r-Process Lanthanide Production and Heating Rates in Kilonovae

r-Process nucleosynthesis in material ejected during neutron star mergers may lead to radioactively powered transients called kilonovae. The timescale and peak luminosity of these transients depend on the composition of the ejecta, which determines the local heating rate from nuclear decays and the opacity. Kasen et al. (2013, ApJ, 774, 25) and Tanaka & Hotokezaka (2013, ApJ, 775, 113) pointed out that lanthanides can drastically increase the opacity in these outflows. We use the new general-purpose nuclear reaction network SkyNet to carry out a parameter study of r-process nucleosynthesis for a range of initial electron fractions $Y_e$, initial specific entropies $s$, and expansion timescales $\tau$. We find that the ejecta is lanthanide-free for $Y_e \gtrsim 0.22 - 0.30$, depending on $s$ and $\tau$. The heating rate is insensitive to $s$ and $\tau$, but certain, larger values of $Y_e$ lead to reduced heating rates, due to individual nuclides dominating the heating. We calculate approximate light curves with a simplified gray radiative transport scheme. The light curves peak at about a day (week) in the lanthanide-free (-rich) cases. The heating rate does not change much as the ejecta becomes lanthanide-free with increasing $Y_e$, but the light curve peak becomes about an order of magnitude brighter because it peaks much earlier when the heating rate is larger. We also provide parametric fits for the heating rates between 0.1 and $100\,\text{days}$, and we provide a simple fit in $Y_e$, $s$, and $\tau$ to estimate whether the ejecta is lanthanide-rich or not.Comment: 19 pages, 9 figure

SkyNet: A modular nuclear reaction network library

Almost all of the elements heavier than hydrogen that are present in our solar system were produced by nuclear burning processes either in the early universe or at some point in the life cycle of stars. In all of these environments, there are dozens to thousands of nuclear species that interact with each other to produce successively heavier elements. In this paper, we present SkyNet, a new general-purpose nuclear reaction network that evolves the abundances of nuclear species under the influence of nuclear reactions. SkyNet can be used to compute the nucleosynthesis evolution in all astrophysical scenarios where nucleosynthesis occurs. SkyNet is free and open-source and aims to be easy to use and flexible. Any list of isotopes can be evolved and SkyNet supports various different types of nuclear reactions. SkyNet is modular so that new or existing physics, like nuclear reactions or equations of state, can easily be added or modified. Here, we present in detail the physics implemented in SkyNet with a focus on a self-consistent transition to and from nuclear statistical equilibrium (NSE) to non-equilibrium nuclear burning, our implementation of electron screening, and coupling of the network to an equation of state. We also present comprehensive code tests and comparisons with existing nuclear reaction networks. We find that SkyNet agrees with published results and other codes to an accuracy of a few percent. Discrepancies, where they exist, can be traced to differences in the physics implementations.Comment: 39 pages, 11 figures, published in ApJ Supplement Serie

r-Process Nucleosynthesis in Neutron Star Mergers with the New Nuclear Reaction Network SkyNet

At the Big Bang, only the lightest elements, mainly hydrogen and helium, were produced. Stars synthesize heavier elements, such as helium, carbon, and oxygen, from lighter ones through nuclear fusion. Iron-group elements are created in supernovae (both type Ia and core-collapse). It has been known for 60 years that the slow and rapid neutron capture processes (s- and r-process) are each responsible for creating about half of the elements beyond the iron group. The s-process is known to occur in asymptotic giant branch stars, but the astrophysical site of the r-process is still a mystery. Based on observations of heavy elements in old stars, it was theorized that r-process nucleosynthesis takes place in core-collapse supernovae (CCSNe). However, recent CCSN simulations indicate that the conditions required for the r-process are not obtained in CCSN. The focus has thus shifted to neutron star mergers (both binary neutron star and black hole-neutron star mergers), where the r-process easily synthesizes all the known heavy elements. Neutron star mergers are expected to be detected by the Laser Interferometer Gravitational Wave Observatory (LIGO) in the near future, which should either confirm or rule out their proposed association with radioactively powered transients called kilonovae or macronovae that are the observational signatures of r-process nucleosynthesis. To understand how the r-process operates in different astrophysical scenarios and what relative abundance patterns it produces, detailed nuclear reaction network calculations are needed that track thousands of isotopes and tens of thousands of nuclear reactions. In this thesis, I present SkyNet, a new general-purpose nuclear reaction network that can evolve an arbitrary list of nuclear species with an arbitrary set of nuclear reactions. I describe in detail the different physics that is implemented in SkyNet and I perform code tests and comparisons to other nuclear reaction networks. Then I use SkyNet to systematically investigate r-process nucleosynthesis as a function of the initial electron fraction, initial entropy, and expansion timescale of the fluid. Further, I present r-process nucleosynthesis calculations with SkyNet in the dynamical ejecta of a black hole–neutron star merger with varying levels of neutrino irradiation. Finally, I study the r-process in the outflow of a neutron star merger remnant disk as a function of the lifetime of the central hypermassive neutron star (HMNS). SkyNet is easy to use and flexible and it is publicly available as open-source software. Multiple researchers are already using SkyNet for their work, and I hope that SkyNet will be a useful tool for the broader nuclear astrophysics community

