Abstract
We have investigated a highly energetic H-ingestion event during shell He burning leading to H-burning luminosities of log (LH/L⊙) ∼ 13 in a 45 M⊙ Pop III massive stellar model. In order to ...track the nucleosynthesis which may occur in such an event, we run a series of single-zone nucleosynthesis models for typical conditions found in the stellar evolution model. Such nucleosynthesis conditions may lead to i-process neutron densities of up to ∼1013 cm−3. The resulting simulation abundance pattern, where Mg comes from He burning and Ca from the i process, agrees with the general observed pattern of the most iron-poor star currently known, SMSS J031300.36−670839.3. However, Na is also efficiently produced in these i-process conditions, and the prediction exceeds observations by ∼2.5 dex. While this probably rules out this model for SMSS J031300.36−670839.3, the typical i-process signature of combined He burning and i process of higher than solar Na/Mg, Mg/Al, and low Ca/Mg is reproducing abundance features of the two next most iron-poor stars HE 1017−5240 and HE 1327−2326 very well. The i process does not reach Fe which would have to come from a low level of additional enrichment. i process in hyper-metal-poor or Pop III massive stars may be able to explain certain abundance patterns observed in some of the most metal-poor CEMP-no stars.
For the origin of heavy rapid neutron capture process (r-process) elements, different sources have been proposed, e.g. core-collapse supernovae or neutron star mergers. Old metal-poor stars carry the ...signature of the astrophysical source(s). Among the elements dominantly made by the r-process, europium (Eu) is relatively easy to observe. In this work we simulate the evolution of Eu in our Galaxy with the inhomogeneous chemical evolution (ICE) model, and compare our results with spectroscopic observations. We test the most important parameters affecting the chemical evolution of Eu: (a) for neutron star mergers the coalescence time-scale of the merger (t
coal) and the probability to experience a neutron star merger event after two supernova explosions occurred and formed a double neutron star system (P
NSM) and (b) for the subclass of magnetorotationally driven supernovae (‘Jet-SNe’), their occurrence rate compared to standard supernovae (P
Jet-SN). We find that the observed Eu/Fe pattern in the Galaxy can be reproduced by a combination of neutron star mergers and Jet-SNe as r-process sources. While neutron star mergers alone seem to set in at too high metallicities, Jet-SNe provide a cure for this deficiency at low metallicities. Furthermore, we confirm that local inhomogeneities can explain the observed large spread in the Eu abundances at low metallicities. We also predict the evolution of O/Fe to test whether the spread in α-elements for inhomogeneous models agrees with observations and whether this provides constraints on supernova explosion models and their nucleosynthesis.
We provide here a significant extension of the NuGrid Set 1 models in mass coverage and towards lower metallicity, adopting the same physics assumptions. The combined data set now includes the ...initial masses MZAMS/M⊙ = 1, 1.65, 2, 3, 4, 5, 6, 7, 12, 15, 20, 25 for Z = 0.02, 0.01, 0.006, 0.001, 0.0001 with α-enhanced composition for the lowest three metallicities. These models are computed with the MESA stellar evolution code and are evolved up to the AGB, the white dwarf stage, or until core collapse. The nucleosynthesis was calculated for all isotopes in post-processing with the NuGrid MPPNP code. Explosive nucleosynthesis is based on semi-analytic 1D shock models. Metallicity-dependent mass-loss, convective boundary mixing in low- and intermediate-mass models and H and He core burning massive star models are included. Convective O-C shell mergers in some stellar models lead to the strong production of odd-Z elements P, Cl, K, and Sc. In AGB models with hot dredge-up, the convective boundary mixing efficiency is reduced to accommodate for its energetic feedback. In both low-mass and massive star models at the lowest metallicity, H-ingestion events are observed and lead to i-process nucleosynthesis and substantial 15N production. Finally, complete yield data tables, derived data products and online analytic data access are provided.
