Evolution and nucleosynthesis of asymptotic giant branch stars and accreting white dwarfs

Type Ia supernovae (SNIa) are luminous stellar explosions which mark the fatal disruption of white dwarfs in a binary system. They are the major producers of Iron group elements in the solar system and also give relevant contribution to the alpha-elements Silicon, Solfur, Calcium and Titanium. Within specific conditions SNIa may also produce about 30 proton-rich isotopes heavier than iron. It is controversial what is the relevance of this p-process component for the abundance of these isotopes in the Galaxy and in our solar system. Its efficiency depends on the products of neutron capture processes active during the accretion phase to reach the Chandrasekhar mass. The aim of this thesis is to provide for the first time comprehensive stellar simulations for investigating the possibility of producing this seeds distribution for p-process nucleosynthesis, calculating it modelling the accretion phase onto a white-dwarfs increasing mass toward the Chandrasekhar limit. The main stellar model properties during the accretion phase are not so different from the asymptotic giant branch phase, before the star becomes a WD and the accretion phase starts. We have used the same stellar code MESA (revision 4219) to produce AGB stellar models, implementing the best known physics and producing eleven one-dimensional AGB stellar models with initial mass M = 2 and 3 solar masses, and with initial metal content Z=0.01 and Z=0.02. The convective boundary-mixing below Thermal Pulses and the Third-Dredge Up is included directly in stellar calculations to take into account Kelvin-Helmholtz instability and gravity waves. Rotation and magnetic field are not included. The same parameterization adopted for AGB models was consistently used for the accretion models, calculating 4 WD models with initial mass 0.856, 1.025, 1.259 and 1.376 solar masses accreting Z=0.01 metal content material. Post-processing calculations are finally done with the Mppnp NuGrid code

i-process Nucleosynthesis and Mass Retention Efficiency in He-shell Flash Evolution of Rapidly Accreting White Dwarfs

© 2017. The American Astronomical Society. All rights reserved. Based on stellar evolution simulations, we demonstrate that rapidly accreting white dwarfs (WDs) in close binary systems are an astrophysical site for the intermediate neutron-capture process. During recurrent and very strong He-shell flashes in the stable H-burning accretion regime H-rich material enters the He-shell flash convection zone. 12 C(p, γ) 13 N reactions release enough energy to potentially impact convection, and i process is activated through the 13 C(α, n) 16 O reaction. The H-ingestion flash may not cause a split of the convection zone as it was seen in simulations of He-shell flashes in post-AGB and low-Z asymptotic giant branch (AGB) stars. We estimate that for the production of first-peak heavy elements this site can be of similar importance for galactic chemical evolution as the s-process production by low-mass AGB stars. The He-shell flashes result in the expansion and, ultimately, ejection of the accreted and then i-process enriched material, via super-Eddington-luminosity winds or Roche-lobe overflow. The WD models do not retain any significant amount of the accreted mass, with a He retention efficiency of ≲ 10% depending on mass and convective boundary mixing assumptions. This makes the evolutionary path of such systems to supernova Ia explosion highly unlikely

Mixing Uncertainties in Low-Metallicity AGB Stars: The Impact on Stellar Structure and Nucleosynthesis

The slow neutron-capture process (s-process) efficiency in low-mass 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 low-mass 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 post-processing 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 low-metallicity stars. We find that mixing at the interface between the He-intershell and the CO-core has a critical impact on the s-process 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 13C-pocket. Additionally, our results indicate that models with very low to no mixing below the He-intershell during thermal pulses, and with a 13C-pocket size of at least ∼3 × 10−4 M⊙, are strongly favored in reproducing observations. Online access to complete yield data tables is also provided

MESA and NuGrid simulations of classical novae: CO and ONe nova nucleosynthesis

Classical novae are the result of thermonuclear flashes of hydrogen accreted by CO or ONe white dwarfs, leading eventually to the dynamic ejection of the surface layers. These are observationally known to be enriched in heavy elements, such as C, O and Ne that must originate in layers below the H-flash convection zone. Building on our previous work, we now present stellar evolution simulations of ONe novae and provide a comprehensive comparison of our models with published ones. Some of our models include exponential convective boundary mixing to account for the observed enrichment of the nova ejecta even when accreted material has a solar abundance distribution. Our models produce maximum temperature evolution profiles and nucleosynthesis yields in good agreement with models that generate enriched ejecta by assuming that the accreted material was pre-mixed. We confirm for ONe novae the result we reported previously, i.e.\ we found that $^3$He could be produced {\it in situ} in solar-composition envelopes accreted with slow rates (\dot{M} < 10^{-10}\,M_\odot/\mbox{yr}) by cold ($T_{\rm WD} < 10^7$ K) CO WDs, and that convection was triggered by $^3$He burning before the nova outburst in that case. In addition, we now find that the interplay between the $^3$He production and destruction in the solar-composition envelope accreted with an intermediate rate, e.g.\ \dot{M} = 10^{-10}\,M_\odot/\mbox{yr}, by the $1.15\,M_\odot$ ONe WD with a relatively high initial central temperature, e.g.\ $T_{\rm WD} = 15\times 10^6$ K, leads to the formation of a thick radiative buffer zone that separates the bottom of the convective envelope from the WD surface. (Abridged)Comment: 19 pages, 23 figures, 2 tables, accepted to publication by MNRA

Impact of newly measured 26Al(n, p)26Mg and 26Al(n, α)23Na reaction rates on the nucleosynthesis of 26Al in stars

