Stellar Evolution Code Research Articles

A new 12C+12C reaction rate: Impact on stellar evolution

Among the reactions driving stellar evolution during carbon burning, 12C + 12C fusion provides the key ingredients. This system reveals many resonances, but also regions with suppressed fusion cross-sections. The reaction was recently measured by the STELLA collaboration utilizing the gammaparticle coincidence technique for precise cross-section measurements reaching down to the Gamow window of massive stars. From the experimental data, reaction rates were determined by approximating a hindrance parametrization and by adding on top a resonance at the lowest measured energy. The impact of these reaction rates on the evolution of massive stars was explored with models of 12 and 25 M⊙ using the stellar evolution code GENEC, and a detailed study of the resulting nucleosynthesis with a 1454 elements network was performed. The sensitivity of the STELLA experimental cross-sections on the temperature range for C-burning for the stellar models studied were presented. The final abundances and their impacts on stellar evolution were discussed.

Asteroseismic determination of fundamental parameters for exoplanet host stars with deep learning

As the number of exoplanets discovered by space missions, such as NASA’s Kepler and TESS, has been increasing rapidly during the past decades, the need for an efficient approach to determining the properties of the exoplanet hosts as well as the exoplanets is urgent. In this work, we developed a neural network approach based on Gaussian mixture models to directly determine stellar properties and corresponding uncertainties for exoplanet host stars. Over 16 million stellar models with different initial parameters generated by the Aarhus STellar Evolution Code were used to train the neural networks. To validate the performance of our neural networks, we compared the neural network predicted stellar parameters with results from other methods, e.g., scaling relations, machine learning, and grid-based modeling, for 47 main-sequence, subgiant, and red-giant stars that have exoplanets orbiting around them. The fractional differences between our neural networks and other methods for masses, radii, and ages are generally small, though systematic deviations for masses and ages are seen during comparisons due to the different input physics and seismic parameters used in the modeling. The average uncertainties predicted by our networks for these stars are of the order of 2.4%, 1.1%, and 17.4% for mass, radius, and age, respectively. Overall, our neural network method is able to determine stellar parameters accurately and precisely in a short amount of time, indicating a promising future for its application in the study of exoplanets and their host stars.

New insights into the helium star formation channel of AM CVn systems with explanations of Gaia14aae and ZTFJ1637+49

ABSTRACT We model helium-rich stars with solar metallicity (X = 0.7, Z = 0.02) progenitors that evolve to form AM Canum Venaticorum systems through a helium-star formation channel, with the aim to explain the observed properties of Gaia14aae and ZTFJ1637+49. We show that semidegenerate, H-exhausted (X ≤ 10−5), He-rich (Y ≈ 0.98) donors can be formed after a common envelope evolution (CEE) phase if either additional sources of energy are used to eject the common envelope, or a different formalism of CEE is implemented. We follow the evolution of such binary systems after the CEE phase using the Cambridge stellar evolution code when they consist of a He-star and a white dwarf accretor, and report that the mass, radius, and mass-transfer rate of the donor, the orbital period of the system, and the lack of hydrogen in the spectrum of Gaia14aae and ZTFJ1637+49 match well with our modelled trajectories wherein, after the CEE phase Roche lobe overflow is governed not only by the angular momentum loss (AML) owing to gravitational wave radiation (AMLGR) but also an additional AML owing to α–Ω dynamos in the donor. This additional AML is modelled with our double-dynamo (DD) model of magnetic braking in the donor star. We explain that this additional AML is just a consequence of extending the DD model from canonical cataclysmic variable donors to evolved donors. We show that none of our modelled trajectories match with Gaia14aae or ZTFJ1637+49 if the systems are modelled only with AMLGR.