Dynamical Mass Ejection from Binary Neutron Star Mergers

We present fully general-relativistic simulations of binary neutron star mergers with a temperature and composition dependent nuclear equation of state. We study the dynamical mass ejection from both quasi-circular and dynamical-capture eccentric mergers. We systematically vary the level of our treatment of the microphysics to isolate the effects of neutrino cooling and heating and we compute the nucleosynthetic yields of the ejecta. We find that eccentric binaries can eject significantly more material than quasi-circular binaries and generate bright infrared and radio emission. In all our simulations the outflow is composed of a combination of tidally- and shock-driven ejecta, mostly distributed over a broad $\sim 60^\circ$ angle from the orbital plane, and, to a lesser extent, by thermally driven winds at high latitudes. Ejecta from eccentric mergers are typically more neutron rich than those of quasi-circular mergers. We find neutrino cooling and heating to affect, quantitatively and qualitatively, composition, morphology, and total mass of the outflows. This is also reflected in the infrared and radio signatures of the binary. The final nucleosynthetic yields of the ejecta are robust and insensitive to input physics or merger type in the regions of the second and third r-process peaks. The yields for elements on the first peak vary between our simulations, but none of our models is able to explain the Solar abundances of first-peak elements without invoking additional first-peak contributions from either neutrino and viscously-driven winds operating on longer timescales after the mergers, or from core-collapse supernovae.Comment: 19 pages, 10 figures. We corrected a problem in the formulation of the neutrino heating scheme and re-ran all of the affected models. The main conclusions are unchanged. This version also contains one more figure and a number of improvements on the tex

R-process Nucleosynthesis from Three-Dimensional Magnetorotational Core-Collapse Supernovae

We investigate r-process nucleosynthesis in three-dimensional (3D) general-relativistic magnetohydrodynamic simulations of rapidly rotating strongly magnetized core collapse. The simulations include a microphysical finite-temperature equation of state and a leakage scheme that captures the overall energetics and lepton number exchange due to postbounce neutrino emission and absorption. We track the composition of the ejected material using the nuclear reaction network SkyNet. Our results show that the 3D dynamics of magnetorotational core-collapse supernovae (CCSN) are important for their nucleosynthetic signature. We find that production of r-process material beyond the second peak is reduced by a factor of 100 when the magnetorotational jets produced by the rapidly rotating core undergo a kink instability. Our results indicate that 3D magnetorotationally powered CCSNe are a robust r-process source only if they are obtained by the collapse of cores with unrealistically large precollapse magnetic fields of order $10^{13}$G. Additionally, a comparison simulation that we restrict to axisymmetry, results in overly optimistic r-process production for lower magnetic field strengths.Comment: 10 pages, 9 figures, 2 tables. submitted to Ap

Signatures of hypermassive neutron star lifetimes on r-process nucleosynthesis in the disk ejecta from neutron star mergers

