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The astrophysical rapid neutron capture process or 'r process' of nucleosynthesis is believed to be responsible for the production of approximately half the heavy element abundances found in nature. This multifaceted problem remains one of the greatest open challenges in all of physics. Knowledge of nuclear physics properties such as masses, β-decay and neutron capture rates, as well as β-delayed neutron emission probabilities are critical inputs that go into calculations of r-process nucleosynthesis. While properties of nuclei near stability have been established, much still remains unknown regarding neutron-rich nuclei far from stability that may participate in the r process. Sensitivity studies gauge the astrophysical response of a change in nuclear physics input(s) which allows for the isolation of the most important nuclear properties that shape the final abundances observed in nature. This review summarizes the extent of recent sensitivity studies and highlights how these studies play a key role in facilitating new insight into the r process. The development of these tools promotes a focused effort for state-of-the-art measurements, motivates construction of new facilities and will ultimately move the community towards addressing the grand challenge of 'How were the elements from iron to uranium made?'. IntroductionOne of the major open questions in all of physics is the identification of the sites responsible for the production of the heaviest elements [1,2]. It has been understood since the 1950s that the solar system abundances of nuclei heavier than iron can be divided roughly in half based on the nucleosynthesis processes that create them. Slow neutron capture process, or s-process, nuclei lie along the middle of the valley of stability, and rapid neutron capture process, or r-process, nuclei are found on the neutron-rich side, with a third process, the p process, responsible for the significantly less abundant nuclei on the proton-rich side of stability [3,4]. Since that time much progress has been made, e.g. [5], and the basic mechanisms of and astrophysical sites for the creation of the s-process [6] and heavy p-process [7,8] nuclei are on a firm footing. The site or sites of the r process still evade definitive determination [9,10,11].The r-process pattern is extracted from the solar system abundances by subtracting the s-process and p-process contributions [12,13]. The residual pattern consists of three main abundance peaks at A ∼ 80, 130, and 195, associated with the N = 50, 82, and 126 closed shells. Building up to the heaviest r-process elements requires on the order of 100 neutrons per seed nucleus. Additional constraints come from meteoritic data, e.g. [14], and observations of r-process elements in old stars in the galactic halo, e.g. [15,16]. This data points to distinct origins for the light A < 120 ('weak') and heavy A > 120 ('main') r-process nuclei. The pattern of main r-process elements is remarkably similar among r-process enhanced halo stars and is a match to the solar residuals. Thi...

We examine the nucleosynthesis products that are produced in the outflow from rapidly accreting disks. We find that the type of element synthesis varies dramatically with the degree of neutrino trapping in the disk and therefore the accretion rate of the disk. Disks with relatively high accretion rates such asṀ ¼ 10 M s À1 can produce very neutron-rich nuclei that are found in the r-process. Disks with more moderate accretion rates can produce copious amounts of nickel, as well as the light elements such as lithium and boron. Disks with lower accretion rates such aṡ M ¼ 1 M s À1 produce large amounts of nickel, as well as some unusual nuclei such as 49 Ti, 45 Sc, 64 Zn, and 92 Mo. This wide array of potential nucleosynthesis products is due to the varying influence of electron neutrinos and antineutrinos emitted from the disk on the neutron-to-proton ratio in the outflow. We use a parameterization for the outflow and discuss our results in terms of entropy and outflow acceleration.

We consider hot accretion disk outflows from black hole-neutron star mergers in the context of the nucleosynthesis they produce. We begin with a three-dimensional numerical model of a black hole-neutron star merger and calculate the neutrino and antineutrino fluxes emitted from the resulting accretion disk. We then follow the element synthesis in material outflowing the disk along parameterized trajectories. We find that at least a weak r-process is produced, and in some cases a main r-process as well. The neutron-rich conditions required for this production of r-process nuclei stem directly from the interactions of the neutrinos emitted by the disk with the free neutrons and protons in the outflow.