A promising formation channel for symbiotic X-ray binaries: cases of IGR J17329−2731 and 4U 1700+24

ABSTRACT Recent observations demonstrate that the symbiotic X-ray binary (SyXB) IGR J17329−2731 contains a highly magnetized neutron star (NS), which accretes matter through the wind from its giant star companion, and suggest that 4U 1700+24 may also have a highly magnetized NS. Accretion-induced collapse (AIC) from oxygen–neon–magnesium white dwarf (ONeMg WD) + red giant (RG) star binaries is one promising channel to form these SyXBs, while other long standing formation channels have difficulties to produce these SyXBs. By considering non-magnetic and magnetic ONeMg WDs, I investigate the evolution of ONeMg WD + RG binaries with the mesa stellar evolution code for producing SyXBs with non-magnetic or magnetized NSs. In the pre-AIC evolution with magnetic confinement, the mass accumulation efficiency of the accreting WD is increased at low-mass transfer rate compared with the non-magnetic case. The newborn NSs formed via AIC of highly magnetized WDs could inherit the large magnetic field through conservation of magnetic flux, and the systems could have a long age compatible with that of the red giant companions. These young and highly magnetized NSs could accrete matters from the stellar wind of the giant companions to that shine as those observed SyXBs, and could preserve their high magnetic field during this time. The mesa calculation results show that the initial parameter (initial RG mass and orbital period) space for the AIC with magnetic confinement to form SyXBs with highly magnetized NSs shifts to be lower and narrower compared with that of the no magnetic confinement case.

Modelling of the post-asymptotic giant branch phase as a tool to understand asymptotic giant branch evolution and nucleosynthesis

ABSTRACT We study a sample of single Galactic post-asymptotic giant branch (post-AGB) stars with known surface chemical composition. Gaia EDR3 and Gaia DR3 parallaxes have enabled accurate determinations of the luminosities of these objects, thus making it possible to characterize them in terms of their initial masses, chemical compositions, and progenitor ages. We used extant evolutionary sequences of asymptotic giant branch (AGB) stars produced using the ATON stellar evolutionary code extended to the post-AGB phase, specifically for this study. The luminosity and surface carbon abundance together prove to be the most valuable indicator of the previous evolution and nucleosynthetic history of the star, particularly regarding the efficiency of mixing mechanisms and the growth of the core. This analysis allows us to place the post-AGB sample into classes based on their evolutionary history on the AGB. This includes low-mass objects that evolved as M-type stars without third dredge-up, carbon stars, and intermediate-mass AGB stars with hot bottom burning (HBB). Additionally, our analysis reveals that AGB nucleosynthesis is not homogeneous, even for stars with similar initial masses and metallicities. The approach of using observations of post-AGB stars to constrain AGB and post-AGB models shed new light on still debated issues related to the AGB evolution, such as the threshold mass required to become a C-star, the minimum mass for HBB and the amount of carbon that can be accumulated in the surface regions of the star during the AGB lifetime, as well as deep mixing experienced during the red giant branch phase.

Li-rich and super Li-rich giants produced by element diffusion

Context.About 0.2−2% of giant stars are Li rich, that is to say their lithium abundance (A(Li)) is higher than 1.5 dex. Nearly 6% of these Li-rich giant stars are super Li rich, with an A(Li) exceeding 3.2 dex. Meanwhile, the formation mechanism of these Li-rich and super Li-rich giants is still under debate.Aims.Considering the compact He core of red giants, attention is paid to the effect of element diffusion on A(Li). In particular, when the He-core flash occurs, element diffusion makes the thermohaline mixing zone extend inward and connect to the inner convection region of stars. Then, a large amount of7Be produced by the He flash can be transferred to the stellar surface, finally turning into7Li. Thus, the goal of this work is to propose the mechanism of A(Li) enrichment and achieve consistency between the theoretical and observation data.Methods.Using the Modules for Experiments in Stellar Astrophysics (MESA) stellar evolution code, we simulated the evolution of low-mass stars, considering the effects of element diffusion on the Li abundances. The timescale ratio of Li-rich giants to normal giants was estimated using the population synthesis method. Then we obtained the theoretical value of A(Li) and made a comparison with observations.Results.Considering the influence of element diffusion in the model results in the increase of the lithium abundance up to about 1.8 dex, which can reveal Li-rich giants. Simultaneously, introducing high constant diffusive mixing coefficients (Dmix) with the values from 1011to 1015cm2s−1in the model allows the A(Li) to increase from 2.4 to 4.5 dex, which can explain most of the Li-rich and super Li-rich giant stars. The population synthesis method reveals that the amount of Li-rich giants is about 0.2−2% of all giants, which is consistent with observation estimated levels.Conclusions.In our models the element diffusion, mainly triggered by the gravity field, changes the mean molecular weight at the junction zone between the stellar envelope and the He core, which makes the thermohaline mixing region expand to the inner convection region of stars. A transport channel, efficiently transporting7Be in the hydrogen-burning region of the star to the convective envelope where7Be decays into7Li, is formed. Combining high constant diffusive mixing coefficients, the transport channel can explain the origin of Li-rich and super Li-rich giants, even the most super Li-rich giants.

