The production of proton-rich isotopes beyond iron: The ?-process in stars

© 2016 World Scientific Publishing Company. Beyond iron, a small fraction of the total abundances in the Solar System is made of proton-rich isotopes, the p-nuclei. The clear understanding of their production is a fundamental challenge for nuclear astrophysics. The p-nuclei constrain the nucleosynthesis in core-collapse and thermonuclear supernovae. The γ-process is the most established scenario for the production of the p-nuclei, which are produced via different photodisintegration paths starting on heavier nuclei. A large effort from nuclear physics is needed to access the relevant nuclear reaction rates far from the valley of stability. This review describes the production of the heavy proton-rich isotopes by the γ-process in stars, and explores the state of the art of experimental nuclear physics to provide nuclear data for stellar nucleosynthesis

The diverse origins of neutron-capture elements in the metal-poor star HD 94028 : possible detection of products of i-process nucleosynthesis

We present a detailed analysis of the composition and nucleosynthetic origins of the heavy elements in the metal-poor ([Fe/H] = −1.62 ± 0.09) star HD 94028. Previous studies revealed that this star is mildly enhanced in elements produced by the slow neutron-capture process (s process; e.g., [Pb/Fe] = +0.79 ± 0.32) and rapid neutron-capture process (r process; e.g., [Eu/Fe] = +0.22 ± 0.12), including unusually large molybdenum ([Mo/Fe] = +0.97 ± 0.16) and ruthenium ([Ru/Fe] = +0.69 ± 0.17) enhancements. However, this star is not enhanced in carbon ([C/Fe] = −0.06 ± 0.19). We analyze an archival near-ultraviolet spectrum of HD 94028, collected using the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope, and other archival optical spectra collected from ground-based telescopes. We report abundances or upper limits derived from 64 species of 56 elements. We compare these observations with s-process yields from low-metallicity AGB evolution and nucleosynthesis models. No combination of s- and r-process patterns can adequately reproduce the observed abundances, including the super-solar [As/Ge] ratio (+0.99 ± 0.23) and the enhanced [Mo/Fe] and [Ru/Fe] ratios. We can fit these features when including an additional contribution from the intermediate neutron-capture process (i process), which perhaps operated through the ingestion of H in He-burning convective regions in massive stars, super-AGB stars, or low-mass AGB stars. Currently, only the i process appears capable of consistently producing the super-solar [As/Ge] ratios and ratios among neighboring heavy elements found in HD 94028. Other metal-poor stars also show enhanced [As/Ge] ratios, hinting that operation of the i process may have been common in the early Galaxy

MESA and NuGrid Simulations of Classical Nova Outbursts and Nucleosynthesis

Classical novae are the results of surface thermonuclear explosions of hydrogen accreted by white dwarfs (WDs) from their low-mass main-sequence or red-giant binary companions. Chemical composition analysis of their ejecta shows that nova outbursts occur on both carbon-oxygen (CO) and more massive oxygen-neon (ONe) WDs, and that there is cross-boundary mixing between the accreted envelope and underlying WD. We demonstrate that the state-of-the-art stellar evolution code MESA and post-processing nucleosynthesis tools of NuGrid can successfully be used for modeling of CO and ONe nova outbursts and nucleosynthesis. The convective boundary mixing (CBM) in our 1D numerical simulations is implemented using a diffusion coefficient that is exponentially decreasing with a distance below the bottom of the convective envelope. We show that this prescription produces maximum temperature evolution profiles and nucleosynthesis yields in good agreement with those obtained using the commonly adopted 1D nova model in which the CBM is mimicked by assuming that the accreted envelope has been pre-mixed with WD's material. In a previous paper, we have found that 3He can be produced in situ in solar-composition envelopes accreted with slow rates (dM/dt < 1e-10 M_sun/yr) by cold (T_WD < 1d7 K) CO WDs, and that convection is triggered by 3He burning before the nova outburst in this case. Here, we confirm this result for ONe novae. Additionally, we find that the interplay between the 3He production and destruction in the solar-composition envelope accreted with an intermediate rate, e.g. dM/dt = 1e-10 M_sun/yr, by the 1.15 M_sun ONe WD with a relatively high initial central temperature, e.g. T_WD = 15e6 K, leads to the formation of a thick radiative buffer zone that separates the bottom of the convective envelope from the WD surface.Comment: 6 pages, 4 figures, STELLA NOVAE: FUTURE AND PAST DECADES Conference Proceedings, Submitted to ASP Conference Serie