Some of the heavy elements, such as gold and europium (Eu), are almost exclusively formed by the rapid neutron capture process (r-process). However, it is still unclear which astrophysical site between core-collapse supernovae and neutron star -neutron star (NS-NS) mergers produced most of the r-process elements in the universe. Galactic chemical evolution (GCE) models can test these scenarios by quantifying the frequency and yields required to reproduce the amount of europium (Eu) observed in galaxies. Although NS-NS mergers have become popular candidates, their required frequency (or rate) needs to be consistent with that obtained from gravitational wave measurements. Here we address the first NS-NS merger detected by LIGO/Virgo (GW170817) and its associated Gamma-ray burst and analyze their implication on the origin of r-process elements. The range of NS-NS merger rate densities of 320−4740 Gpc −3 yr −1 provided by LIGO/Virgo is remarkably consistent with the range required by GCE to explain the Eu abundances in the Milky Way with NS-NS mergers, assuming the solar r-process abundance pattern for the ejecta. Under the same assumption, this event has produced about 1 − 5 Earth masses of Eu, and 3 − 13 Earth masses of gold. When using theoretical calculations to derive Eu yields, constraining the role of NS-NS mergers becomes more challenging because of nuclear astrophysics uncertainties. This is the first study that directly combines nuclear physics uncertainties with GCE calculations. If GW170817 is a representative event, NS-NS mergers can produce Eu in sufficient amounts and are likely to be the main r-process site.

We use network calculations of r-process nucleosynthesis to pin down the origin of the peak in the solar r-process abundance distribution near nuclear mass number A ഠ 160. The peak is due to a subtle interplay of nuclear deformation and b decay, and forms not in the steady phase of the r process, but only just prior to freeze-out, as the free neutrons rapidly disappear. Its existence should therefore help constrain the conditions under which the r process freezes out. [S0031-9007(97)04032-5] PACS numbers: 26.30. + k The r process is responsible for synthesizing roughly half the heavy nuclei in the solar system [1]. It is widely believed to occur in type II supernovae, beginning at a time when the density of free neutrons is so high that neutron capture by nuclei occurs much more rapidly than nuclear b decay. Under these conditions equilibrium between neutron capture and photodisintegration [called ͑n, g͒-͑g, n͒ equilibrium] establishes itself so that before long very neutron-rich isotopes of each element are populated with their relative abundances set only by their neutron separation energies and partition functions, and the temperature and density of the environment. Towards the end of this "steady" phase of the r process-so called because creation and destruction reactions balance and abundances change slowly-the neutrons begin to disappear, making ͑n, g͒-͑g, n͒ equilibrium more difficult to maintain and causing nuclear abundances to change rapidly. Eventually the neutron capture and photodisintegration reactions "freeze out," and the nuclei simply b decay back to the stability line to give the final r-process abundances.The dominant features in the solar r-process abundance distribution are large peaks at nuclear mass numbers A ഠ 80, 130, and 195. These form as the nuclear flow passes through neutron-closed-shell nuclei, which capture neutrons reluctantly and have long beta-decay lifetimes, acting as bottlenecks at which abundances build up. This mechanism for peak formation has been well understood for many years [2]. The second most pronounced feature in the abundance distribution is a smaller but still very distinct peak at A ഠ 160, the region of the rare-earth elements (REEs). In contrast to the major peaks, its origin has remained something of a mystery since its presence was noted in the first detailed compilations of solar-system abundances [3,4]. In this paper we show that the REE peak is due to deformation in nuclei created after the steady phase of the r process ends. Previous r-process studies have usually limited themselves to the steady phase. Thus, although deformation has long been suspected to play a role in the formation of the REE peak [2], no compelling connection has been made until now.Nuclei deform when deformation increases stability. Up to a point, increasingly neutron-rich isotopes of an element can thus gain relative stability by increasing their deformation. At some isotope, however, deformation can increase no more; the next isotope is then less stable. Because of the sudden drop i...

Black hole accretion disks can form through the collapse of rotating massive stars. These disks produce large numbers of neutrinos and antineutrinos of electron flavor that can influence energetics and nucleosynthesis. Neutrinos are produced in sufficient numbers that, after they are emitted, they can undergo flavor transformation facilitated by the neutrino self interaction. We show that some of the neutrino flavor transformation phenomenology for accretion disks is similar to that of the supernova case, but also, we find the disk geometry lends itself to different transformation behaviors. These transformations strongly influence the nucleosynthetic outcome of disk winds.

Matter-neutrino resonances (MNR) can occur in environments where the flux of electron antineutrinos is greater than the flux of electron neutrinos. These resonances may result in dramatic neutrino flavor transformation. Compact object merger disks are an example of an environment where electron antineutrinos outnumber neutrinos. We study MNR resonances in several such disk configurations and find two qualitatively different types of matter-neutrino resonances: a standard MNR and a symmetric MNR. We examine the transformation that occurs in each type of resonance and explore the consequences for nucleosynthesis.