A systematic study of super-Eddington layers in the envelopes of massive stars

Context. The proximity to the Eddington luminosity has been attributed as the cause of several observed effects in massive stars. Computationally, if the luminosity carried through radiation exceeds the local Eddington luminosity in the low-density envelopes of massive stars, it can result in numerical difficulties, inhibiting further computation of stellar models. This problem is exacerbated by the fact that very few massive stars are observed beyond the Humphreys-Davidson limit, the same region in the Hertzsprung-Russell diagram where the aforementioned numerical issues relating to the Eddington luminosity occur in stellar models. Aims. One-dimensional stellar evolution codes have to use pragmatic solutions to evolve massive stars through this computationally difficult phase. In this work, we quantify the impact of these solutions on the evolutionary properties of massive stars. Methods. We used the stellar evolution code MESA with commonly used input parameters for massive stellar models to compute the evolution of stars in the initial mass range of 10–110 M⊙ at one-tenth of solar metallicity. Results. We find that numerical difficulties in stellar models with initial masses greater than or equal to 30 M⊙ cause these models to fail before the end of core helium burning. Recomputing these models using the same physical inputs but three different pragmatic solutions to treat the numerical instability, we find that the maximum radial expansion achieved by stars can vary by up to 2000 R⊙, while the remnant mass of the stars can vary by up to 14 M⊙ between the sets. These differences can have implications on studies such as binary population synthesis.

White Dwarf—Red Giant Star Binaries as Type Ia Supernova Progenitors: With and without Magnetic Confinement

Various white-dwarf (WD) binary scenarios have been proposed trying to understand the nature and the diversity of type Ia supernovae (SNe Ia). In this work, we study the evolution of carbon–oxygen WD—red giant (RG) binaries (including the role of magnetic confinement) as possible SN Ia progenitors (the so-called symbiotic progenitor channel). Using the mesa stellar evolution code, we calculate the time dependence of the structure of the RG star, the wind mass loss, the Roche lobe-overflow mass-transfer rate, the polar mass-accretion rate (in the case of magnetic confinement), and the orbital and angular-momentum evolution. We consider cases where the WD is nonmagnetic and cases where the magnetic field is strong enough to force accretion onto the two small polar caps of the WD. Confined accretion onto a small area allows for more efficient hydrogen burning, potentially suppressing nova outbursts. This makes it easier for the WD to grow in mass toward the Chandrasekhar-mass limit and explode as a SN Ia. With magnetic confinement, the initial parameter space of the symbiotic channel for SNe Ia is shifted toward shorter orbital periods and lower donor masses compared to the case without magnetic confinement. Searches for low-mass He WDs or relatively low-mass giants with partially stripped envelopes that survived the supernova explosion and are found in SN remnants will provide crucial insights for our understanding of the contribution of this symbiotic channel.