Stellar origin of 15N-rich presolar SiC grains of type AB: Supernovae with explosive hydrogen burning

© 2017. The American Astronomical Society. All rights reserved. We report C, N, and Si isotopic data for 59 highly 13 C-enriched presolar submicron-to micron-sized SiC grains from the Murchison meteorite, including eight putative nova grains (PNGs) and 29 15 N-rich ( 14 N/ 15 N ≤ solar) AB grains, and their Mg-Al, S, and Ca-Ti isotope data when available. These 37 grains are enriched in 13 C, 15 N, and 26 Al with the PNGs showing more extreme enhancements. The 15 N-rich AB grains show systematically higher 26 Al and 30 Si excesses than the 14 N-rich AB grains. Thus, we propose to divide the AB grains into groups 1 ( 14 N/ 15 N < solar) and 2 ( 14 N/ 15 N ≥ solar). For the first time, we have obtained both S and Ti isotopic data for five AB1 grains and one PNG and found 32 S and/or 50 Ti enhancements. Interestingly, one AB1 grain had the largest 32 S and 50 Ti excesses, strongly suggesting a neutron-capture nucleosynthetic origin of the 32 S excess and thus the initial presence of radiogenic 32 Si (t 1/2 = 153 years). More importantly, we found that the 15 N and 26 Al excesses of AB1 grains form a trend that extends to the region in the N-Al isotope plot occupied by C2 grains, strongly indicating a common stellar origin for both AB1 and C2 grains. Comparison of supernova models with the AB1 and C2 grain data indicates that these grains came from supernovae that experienced H ingestion into the He/C zones of their progenitors

Origin of the p-process radionuclides ⁹²Nb and ¹⁴⁶Sm in the early solar system and inferences on the birth of the Sun

The abundances of ⁹²Nb and ¹⁴⁶Sm in the early solar system are determined from meteoritic analysis, and their stellar production is attributed to the p process. We investigate if their origin from thermonuclear supernovae deriving from the explosion of white dwarfs with mass above the Chandrasekhar limit is in agreement with the abundance of ⁵³Mn, another radionuclide present in the early solar system and produced in the same events. A consistent solution for ⁹²Nb and ⁵³Mn cannot be found within the current uncertainties and requires the ⁹²Nb/⁹²Mo ratio in the early solar system to be at least 50% lower than the current nominal value, which is outside its present error bars. A different solution is to invoke another production site for ⁹²Nb, which we find in the α-rich freezeout during core-collapse supernovae from massive stars. Whichever scenario we consider, we find that a relatively long time interval of at least ∼10 My must have elapsed from when the star-forming region where the Sun was born was isolated from the interstellar medium and the birth of the Sun. This is in agreement with results obtained from radionuclides heavier than iron produced by neutron captures and lends further support to the idea that the Sun was born in a massive star-forming region together with many thousands of stellar siblings

Re-evaluation of the ¹⁶O(n, γ)¹⁷O cross section at astrophysical energies and its role as a neutron poison in the s-process

The doubly magic nucleus ¹⁶O has a small neutron-capture cross section of just a few tens of microbarns in the astrophysical energy region. Despite this, ¹⁶O plays an important role as a neutron poison in the astrophysical slow neutron capture (s) process due to its high abundance. We present in this paper a re-evaluation of the available experimental data for ¹⁶O(n, γ)¹⁷O and derive a new recommendation for the Maxwellian-averaged cross sections between kT = 5 and 100 keV. Our new recommendations are lower up to kT = 60 keV compared to the previously recommended values but up to 14% higher at kT = 100 keV. We explore the impact of this different energy dependence on the weak s-process during core helium burning (kT = 26 keV) and shell carbon burning (kT = 90 keV) in massive stars where ¹⁶O is the most abundant isotope

Sulfur isotopic compositions of submicrometer SiC grains from the Murchison meteorite

