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Register a new userImpact of weak interactions of free nucleons on the r-process in dynamical ejecta from neutron-star mergers
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We investigate beta-interactions of free nucleons and their impact on the electron fraction (Y_e) and r-process nucleosynthesis in ejecta characteristic of binary neutron star mergers (BNSMs). For that we employ trajectories from a relativistic BNSM model to represent the density-temperature evolutions in our parametric study. In the high-density environment, positron captures decrease the neutron richness at the high temperatures predicted by the hydrodynamic simulation. Circumventing the complexities of modelling three-dimensional neutrino transport, (anti)neutrino captures are parameterized in terms of prescribed neutrino luminosities and mean energies, guided by published results and assumed as constant in time. Depending sensitively on the adopted neutrino-antineutrino luminosity ratio, neutrino processes increase Y_e to values between 0.25 and 0.40, still allowing for a successful r-process compatible with the observed solar abundance distribution and a significant fraction of the ejecta consisting of r-process nuclei. If the electron neutrino luminosities and mean energies are relatively large compared to the antineutrino properties, the mean Y_e might reach values >0.40 so that neutrino captures seriously compromise the success of the r-process. In this case, the r-abundances remain compatible with the solar distribution, but the total amount of ejected r-material is reduced to a few percent, because the production of iron-peak elements is favored. Proper neutrino physics, in particular also neutrino absorption, have to be included in BNSM simulations before final conclusions can be drawn concerning r-processing in this environment and concerning observational consequences like kilonovae, whose peak brightness and color temperature are sensitive to the composition-dependent opacity of the ejecta.
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Heavy elements like gold, platinum or uranium are produced in the r-process, which needs neutron-rich and explosive environments. Neutron star mergers are a promising candidate for an r-process site. They exhibit three different channels for matter ejection fulfilling these conditions: dynamic ejecta due to tidal torques, neutrino-driven winds and evaporating matter from the accretion disk. We present a first study of the integrated nucleosynthesis for a neutrino-driven wind from a neutron star merger with a hyper-massive neutron star. Trajectories from a recent hydrodynamical simulation are divided into four different angle regions and post-processed with a reaction network. We find that the electron fraction varies around $Y_e approx 0.1 - 0.4$, but its distribution differs for every angle of ejection. Hence, the wind ejecta do not undergo a robust r-process, but rather possess distinct nucleosynthesis yields depending on the angle range. Compared to the dynamic ejecta, a smaller amount of neutron-rich matter gets unbound, but the production of lighter heavy elements with $A lesssim 130$ in the neutrino-driven wind can complement the strong r-process of the dynamic ejecta.
Material ejected during (or immediately following) the merger of two neutron stars may assemble into heavy elements by the r-process. The subsequent radioactive decay of the nuclei can power electromagnetic emission similar to, but significantly dimmer than, an ordinary supernova. Identifying such events is an important goal of future transient surveys, offering new perspectives on the origin of r-process nuclei and the astrophysical sources of gravitational waves. Predictions of the transient light curves and spectra, however, have suffered from the uncertain optical properties of heavy ions. Here we consider the opacity of expanding r-process material and argue that it is dominated by line transitions from those ions with the most complex valence electron structure, namely the lanthanides. For a few representative ions, we run atomic structure models to calculate radiative data for tens of millions of lines. We find that the resulting r-process opacities are orders of magnitude larger than that of ordinary (e.g., iron-rich) supernova ejecta. Radiative transport calculations using these new opacities indicate that the transient emission should be dimmer and redder than previously thought. The spectra appear pseudo-blackbody, with broad absorption features, and peak in the infrared (~1 micron). We discuss uncertainties in the opacities and attempt to quantify their impact on the spectral predictions. The results have important implications for observational strategies to find and study the radioactively powered electromagnetic counterparts to compact object mergers.
Although the rapid neutron-capture process, or r-process, is fundamentally important for explaining the origin of approximately half of the stable nuclei with A > 60, the astrophysical site of this process has not been identified yet. Here we study r-process nucleosynthesis in material that is dynamically ejected by tidal and pressure forces during the merging of binary neutron stars (NSs) and within milliseconds afterwards. For the first time we make use of relativistic hydrodynamical simulations of such events, defining consistently the conditions that determine the nucleosynthesis, i.e., neutron enrichment, entropy, early density evolution and thus expansion timescale, and ejecta mass. We find that 10^{-3}-10^{-2} solar masses are ejected, which is enough for mergers to be the main source of heavy (A > 140) galactic r-nuclei for merger rates of some 10^{-5} per year. While asymmetric mergers eject 2-3 times more mass than symmetric ones, the exact amount depends weakly on whether the NSs have radii of ~15 km for a stiff nuclear equation of state (EOS) or ~12 km for a soft EOS. R-process nucleosynthesis during the decompression becomes largely insensitive to the detailed conditions because of efficient fission recycling, producing a composition that closely follows the solar r-abundance distribution for nuclei with mass numbers A > 140. Estimating the light curve powered by the radioactive decay heating of r-process nuclei with an approximative model, we expect high emission in the B-V-R bands for 1-2 days with potentially observable longer duration in the case of asymmetric mergers because of the larger ejecta mass.
Weak reactions are critical for the neutron richness of the matter dynamically ejected after the merger of two neutron stars. The neutron richness, defined by the electron fraction (Ye), determines which heavy elements are produced by the r-process and thus directly impacts the kilonova light curve. In this work, we have performed a systematic and detailed post-processing study of the impact of weak reactions on the distribution of the electron fraction and of the entropy on the dynamic ejecta obtained from an equal mass neutron star binary merger simulated in full general relativity and with microscopic equation of state. Previous investigations indicated that shocks increase Ye, however our results show that shocks can also decrease Ye, depending on their thermodynamical conditions. Moreover, we have found that neutrino absorption are key and need to be considered in future simulations. We also demonstrated that the angular dependence of the neutrino luminosity and the spatial distribution of the ejecta can lead to significant difference in the electron fraction distribution. In addition to the detailed study of the Ye evolution and its dependences, we have performed nucleosynthesis calculations. They clearly point to the necessity of improving the neutrino treatment in current simulations to be able to predict the contribution of neutron star mergers to the chemical history of the universe and to reliable calculate their kilonova light curves.
With the recent advent of multi-messenger gravitational-wave astronomy and in anticipation of more sensitive, next-generation gravitational-wave detectors, we investigate the dynamics, gravitational-wave emission, and nucleosynthetic yields of numerous eccentric binary neutron-star mergers having different equations of state. For each equation of state we vary the orbital properties around the threshold of immediate merger, as well as the binary mass ratio. In addition to a study of the gravitational-wave emission including $f$-mode oscillations before and after merger, we couple the dynamical ejecta output from the simulations to the nuclear-reaction network code texttt{SkyNet} to compute nucleosynthetic yields and compare to the corresponding results in the case of a quasi-circular merger. We find that the amount and velocity of dynamically ejected material is always much larger than in the quasi-circular case, reaching maximal values of $M_{rm ej, max} sim 0.1 , M_{odot}$ and $v_{rm max}/c sim 0.75$. At the same time, the properties of this material are rather insensitive to the details of the orbit, such as pericenter distance or post-encounter apoastron distance. Furthermore, while the composition of the ejected matter depends on the orbital parameters and on the equation of state, the relative nucleosynthetic yields do not, thus indicating that kilonova signatures could provide information on the orbital properties of dynamically captured neutron-star binaries.
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