Structure and evolution of ultra-massive white dwarfs in general relativity

Context. Ultra-massive white dwarfs (M⋆ ≳ 1.05 M⊙) are of utmost importance in view of the role they play in type Ia supernovae explosions, merger events, the existence of high-magnetic -field white dwarfs, and the physical processes in the super asymptotic giant branch phase. Aims. We aim to present the first set of constant rest-mass ultra-massive oxygen-neon white dwarf cooling tracks with masses of M⋆ > 1.29 M⊙ which fully take into account the effects of general relativity on their structural and evolutionary properties. Methods. We computed the full evolution sequences of 1.29, 1.31, 1.33, 1.35, and 1.369 M⊙ white dwarfs with the La Plata stellar evolution code, LPCODE. For this work, the standard equations of stellar structure and evolution have been modified to include the effects of general relativity. Specifically, the fully general relativistic partial differential equations governing the evolution of a spherically symmetric star are solved in a way so that they resemble the standard Newtonian equations of stellar structure. For comparison purposes, the same sequences have been computed for the Newtonian case. Results. According to our calculations, the evolutionary properties of the most massive white dwarfs are strongly modified by general relativity effects. In particular, the resulting stellar radius is markedly smaller in the general relativistic case, being up to 25% smaller than predicted by the Newtonian treatment for the more massive ones. We find that oxygen-neon white dwarfs more massive than 1.369 M⊙ become gravitationally unstable with respect to general relativity effects. When core chemical distribution due to phase separation on crystallization is considered, such instability occurs at somewhat lower stellar masses, ≳1.36 M⊙. In addition, cooling times for the most massive white dwarf sequences are about a factor of two smaller than in the Newtonian case at advanced stages of evolution. Finally, a sample of white dwarfs have been identified as ideal candidates to test these general relativistic effects. Conclusions. We conclude that the general relativity effects should be taken into account for an accurate assessment of the structural and evolutionary properties of the most massive white dwarfs. These new ultra-massive white dwarf models constitute a considerable improvement over those computed in the framework of the standard Newtonian theory of stellar interiors.

Nucleosynthetic yields of intermediate-mass primordial to extremely metal-poor stars

Context. Stellar models and nucleosynthetic yields of primordial to extremely metal-poor (EMP) stars are crucial to interpret the surface abundances of the most metal-poor stars observed and, ultimately, to better understand the earliest stellar populations. In addition, they are key ingredients of Galactic chemical evolution models. Aims. We aim to better characterise the evolution and fates, and determine updated nucleosynthetic yields of intermediate-mass stars between primordial and EMP metallicity (Z = 10−10, 10−8, 10−7, 10−6, and 10−5). We also probed uncertainties in the nucleosynthesis of the oldest intermediate-mass stars, namely those related to the treatment of convection and convective boundaries and those related to wind prescriptions during the asymptotic giant branch (AGB) phase. Methods. We analyse the evolution of models from their main sequence, through the thermally pulsing AGB (TP-AGB), to the latest stages of their evolution, using the Monash-Mount Stromlo stellar evolution code MONSTAR. The results were post-processed with the code MONSOON, which allowed for the determination of the nucleosynthetic yields of 77 species up to 62Ni. By comparing them to similar calculations existing in the literature, we inspected the effects of input physics on the nucleosynthesis of EMP models. Results. From the evolutionary point of view, as reported in former works, we identified proton ingestion episodes (PIEs) in our lowest-mass lowest-metallicity models. Models of Z = 10−10 and Z = 10−8 in a narrow initial mass range around 5 M⊙ experience the cessation of thermal pulses, and their final fates as type-I1/2 supernovae cannot be discarded. However, the initial mass range of models eventually leading to the formation of type-I1/2 and electron-capture supernovae is considerably reduced compared to former works. All the models of initial mass ≳6–7 M⊙ experience a corrosive second dredge-up and, analogously to those experiencing PIEs, undergo significant metal enrichment in their envelopes. The associated increase in their opacities allows them to develop a solar-like TP-AGB or TP-super-AGB, ultimately becoming white dwarfs. Except for those undergoing the cessation of thermal pulses, all of our models show the nucleosynthetic signatures of both efficient third dredge-up and hot-bottom burning, with the activation of the NeNa cycle and the MgAlSi chains. This leads to the creation of vast amounts of CNO, with typical [N/Fe] > 4), and the characteristic abundance signature [N/Fe] > [C/Fe] > [O/Fe]. Our nucleosynthetic yields present dramatic differences with respect to recent results existing in the literature for intermediate-mass models of similar metallicities. The reason for these discrepancies lay in the poorly known input physics related to stellar winds and, above all, the treatment of convection and convective boundaries.