We report C, Si, N, S, Mg-Al, and Ca-Ti isotopic compositions of presolar silicon carbide (SiC) grains from the SiC-rich KJE size fraction (0.5-0.8 μm) of the Murchison meteorite. One thousand one hundred thirteen SiC grains were identified based on their C and Si isotopic ratios. Mainstream, AB, C, X, Y, and Z subtypes of SiC, and X-type silicon nitride (Si₃N₄) account for 81.4%, 5.7%, 0.1%, 1.5%, 5.8%, 4.9%, and 0.4%, respectively. Twenty-five grains with unusual Si isotopic ratios, including one C grain, 16 X grains, 1 Y grain, 5 Z grains, and 2 X-type Si₃N₄ grains were selected for N, S, Mg-Al, and Ca-Ti isotopic analysis. The C grain is highly enriched in ²⁹Si and ³⁰Si (δ²⁹Si = 1345‰ ± 19‰, δ³⁰Si = 1272‰ ± 19‰). It has a huge ³²S excess, larger than any seen before, and larger than that predicted for the Si/S supernova (SN) zone, providing evidence against the elemental fractionation model by Hoppe et al. Two SN models investigated here present a more satisfying explanation in terms of a radiogenic origin of ³²S from the decay of short-lived ³²Si (τ1/2 = 153 yr). Silicon-32 as well as ²⁹Si and ³⁰Si can be produced in SNe by short neutron bursts; evidence for initial 44Ti (τ1/2 = 60 yr) in the C grain is additional evidence for an SN origin. The X grains have marginal ³²S excesses, much smaller than expected from their large ²⁸Si excesses. Similarly, the Y and Z grains do not show the S-isotopic anomalies expected from their large Si isotopic anomalies. Low intrinsic S contents and contamination with isotopically normal S are the most likely explanations

Uncertainties in Galactic Chemical Evolution Models

We use a simple one-zone galactic chemical evolution model to quantify the uncertainties generated by the input parameters in numerical predictions for a galaxy with properties similar to those of the Milky Way. We compiled several studies from the literature to gather the current constraints for our simulations regarding the typical value and uncertainty of the following seven basic parameters: the lower and upper mass limits of the stellar initial mass function (IMF), the slope of the high-mass end of the stellar IMF, the slope of the delay-time distribution function of Type Ia supernovae (SNe Ia), the number of SNe Ia per M ⊙ formed, the total stellar mass formed, and the final mass of gas. We derived a probability distribution function to express the range of likely values for every parameter, which were then included in a Monte Carlo code to run several hundred simulations with randomly selected input parameters. This approach enables us to analyze the predicted chemical evolution of 16 elements in a statistical manner by identifying the most probable solutions, along with their 68% and 95% confidence levels. Our results show that the overall uncertainties are shaped by several input parameters that individually contribute at different metallicities, and thus at different galactic ages. The level of uncertainty then depends on the metallicity and is different from one element to another. Among the seven input parameters considered in this work, the slope of the IMF and the number of SNe Ia are currently the two main sources of uncertainty. The thicknesses of the uncertainty bands bounded by the 68% and 95% confidence levels are generally within 0.3 and 0.6 dex, respectively. When looking at the evolution of individual elements as a function of galactic age instead of metallicity, those same thicknesses range from 0.1 to 0.6 dex for the 68% confidence levels and from 0.3 to 1.0 dex for the 95% confidence levels. The uncertainty in our chemical evolution model does not include uncertainties relating to stellar yields, star formation and merger histories, and modeling assumptions

JINA-NuGrid Galactic Chemical Evolution Pipeline

Galactic chemical evolution is a topic that involves nuclear physics, stellar evolution, galaxy evolution, observation, and cosmology. Continuous communication and feedback between these fields is a key component in improving our understanding of how galaxies form and how elements are created and recycled in galaxies and intergalactic space. In this proceedings, we present the current state of the JINA-NuGrid chemical evolution pipeline. It is designed to probe the impact of nuclear astrophysics uncertainties on galactic chemical evolution, to improve our knowledges regarding the origin of the elements in a cosmological context, and to create the required interdisciplinary connections.Comment: 3 pages, 2 figures, submitted to JPS Conference Proceedings, Nuclei in the Cosmos XI