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Register a new userThe impact of r-process heating on the dynamics of neutron star merger accretion disc winds and their electromagnetic radiation
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Neutron star merger accretion discs can launch neutron-rich winds of $>10^{-2},mathrm{M}_odot$. This ejecta is a prime site for r-process nucleosynthesis, which will produce a range of radioactive heavy nuclei. The decay of these nuclei releases enough energy to accelerate portions of the wind by ~0.1c. Here, we investigate the effect of r-process heating on the dynamical evolution of disc winds. We extract the wind from a 3D general relativistic magnetohydrodynamic simulation of a disc from a post-merger system. This is used to create inner boundary conditions for 2D hydrodynamic simulations that continue the original 3D simulation. We perform two such simulations: one that includes the r-process heating, and another one that does not. We follow the hydrodynamic simulations until the winds reach homology (60 seconds). Using time-dependent multi-frequency multi-dimensional Monte Carlo radiation transport simulations, we then calculate the kilonova light curves from the winds with and without dynamical r-process heating. We find that the r-process heating can substantially alter the velocity distribution of the wind, shifting the mass-weighted median velocity from 0.06c to 0.12c. The inclusion of the dynamical r-process heating makes the light curve brighter and bluer at ~1 d post-merger. However, the high-velocity tail of the ejecta distribution and the early light curves are largely unaffected.
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We study here the formation of heavy r-process nuclei in the high-entropy environment of rapidly expanding neutrino-driven winds from compact objects. In particular, we explore the sensitivity of the element creation in the A>130 region to the low-temperature behavior of the outflows. For this purpose we employ a simplified model of the dynamics and thermodynamical evolution for radiation dominated, adiabatic outflows. It consists of a first stage of fast, exponential cooling, followed by a second phase of slower evolution, either assuming constant density and temperature or a power-law decay of these quantities. These cases are supposed to capture the most relevant effects of a strong deceleration or decreasing acceleration of the transsonic outflows, respectively, e.g. in a wind termination shock caused by the collision with the slower, preceding supernova ejecta. We find that not only the transition temperature between the two expansion phases can make a big difference in the formation of the platinum peak, but also the detailed cooling law during the later phase. Unless the transition temperature and corresponding (free neutron) density become too small (T < 2*10^8 K), a lower temperature or faster temperature decline during this phase allow for a stronger appearance of the third abundance peak. Since the nuclear photodisintegration rates between ~2*10^8 K and ~10^9 K are more sensitive to the temperature than the n-capture rates are to the free neutron density, a faster cooling in this temperature regime shifts the r-process path closer to the n-drip line. With low (gamma,n)- but high beta-decay rates, the r-processing then does not proceed through a (gamma,n)-(n,gamma) equilibrium but through a quasi-equilibrium of (n,gamma)-reactions and beta-decays, as recently also pointed out by Wanajo.
We follow the longterm evolution of the dynamic ejecta of neutron star mergers for up to 100 years and over a density range of roughly 40 orders of magnitude. We include the nuclear energy input from the freshly synthesized, radioactively decaying nuclei in our simulations and study its effects on the remnant dynamics. Although the nuclear heating substantially alters the longterm evolution, we find that running nuclear networks over purely hydrodynamic simulations (i.e. without heating) yields actually acceptable nucleosynthesis results. The main dynamic effect of the radioactive heating is to quickly smooth out inhomogeneities in the initial mass distribution, subsequently the evolution proceeds self-similarly and after 100 years the remnant still carries the memory of the initial binary mass ratio. We also explore the nucleosynthetic yields for two mass ejection channels. The dynamic ejecta very robustly produce strong r-process elements with $A > 130$ with a pattern that is essentially independent of the details of the merging system. From a simple model we find that neutrino-driven winds yield weak r-process contributions with $50 < A < 130$ whose abundance patterns vary substantially between different merger cases. This is because their electron fraction, set by the ratio of neutrino luminosities, varies considerably from case to case. Such winds do not produce any $^{56}{rm Ni}$, but a range of radioactive isotopes that are long-lived enough to produce a second, radioactively powered electromagnetic transient in addition to the macronova from the dynamic ejecta. While our wind model is very simple, it nevertheless demonstrates the potential of such neutrino-driven winds for electromagnetic transients and it motivates further, more detailed neutrino-hydrodynamic studies. The properties of the mentioned transients are discussed in more detail in a companion paper.
Comparing observational abundance features with nucleosynthesis predictions of stellar evolution or explosion simulations can scrutinize two aspects: (a) the conditions in the astrophysical production site and (b) the quality of the nuclear physics input utilized. We test the abundance features of r-process nucleosynthesis calculations for the dynamical ejecta of neutron star merger simulations based on three different nuclear mass models: The Finite Range Droplet Model (FRDM), the (quenched version of the) Extended Thomas Fermi Model with Strutinsky Integral (ETFSI-Q), and the Hartree-Fock-Bogoliubov (HFB) mass model. We make use of corresponding fission barrier heights and compare the impact of four different fission fragment distribution models on the final r-process abundance distribution. In particular, we explore the abundance distribution in the second r-process peak and the rare-earth sub-peak as a function of mass models and fission fragment distributions, as well as the origin of a shift in the third r-process peak position. The latter has been noticed in a number of merger nucleosynthesis predictions. We show that the shift occurs during the r-process freeze-out when neutron captures and {beta}-decays compete and an (n,{gamma})-({gamma},n) equilibrium is not maintained anymore. During this phase neutrons originate mainly from fission of material above A = 240. We also investigate the role of {beta}-decay half-lives from recent theoretical advances, which lead either to a smaller amount of fissioning nuclei during freeze-out or a faster (and thus earlier) release of fission neutrons, which can (partially) prevent this shift and has an impact on the second and rare-earth peak as well.
The determination of the mass, composition, and geometry of matter outflows in black hole-neutron star and neutron star-neutron star binaries is crucial to current efforts to model kilonovae, and to understand the role of neutron star merger in r-process nucleosynthesis. In this manuscript, we review the simple criteria currently used in merger simulations to determine whether matter is unbound and what the asymptotic velocity of ejected material will be. We then show that properly accounting for both heating and cooling during r-process nucleosynthesis is important to accurately predict the mass and kinetic energy of the outflows. We also derive a model accounting for both of these effects that can be easily implemented in merger simulations. We show, however, that the detailed velocity distribution and geometry of the outflows can currently only be captured by full 3D fluid simulations of the outflows, as non-local effect ignored by the simple criteria used in merger simulations cannot be safely neglected when modeling these effects. Finally, we propose the introduction of simple source terms in the fluid equations to approximately account for heating/cooling from r-process nucleosynthesis in future seconds-long 3D simulations of merger remnants, without the explicit inclusion of out-of-nuclear statistical equilibrium reactions in the simulations.
We have collected the parameter of 38 neutron stars (NSs) in binary systems with spin periods and measured masses. By adopting the Boot-strap method, we reproduced the procedure of mass calculated for each system separately, to determine the truly mass distribution of the NS that obtained from observation. We also applied the Monte-Carlo simulation and introduce the characteristic spin period 20 ms, in order to distinguish between millisecond pulsars (MSPs) and less recycled pulsars. The mass distributions of MSPs and the less recycled pulsars could be fitted by a Gaussian function as $rm 1.45pm0.42 M_{odot}$ and $rm 1.31pm0.17 M_{odot} (rm with ~ 1sigma)$ respectively. As such, the MSP masses are heavier than those in less recycled systems by factor of $rm sim 0.13M_{odot}$, since the accretion effect during the recycling process.
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