Characterising the AGB bump and its potential to constrain mixing processes in stellar interiors

Context. In the 1990s, theoretical studies motivated the use of the asymptotic giant branch bump (AGBb) as a standard candle given the weak dependence between its luminosity and stellar metallicity. Because of the small size of observed asymptotic giant branch (AGB) samples, detecting the AGBb is not an easy task. However, this has now been made possible thanks to the wealth of data collected by the CoRoT, Kepler, and TESS space-borne missions. Aims. It is well-known that the AGB bump provides valuable information on the internal structure of low-mass stars, particularly on mixing processes such as core overshooting during the core He-burning phase. Here, we investigate the dependence of the AGBb position on stellar parameters such as the stellar mass and metallicity based on the calibration of stellar models to observations. Methods. In this context, we analysed ∼4000 evolved giants observed by Kepler and TESS, including red giant branch (RGB) stars and AGB stars, for which asteroseismic and spectrometric data are available. By using statistical mixture models, we detected the AGBb both in frequency at maximum oscillation power, νmax, and in effective temperature, Teff. Then, we used the Modules for Experiments in Stellar Astrophysics (MESA) stellar evolution code to model AGB stars and match the AGBb occurrence with observations. Results. From the observations, we were able to derive the AGBb location in 15 bins of mass and metallicity. We noted that the higher the mass, the later the AGBb occurs in the evolutionary track, which agrees with theoretical works. Moreover, we found a slight increase in the luminosity at the AGBb when the metallicity increases. By fitting those observations with stellar models, we noticed that low-mass stars (M ≤ 1.0 M⊙) require a small core overshooting region during the core He-burning phase. This core overshooting extent increases toward high mass; however, above M ≥ 1.5 M⊙, we found that the AGBb location cannot be reproduced with a realistic He-core overshooting alone. Thus, additional mixing processes have to be invoked instead. Conclusions. The observed dependence on metallicity complicates the application of the AGBb as a standard candle. Moreover, different mixing processes may occur according to stellar mass. At low mass (M ≤ 1.5 M⊙), the AGBb location can be used to constrain the He-core overshooting. At high mass (M ≥ 1.5 M⊙), an additional mixing induced, for instance, by rotation is needed to reproduce what is seen in observations.

Binary Evolution and Blue Stragglers in Different Regions of the Color–Magnitude Diagrams of Globular Clusters with Different Ages

Blue stragglers (BSs) in different regions (e.g., two sequences) of the color–magnitude diagram (CMD) have been suggested to provide a strong constraint on the origin of BSs in globular clusters (GCs). However, our understanding of the locations of binary-induced BSs is incomplete, e.g., its low-luminosity boundary. By employing Eggleton’s stellar evolution code, the locations of mass-transferring (MT) BS binaries and post-mass-transfer (post-MT) BS binaries are investigated in the CMDs of GCs with different ages. The results show that post-MT BS binaries always populate the region of the CMD below the ZAMS+0.75 for GCs older than 10 Gyr and can also populate the region above the ZAMS+0.75 (the region of MT BS binaries). The low-luminosity boundary of the binary-induced BSs is located near the ZAMS in a way that is independent of the age of old GCs. The BS binaries draw a narrow sequence below the ZAMS+0.75 and cover most of the BSs in NGC 362. These results support that binary evolution can explain the appearance of BSs in different regions of the CMDs of GCs with different ages, and at least a fraction of BSs along the blue sequence could be post-MT BSs. However, the theoretical gap does not exactly correspond to the observed gap in NGC 362, and the models predict an excess of BSs below ZAMS+0.75 with respect to the observations in NGC 362. Therefore, more theoretical and observational work is needed to clarify the origin of these discrepancies.