We investigate the nucleosynthesis of heavy elements in the winds ejected by accretion disks formed in neutron star mergers. We compute the element formation in disk outflows from hypermassive neutron star (HMNS) remnants of variable lifetime, including the effect of angular momentum transport in the disk evolution. We employ long-term axisymmetric hydrodynamic disk simulations to model the ejecta, and compute r-process nucleosynthesis with tracer particles using a nuclear reaction network containing $\sim 8000$ species. We find that the previously known strong correlation between HMNS lifetime, ejected mass, and average electron fraction in the outflow is directly related to the amount of neutrino irradiation on the disk, which dominates mass ejection at early times in the form of a neutrino-driven wind. Production of lanthanides and actinides saturates at short HMNS lifetimes ($\lesssim 10$ ms), with additional ejecta contributing to a blue optical kilonova component for longer-lived HMNSs. We find good agreement between the abundances from the disk outflow alone and the solar r-process distribution only for short HMNS lifetimes ($\lesssim 10$ ms). For longer lifetimes, the rare-earth and third r-process peaks are significantly under-produced compared to the solar pattern, requiring additional contributions from the dynamical ejecta. The nucleosynthesis signature from a spinning black hole (BH) can only overlap with that from a HMNS of moderate lifetime ($\lesssim 60$ ms). Finally, we show that angular momentum transport not only contributes with a late-time outflow component, but that it also enhances the neutrino-driven component by moving material to shallower regions of the gravitational potential, in addition to providing additional heating.Comment: 18 pages, 11 figures, published version with small change

Dynamics, nucleosynthesis, and kilonova signature of black hole—neutron star merger ejecta

We investigate the ejecta from black hole—neutron star mergers by modeling the formation and interaction of mass ejected in a tidal tail and a disk wind. The outflows are neutron-rich, giving rise to optical/infrared emission powered by the radioactive decay of r-process elements (a kilonova). Here we perform an end-to-end study of this phenomenon, where we start from the output of a fully-relativistic merger simulation, calculate the post-merger hydrodynamical evolution of the ejecta and disk winds including neutrino physics, determine the final nucleosynthetic yields using post-processing nuclear reaction network calculations, and compute the kilonova emission with a radiative transfer code. We study the effects of the tail-to-disk mass ratio by scaling the tail density. A larger initial tail mass results in fallback matter becoming mixed into the disk and ejected in the subsequent disk wind. Relative to the case of a disk without dynamical ejecta, the combined outflow has lower mean electron fraction, faster speed, larger total mass, and larger absolute mass free of high-opacity Lanthanides or Actinides. In most cases, the nucleosynthetic yield is dominated by the heavy r-process contribution from the unbound part of the dynamical ejecta. A Solar-like abundance distribution can however be obtained when the total mass of the dynamical ejecta is comparable to the mass of the disk outflows. The kilonova has a characteristic duration of 1 week and a luminosity of  ~10^(41) erg s^(-1), with orientation effects leading to variations of a factor  ~2 in brightness. At early times (<1 d) the emission includes an optical component from the (hot) Lanthanide-rich material, but the spectrum evolves quickly to the infrared thereafter

Neutrino-heated winds from millisecond protomagnetars as sources of the weak r-process

We explore heavy element nucleosynthesis in neutrino-driven winds from rapidly rotating, stronglymagnetized protoneutron stars (‘millisecond protomagnetars’) forwhich themagnetic dipole is aligned with the rotation axis, and the field is assumed to be a static force-free configuration. We process the protomagnetar wind trajectories calculated by Vlasov, Metzger & Thompson through the r-process nuclear reaction network SkyNet using contemporary models for the evolution of the wind electron fraction during the protoneutron star cooling phase. Although we do not find a successful second or third-peak r-process for any rotation period P, we show that protomagnetars with P ∼ 1–5 ms produce heavy element abundance distributions that extend to higher nuclear mass number than from otherwise equivalent spherical winds (with the mass fractions of some elements enhanced by factors of �100–1000). The heaviest elements are synthesized by outflows emerging along flux tubes that graze the closed zone and pass near the equatorial plane outside the light cylinder. Due to dependence of the nucleosynthesis pattern on the magnetic field strength and rotation rate of the protoneutron star, natural variations in these quantities between core collapse events could contribute to the observed diversity of the abundances of weak r-process nuclei in metal-poor stars. Further diversity, including possibly even a successful third-peak r-process, could be achieved for misaligned rotators with non-zero magnetic inclination with respect to the rotation axis. If protomagnetars are central engines for GRBs, their relativistic jets should contain a high-mass fraction of heavy nuclei of characteristic mass number ¯A ≈ 100, providing a possible source for ultrahigh energy cosmic rays comprised of heavy nuclei with an energy spectrum that extends beyond the nominal Grezin–Zatsepin–Kuzmin cut-off for protons or iron nuclei