The production of the heavy stable proton-rich isotopes between 74Se and 196Hg-the p nuclides-is due to the contribution from different nucleosynthesis processes, activated in different types of ...stars. Whereas these processes have been subject to various studies, their relative contributions to Galactic chemical evolution (GCE) are still a matter of debate. Here we investigate for the first time the nucleosynthesis of p nuclides in GCE by including metallicity and progenitor mass-dependent yields of core-collapse supernovae (ccSNe) into a chemical evolution model. We used a grid of metallicities and progenitor masses from two different sets of stellar yields and followed the contribution of ccSNe to the Galactic abundances as a function of time. In combination with previous studies on p-nucleus production in thermonuclear supernovae (SNIa), and using the same GCE description, this allows us to compare the respective roles of SNeIa and ccSNe in the production of p-nuclei in the Galaxy. The γ process in ccSN is very efficient for a wide range of progenitor masses (13 M -25 M ) at solar metallicity. Since it is a secondary process with its efficiency depending on the initial abundance of heavy elements, its contribution is strongly reduced below solar metallicity. This makes it challenging to explain the inventory of the p nuclides in the solar system by the contribution from ccSNe alone. In particular, we find that ccSNe contribute less than 10% of the solar p nuclide abundances, with only a few exceptions. Due to the uncertain contribution from other nucleosynthesis sites in ccSNe, such as neutrino winds or -rich freeze out, we conclude that the light p-nuclides 74Se, 78Kr, 84Sr, and 92Mo may either still be completely or only partially produced in ccSNe. The γ-process accounts for up to twice the relative solar abundances for 74Se in one set of stellar models and 196Hg in the other set. The solar abundance of the heaviest p nucleus 196Hg is reproduced within uncertainties in one set of our models due to photodisintegration of the Pb isotopes 208,207,206Pb. For all other p nuclides, abundances as low as 2% of the solar level were obtained.
ABSTRACT
Rapid neutron capture process (r-process) elements have been detected in a large fraction of metal-poor halo stars, with abundances relative to iron (Fe) that vary by over two orders of ...magnitude. This scatter is reduced to less than a factor of 3 in younger Galactic disc stars. The large scatter of r-process elements in the early Galaxy suggests that the r-process is made by rare events, like compact binary mergers and rare sub-classes of supernovae. Although being rare, neutron star mergers alone have difficulties to explain the observed enhancement of r-process elements in the lowest metallicity stars compared to Fe. The supernovae producing the two neutron stars already provide a substantial Fe abundance where the r-process ejecta from the merger would be injected. In this work we investigate another complementary scenario, where the r-process occurs in neutron star-black hole mergers in addition to neutron star mergers. Neutron star-black hole mergers would eject similar amounts of r-process matter as neutron star mergers, but only the neutron star progenitor would have produced Fe. Furthermore, a reduced efficiency of Fe production from single stars significantly alters the age–metallicity relation, which shifts the onset of r-process production to lower metallicities. We use the high-resolution (20 pc)3/cell inhomogeneous chemical evolution tool ‘ICE’ to study the outcomes of these effects. In our simulations, an adequate combination of neutron star mergers and neutron star-black hole mergers qualitatively reproduces the observed r-process abundances in the Galaxy.
Though the neutrino-driven convection model for the core-collapse explosion mechanism has received strong support in recent years, there are still many uncertainties in the explosion parameters-such ...as explosion energy, remnant mass, and end-of-life stellar abundances as initial conditions. Using a broad set of spherically symmetric core-collapse simulations we examine the effects of these key parameters on explosive nucleosynthesis and final explosion yields. The post-bounce temperature and density evolution of zero-age main-sequence 15, 20, and 25 solar mass progenitors are post-processed through the Nucleosynthesis Grid nuclear network to obtain detailed explosive yields. In particular, this study focuses on radio isotopes that are of particular interest to the next generation of gamma-ray astronomical observations: 43K, 47Ca, 44Sc, 47Sc, 48V, 48Cr, 51Cr, 52Mn, 59Fe, 56Co, 57Co, and 57Ni. These nuclides may be key in advancing our understanding of the inner workings of core-collapse supernovae by probing the parameters of the explosion engine. We find that the isotopes that are strong indicators of explosion energy are 43K, 47Ca, 44Sc, 47Sc, and 59Fe, those that are dependent on the progenitor structure are 48V, 51Cr, and 57Co, and those that probe neither are 48Cr, 52Mn, 57Ni, and 56Co. We discuss the prospects of observing these radionuclides in supernova remnants.
ABSTRACT We provide a set of stellar evolution and nucleosynthesis calculations that applies established physics assumptions simultaneously to low- and intermediate-mass and massive star models. Our ...goal is to provide an internally consistent and comprehensive nuclear production and yield database for applications in areas such as presolar grain studies. Our non-rotating models assume convective boundary mixing (CBM) where it has been adopted before. We include 8 (12) initial masses for Z = 0.01 (0.02). Models are followed either until the end of the asymptotic giant branch phase or the end of Si burning, complemented by simple analytic core-collapse supernova (SN) models with two options for fallback and shock velocities. The explosions show which pre-SN yields will most strongly be effected by the explosive nucleosynthesis. We discuss how these two explosion parameters impact the light elements and the s and p process. For low- and intermediate-mass models, our stellar yields from H to Bi include the effect of CBM at the He-intershell boundaries and the stellar evolution feedback of the mixing process that produces the pocket. All post-processing nucleosynthesis calculations use the same nuclear reaction rate network and nuclear physics input. We provide a discussion of the nuclear production across the entire mass range organized by element group. The entirety of our stellar nucleosynthesis profile and time evolution output are available electronically, and tools to explore the data on the NuGrid VOspace hosted by the Canadian Astronomical Data Centre are introduced.