Population study of astrophysical false positive detections in the southern plato field

ABSTRACT For the upcoming PLAnetary Transits and Oscillation of stars (PLATO) satellite mission, a large number of target stars are required to yield a statistically significant number of planet transits. Locating the centres of the long duration observational phase (LOP) fields closer to the Galactic plane will increase the target star numbers but also the astrophysical false positives (FPs) from blended eclipsing binary systems. We utilize the Binary Stellar Evolution and Population Synthesis code, to create a complete synthetic stellar and planetary population for the proposed southern LOP field (LOPS0), as well as for a representative portion of the northern LOP field (LOPN-sub). For LOPS0, we find an overall low FP rate for planets smaller than Neptunes. The FP rate generally shows little variation with Galactic longitude (l), and a modest increase with decreasing Galactic latitude (|b|). The location of the LOPS field centre within the current allowed region is not strongly constrained by FPs. Analysis of LOPN-sub suggests a markedly increased number of FPs across the full range of planet radii at low |b| resulting in approximately twice the percentage of FP detection rate in the LOPN-sub compared to the corresponding southern field segment in the planet radius range −0.2 < log (R/R⊕) ≤ 0.4. However, only a few per cent of fully eclipsing FPs in LOPS0 in this radius range have periods between 180 and 1000 d so the vast majority of FPs are expected to be outside the period range of interest for PLATO.

Evolution of Carbon-Oxygen White Dwarf Accreting Helium as One of the Progenitors of Accretion-induced Collapse

Carbon-oxygen white dwarf (CO WD) and helium star binary is one of the ways that can lead to accretion-induced collapse (AIC). The continuous accretion may increase the mass of the white dwarf until at a certain condition, carbon burning off-center might be initiated and alter CO into ONe WD. This paper is intended to analyze the long-term evolution of the CO WD accreting helium material. The stellar evolution code used in this research is MESA (Modules for Experiments in Stellar Astrophysics). MESA creates CO WD by evolving ZAMS star with an initial mass of 6 M⊙ to produce 0.9 M⊙ CO WD. The accretion rates are 4×10−6 and 4×10−7 M⊙/year which consist of mostly helium. It shows that for the high accretion rate, the helium burning on the surface of the WD is stable. It is predicted that it will continue to be stable for a very long time because of the high supply of matter. For the lower one, the burning experiences a fluctuation from the beginning of the accretion. For both cases, if carbon burning off-center has occurred, it may lead to the creation of ONe WD and eventually AIC.

Time-dependent, long-term hydrodynamic simulations of the inner protoplanetary disk

Context. The spin evolution of young protostars, surrounded by an accretion disk, still poses problems for observations and theoretical models. In recent studies, the importance of the magnetic star-disk interaction for stellar spin evolution has been elaborated. The accretion disk in these studies, however, is only represented by a simplified model and important features are not considered. Aims. A more realistic representation of the accretion disk is indispensable for a better understanding of the star-disk interaction and the stellar spin evolution. The aim of this study is to investigate the influence of a hydrodynamic disk evolution on the stellar rotational period and vice versa during the accretion phase. Methods. We combined the implicit hydrodynamic TAPIR disk code with a stellar spin evolution model. The influence of stellar magnetic fields on the disk dynamics, the radial position of the inner disk radius, as well as the influence of stellar rotation on the disk were calculated self-consistently. Results. Within a defined parameter space, we can reproduce the majority of fast and slow rotating stars observed in young stellar clusters. Additionally, the back reaction of different stellar spin evolutionary tracks on the disk can be analyzed. Disks around fast rotating stars are located closer to the star. Consequently, the disk midplane temperature in the innermost disk region increases significantly compared to slow rotating stars. We can show the effects of stellar rotation on episodic accretion outbursts. The higher temperatures of disks around fast rotating stars result in more outbursts and a longer outbursting period over the disk lifetime. Conclusions. The combination of a long-term hydrodynamic disk and a stellar spin evolution model allows the inclusion of previously unconsidered effects such as the back-reaction of stellar rotation on the long-term disk evolution and the occurrence of accretion outbursts. However, a wider parameter range has to be studied to further investigate these effects. Additionally, a possible interaction between our model and a more realistic stellar evolution code (e.g., the MESA code) can improve our understanding of the stellar spin evolution and its effects on the pre-main sequence star.