The cosmic production of the short-lived radioactive nuclide 26Al is crucial for our understanding of the evolution of stars and galaxies. However, simulations of the stellar sites producing 26Al are still weakened by significant nuclear uncertainties. We re-evaluate the 26Al(n, p)26Mg, and 26Al(n, α)23Na ground state reactivities from 0.01 GK to 10 GK, based on the recent n TOF measurement combined with theoretical predictions and a previous measurement at higher energies, and test their impact on stellar nucleosynthesis. We computed the nucleosynthesis of low- and high-mass stars using the Monash nucleosynthesis code, the NuGrid mppnp code, and the FUNS stellar evolutionary code. Our low-mass stellar models cover the 2-3 M☉ mass range with metallicities between Z = 0.01 and 0.02, their predicted 26Al/27Al ratios are compared to 62 meteoritic SiC grains. For high-mass stars, we test our reactivities on two 15 M☉ models with Z = 0.006 and 0.02. The new reactivities allow low-mass AGB stars to reproduce the full range of 26Al/27Al ratios measured in SiC grains. The final 26Al abundance in high-mass stars, at the point of highest production, varies by a factor of 2.4 when adopting the upper, or lower limit of our rates. However, stellar uncertainties still play an important role in both mass regimes. The new reactivities visibly impact both low- and high-mass stars nucleosynthesis and allow a general improvement in the comparison between stardust SiC grains and low-mass star models. Concerning explosive nucleosynthesis, an improvement of the current uncertainties between T9∼0.3 and 2.5 is needed for future studies

Measurement of the 70Ge(n,γ) cross section up to 300 keV at the CERN n_TOF facility

Neutron capture data on intermediate mass nuclei are of key importance to nucleosynthesis in the weak component of the slow neutron capture processes, which occurs in massive stars. The (n,γ) cross section on 70Ge, which is mainly produced in the s process, was measured at the neutron time-of-flight facility n_TOF at CERN. Resonance capture kernels were determined up to 40 keV neutron energy and average cross sections up to 300 keV. Stellar cross sections were calculated from kT =5 keV tokT =100 keV and are in very good agreement with a previous measurement by Walter and Beer (1985) and recent evaluations. Average cross sectionsareinagreementwithWalterandBeer(1985)overmostoftheneutronenergyrangecovered,whilethey aresystematicallysmallerforneutronenergiesabove150keV.Wehavecalculatedisotopicabundancesproduced in s-process environments in a 25 solar mass star for two initial metallicities (below solar and close to solar). While the low metallicity model reproduces best the solar system germanium isotopic abundances, the close to solar model shows a good global match to solar system abundances in the range of mass numbers A=60–80.Austrian Science Fund J3503Adolf Messer Foundation ST/M006085/1European Research Council ERC2015-StGCroatian Science Foundation IP-2018-01-857

Measurement of 73Ge(n,γ) cross sections and implications for stellar nucleosynthesis

73Ge(n,γ) cross sections were measured at the neutron time-of-flight facility n_TOF at CERN up to neutron energies of 300 keV, providing for the first time experimental data above 8 keV. Results indicate that the stellar cross section at kT=30 keV is 1.5 to 1.7 times higher than most theoretical predictions. The new cross sections result in a substantial decrease of 73Ge produced in stars, which would explain the low isotopic abundance of 73Ge in the solar system.Fondo de Ciencia de Austria J3503Consejo de Instalaciones de Ciencia y Tecnología Reino Unido ST / M006085 / 1Consejo Europeo de Investigación ERC-2015-StG Nr.677497

Measurement of the 76 Ge ( n , γ ) cross section at the n_TOF facility at CERN

The 76 Ge ( n , γ ) reaction has been measured at the n_TOF facility at CERN via the time-of-flight technique. Neutron capture cross sections on 76 Ge are of interest to a variety of low-background experiments, such as neutrinoless double β decay searches, and to nuclear astrophysics. We have determined resonance capture kernels up to 52 keV neutron energy and used the new data to calculate Maxwellian-averaged neutron capture cross sections for k B T values of 5 to 100 keV.The Austrian Science Fund (FWF) J3503The UK Science and Facilities Council. ST/M006085/1The European Research Council ERC-2015-StG No. 67749

Measurement of the 72 Ge ( n , γ ) cross section over a wide neutron energy range at the CERN n_TOF facility

The 72 Ge ( n , γ ) cross section was measured for neutron energies up to 300 keV at the neutron time-of-flight facility n _ TOF (CERN), Geneva, for the first time covering energies relevant to heavy-element synthesis in stars. The measurement was performed at the high-resolution beamline EAR-1, using an isotopically enriched 72 Ge O 2 sample. The prompt capture γ rays were detected with four liquid scintillation detectors, optimized for low neutron sensitivity. We determined resonance capture kernels up to a neutron energy of 43 keV , and averaged cross sections from 43 to 300 keV . Maxwellian-averaged cross section values were calculated from k T = 5 to 100 keV , with uncertainties between 3.2 % and 7.1 % . The new results significantly reduce uncertainties of abundances produced in the slow neutron capture process in massive stars.Austrian Science Fund (FWF) J3503Science and Technology Facilities Council UK. ST/M006085/1European Research Council (ERC) 2015-STG No.677497Croatian Science Foundation. 8570Ministry of Education, Youth and Sport of the Czech Republic (MSMT) y the Charles University. UNCE/SCI/01

NuGrid stellar data set. 1. Stellar yields from H to Bi for stars with metallicities Z=0.02 and Z=0.01

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 ¹³C 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