and they may eventually explode as SNeIa. This result and the good fits of the iprocess yields from the metalpoor RAWDs to the observed chemical composition of the CEMPr/s stars suggest that some of the presentday CEMPr/s stars could be former distant members of triple systems, orbiting close binary systems with RAWDs that may have later exploded as SNeIa.more » « less