The intermediate neutron capture process

Context.Alongside the slow (s) and rapid (r) neutron capture processes, an intermediate neutron capture process (i-process) is thought to exist. It happens when protons are mixed in a convective helium-burning zone, and is referred to as proton ingestion event (PIE); however, the astrophysical site of thei-process is still a matter of debate. The asymptotic giant branch (AGB) phase of low-mass low-metallicity stars is among the promising sites in this regard.Aims.For the first time, we providei-process yields of a grid of AGB stars experiencing PIEs.Methods.We computed 12 models with initial masses of 1, 2, and 3M⊙and metallicities of [Fe/H] = −3.0, −2.5 −2.3, and −2.0, with the stellar evolution code STAREVOL. We used a nuclear network of 1160 species at maximum, coupled to the chemical transport equations. These simulations do not include any extra mixing process.Results.Proton ingestion takes place preferentially in low-mass and low-metallicity models, arising in six out of our 12 AGB models: the 1 M⊙models with [Fe/H] = −3, −3 andα-enhancement, −2.5, −2.3, and the 2 M⊙models with [Fe/H] = −3 and −2.5. These models experiencei-process nucleosynthesis characterized by neutron densities of ≃1014 − 1015cm−3. Depending on the PIE properties two different evolution paths follow: either the stellar envelope is quickly lost and no more thermal pulses develop or the AGB phase resumes with additional thermal pulses. This behaviour critically depends on the pulse number when the PIE occurs, the mass of the ingested protons, and the extent to which the pulse material is diluted in the convective envelope. We show that the surface enrichment after a PIE is a robust feature of our models and it persists under various convective assumptions. In ouri-process models, elements above iodine (Z = 53) are the most overproduced, particularly Xe, Yb, Ta, Pb, and Bi. Our 3M⊙models do not experience anyi-process, but instead go through a convectives-process in the thermal pulse with a clear signature on their yields.Conclusions.Thus, AGB stars at low-mass and low-metallicity are expected to contribute to the chemical evolution of heavy elements through thes- andi-processes. Our models can synthesise heavy elements up to Pb without any parametrized extra mixing process such as overshoot or inclusion of a13C-pocket. Nevertheless, it remains to be explored how thei-process depends on mixing processes, such as overshoot, thermohaline, or rotation.

Synthesis of thorium and uranium in asymptotic giant branch stars

Context. The intermediate neutron capture process (i-process) operates at neutron densities between those of the slow and rapid neutron-capture processes. It is believed to be triggered by the ingestion of protons in a convective helium-burning region. One possible astrophysical site is low-mass low-metallicity asymptotic giant branch (AGB) stars. Aims. Although it has been widely believed that actinides, and most particularly Th and U, are exclusively produced by explosive r-process nucleosynthesis, we study here the possibility that actinides may also be significantly synthesized through i-process nucleosynthesis in AGB stars. Methods. We computed a 1 M⊙ model at [Fe/H] = −2.5 with the stellar evolution code STAREVOL. We used a nuclear network of 1160 species from H to Cf coupled to the transport processes. Models of various resolutions (temporal and spatial) that use different nuclear datasets are also considered for the analysis. Results. During the proton ingestion event, the neutron density in our AGB model goes up to ∼1015 cm−3 and is shown to be high enough to give rise to the production of actinides. While most of the nuclear flow cycles in the neutron-rich Pb–Bi–Po region, a non-negligible fraction leaks towards heavier elements and eventually synthesizes actinides. The surface enrichment in Th and U is subject to nuclear and astrophysical model uncertainties that could be lowered in the future, in particular by a detailed analysis of the nuclear inputs that affect the neutron capture rates of neutron-rich isotopes between Pb and Pa, along the i-process path. One stellar candidate that may confirm the production of actinides by the i-process is the carbon-enhanced metal-poor (CEMP) r/s star RAVE J094921.8−161722, which shows Th lines in its spectrum. Its surface abundance is shown to be reasonably well reproduced by our AGB model, though abundances of light N ≃ 50 elements remain underestimated. Combined with cosmochronometry, this finding opens the way to dating the i-process event and thus obtaining a lower limit for the age of CEMP-r/s stars. Such a dating is expected to be accurate only if surface abundances of Th and U can be extracted simultaneously. Conclusions. We show that actinides can be synthesized in low-metallicity low-mass AGB stars through the i-process. This astrophysical site therefore potentially contributes to the Galactic enrichment of Th and U, which demonstrates that the r-process may not be the sole mechanism for the production of U and Th.