Recently, an increasing number of studies were devoted to measure the abundances of neutron-capture elements heavier than iron in stars belonging to Galactic Open Clusters (OCs). OCs span a sizeable ...range in metallicity (-0.6 less than or equal to Fe/H less than or equal to +0.4), and they show abundances of light elements similar to disc stars of the same age. A different pattern is observed for heavy elements. A large scatter is observed for Ba, with most OCs showing Ba/Fe and Ba/La overabundant with respect to the Sun. The origin of this overabundance is not clearly understood. With the goal of providing new observational insights, we determined radial velocities, atmospheric parameters and chemical composition of 27 giant stars members of five OCs: Cr 110, Cr 261, NGC 2477, NGC 2506 and NGC 5822. We used high-resolution spectra obtained with the UVES spectrograph at European Southern Observatory Paranal. We perform a detailed spectroscopic analysis of these stars to measure the abundance of up to 22 elements per star. We study the dependence of element abundance on metallicity and age with unprecedented detail, complementing our analysis with data culled from the literature. We confirm the trend of Ba overabundance in OCs, and show its large dispersion for clusters younger than similar to 4 Gyr. Finally, the implications of our results for stellar nucleosynthesis are discussed. We show in this work that the Ba enrichment compared to other neutron-capture elements in OCs cannot be explained by the contributions from the slow neutron-capture process and the rapid neutron-capture process. Instead, we argue that this anomalous signature can be explained by assuming an additional contribution by the intermediate neutron-capture process.
We investigate the evolution of super-AGB (SAGB) thermal pulse (TP) stars for a range of metallicities (Z) and explore the effect of convective boundary mixing (CBM). With decreasing metallicity and ...evolution along the TP phase, the He-shell flash and the third dredge-up (TDU) occur closer together in time. After some time (depending upon the CBM parametrization), efficient TDU begins while the pulse-driven convection zone (PDCZ) is still present, causing a convective exchange of material between the PDCZ and the convective envelope. This results in the ingestion of protons into the convective He-burning pulse. Even small amounts of CBM encourage the interaction of the convection zones leading to transport of protons from the convective envelope into the He layer. H-burning luminosities exceed 109 (in some cases 1010) L⊙. We also calculate models of dredge-out in the most massive SAGB stars and show that the dredge-out phenomenon is another likely site of convective-reactive H-12C combustion. We discuss the substantial uncertainties of stellar evolution models under these conditions. Nevertheless, the simulations suggest that in the convective-reactive H-combustion regime of H ingestion the star may encounter conditions for the intermediate neutron capture process (i-process). We speculate that some CEMP-s/r stars could originate in i-process conditions in the H ingestion phases of low-Z SAGB stars. This scenario would however suggest a very low electron-capture supernova rate from SAGB stars. We also simulate potential outbursts triggered by such H ingestion events, present their light curves and briefly discuss their transient properties.
ABSTRACT
The origin of the proton-rich trans-iron isotopes in the Solar system is still uncertain. Single-degenerate thermonuclear supernovae (SNIa) with n-capture nucleosynthesis seeds assembled in ...the external layers of the progenitor’s rapidly accreting white dwarf (RAWD) phase may produce these isotopes. We calculate the stellar structure of the accretion phase of five white dwarf (WD) models with initial masses ≥ 0.85 $\, \mathrm{M}_\odot$ using the stellar code mesa The near-surface layers of the 1, 1.26, 1.32 and 1.38 $\, \mathrm{M}_\odot$ models are most representative of the regions in which the bulk of the p nuclei are produced during SNIa explosions, and for these models we also calculate the neutron-capture nucleosynthesis in the external layers. Contrary to previous RAWD models at lower mass, we find that the H-shell flashes are the main site of n-capture nucleosynthesis. We find high neutron densities up to several 1015 cm−3 in the most massive WDs. Through the recurrence of the H-shell flashes these intermediate neutron densities can be sustained effectively for a long time leading to high-neutron exposures with a strong production up to Pb. Both the neutron density and the neutron exposure increase with increasing the mass of the accreting WD. Finally, the SNIa nucleosynthesis is calculated using the obtained abundances as seeds. We obtain solar to supersolar abundances for p-nuclei with A > 96. Our models show that SNIa are a viable p-process production site.
PDF