Hydrodynamical simulations for the common-envelope wind model for Type Ia supernovae

Context. The single-degenerate (SD) model is one of the leading models for the progenitors of Type Ia supernovae (SNe Ia). Recently, a new version of the SD model, the common-envelope wind (CEW) model, has been proposed, which, in principle, has the potential to resolve most of the difficulties encountered by previous SD models. This model is still being developed and a number of open issues remain, such as the details of the mass-loss mechanism from the surface of the common envelope (CE), the main observational properties, and the spiral-in timescale of the binary inside the envelope. Aims. In this article, we aim to address these issues by considering hydrodynamical effects on the CE. Methods. Using the stellar evolution code MESA, we carried out a series of 1D hydrodynamical simulations of an asymptotic giant branch (AGB) star undergoing a common-envelope phase with different envelope masses (0.0007 M⊙–0.06 M⊙). The effect of the immersed binary was mimicked by changing the gravitational constant throughout the envelope and injecting an extra heating source at the location of the binary orbit. Results. We found that the envelopes are always dynamically unstable, leading to regular mass ejection events if the envelope is more massive than the critical value of ∼0.003 M⊙. The κ mechanism can naturally explain this phenomenon. We also found that, due to the low mass of the CE, the estimated frictional luminosity caused by the spiral-in of the immersed binary is much less than the nuclear luminosity, and therefore will not affect the structure of the CE significantly. Conclusions. Our results imply that the CE in the CEW model cannot be very massive. We also present a rough estimate for the spiral-in timescale based on a simplified model. We found that, for reasonable assumptions, the timescale may be longer than a few 105 yr; therefore, the white dwarf (WD) may have enough time to increase its mass toward the Chandrasekhar mass, avoiding a merger with the companion.

Stellar evolution models with overshooting based on 3-equation non-local theories

Context. Convective overshoot mixing is a critical ingredient of stellar structure models but is treated in most cases by ad hoc extensions of the mixing-length theory for convection. Advanced theories that are both more physical and numerically treatable are needed. Aims. Convective flows in stellar interiors are highly turbulent. This poses a number of numerical challenges for the modelling of convection in stellar interiors. We included an effective turbulence model in a 1D stellar evolution code in order to treat non-local effects within the same theory. Methods. We used a turbulent convection model that relies on the solution of second order moment equations. We implemented this into a state-of-the-art 1D stellar evolution code. To overcome a deficit in the original form of the model, we took the dissipation due to buoyancy waves in the overshooting zone into account. Results. We compute stellar models of intermediate mass main-sequence stars of between 1.5 and 8 M⊙. Overshoot mixing from the convective core and modified temperature gradients within and above it emerge naturally as a solution of the turbulent convection model equations. Conclusions. For a given set of model parameters, the overshooting extent determined from the turbulent convection model is comparable to other overshooting descriptions, the free parameters of which had been adjusted to match observations. The relative size of the mixed cores decreases with decreasing stellar mass without additional adjustments. We find that the dissipation by buoyancy waves constitutes a necessary and relevant extension of the turbulent convection model in use.

Advancing globular cluster constraints on the axion-photon coupling

We improve the current upper bound on the axion-photon coupling derived from stellar evolution using the R 2 parameter, the ratio of stellar populations on the Asymptotic Giant Branch to Horizontal Branch in Globular Clusters. We compare this with data from simulations using the stellar evolution code MESA which include the effects of axion production. Particular attention is given to quantifying in detail the effects of uncertainties on the R and R 2 parameters due to the modelling of convective core boundaries. Using a semiconvective mixing scheme we constrain the axion-photon coupling to be gaγγ < 0.47 × 10-10 GeV-1. This rules out new regions of QCD axion and axion-like particle parameter space. Complementary evidence from asteroseismology suggests that this could improve to as much as gaγγ < 0.34 × 10-10 GeV-1 as the uncertainties surrounding mixing across convective boundaries are better understood.