Simulating neutron stars with a flexible enthalpy-based equation of state parametrization in SpECTRE

Numerical simulations of neutron star mergers represent an essential step toward interpreting the full complexity of multimessenger observations and constraining the properties of supranuclear matter. Currently, simulations are limited by an array of factors, including computational performance and input physics uncertainties, such as the neutron star equation of state. In this work, we expand the range of nuclear phenomenology efficiently available to simulations by introducing a new analytic parametrization of cold, beta-equilibrated matter that is based on the relativistic enthalpy. We show that the new $\textit{enthalpy parametrization}$ can capture a range of nuclear behavior, including strong phase transitions. We implement the enthalpy parametrization in the $\texttt{SpECTRE}$, code, simulate isolated neutron stars, and compare performance to the commonly used spectral and polytropic parametrizations. We find comparable computational performance for nuclear models that are well represented by either parametrization, such as simple hadronic EoSs. We show that the enthalpy parametrization further allows us to simulate more complicated hadronic models or models with phase transitions that are inaccessible to current parametrizations.Comment: 20 pages, 14 figures, submitted to PRD, additional information on software including input files available at https://github.com/sxs-collaboration/paper-2023-spectre-enthalpy-eo

Initial data for high-compactness black hole–neutron star binaries

For highly compact neutron stars, constructing numerical initial data for black hole–neutron star binary evolutions is very difficult. We describe improvements to an earlier method that enable it to handle these more challenging cases. These improvements were found by invoking a general relaxation principle that may be helpful in improving robustness in other initial data solvers. We examine the case of a 6:1 mass ratio system in inspiral close to merger, where the star is governed by a polytropic Γ =2, an SLy, or an LS220 equation of state (EOS). In particular, we are able to obtain a solution with a realistic LS220 EOS for a star with compactness 0.26 and mass 1.98 M_⊙, which is representative of the highest reliably determined neutron star masses. For the SLy EOS, we can obtain solutions with a comparable compactness of 0.25, while for a family of polytropic equations of state, we obtain solutions with compactness up to 0.21, the largest compactness that is stable in this family. These compactness values are significantly higher than any previously published results

SpECTRE: A Task-based Discontinuous Galerkin Code for Relativistic Astrophysics

We introduce a new relativistic astrophysics code, SpECTRE, that combines a discontinuous Galerkin method with a task-based parallelism model. SpECTRE's goal is to achieve more accurate solutions for challenging relativistic astrophysics problems such as core-collapse supernovae and binary neutron star mergers. The robustness of the discontinuous Galerkin method allows for the use of high-resolution shock capturing methods in regions where (relativistic) shocks are found, while exploiting high-order accuracy in smooth regions. A task-based parallelism model allows efficient use of the largest supercomputers for problems with a heterogeneous workload over disparate spatial and temporal scales. We argue that the locality and algorithmic structure of discontinuous Galerkin methods will exhibit good scalability within a task-based parallelism framework. We demonstrate the code on a wide variety of challenging benchmark problems in (non)-relativistic (magneto)-hydrodynamics. We demonstrate the code's scalability including its strong scaling on the NCSA Blue Waters supercomputer up to the machine's full capacity of 22,380 nodes using 671,400 threads.Comment: 41 pages, 13 figures, and 7 tables. Ancillary data contains simulation input file

Multichannel coupling with supersymmetric quantum mechanics and exactly-solvable model for Feshbach resonance

A new type of supersymmetric transformations of the coupled-channel radial Schroedinger equation is introduced, which do not conserve the vanishing behavior of solutions at the origin. Contrary to usual transformations, these ``non-conservative'' transformations allow, in the presence of thresholds, the construction of potentials with coupled scattering matrices from uncoupled potentials. As an example, an exactly-solvable potential matrix is obtained which provides a very simple model of Feshbach-resonance phenomenon.Comment: 10 pages, 2 figure

Survey of fungal diversity in silages supplied to dairy cattle in Belgium over a two-year period

The fungal diversity in silages for dairy cattle feeding has been assessed by purification and identification of 966 isolates collected in silages during the two 2006 and 2007 winter storage/feed-out periods from farms localized in various geographic regions in Belgium. The relevant fungal species in silos were P. paneum and P. roqueforti (18.2 % and 14.5 % of total isolates, respectively). The proportion between the two species varied significantly from 2006 to 2007 (P<0.05) depending on the type of forage crop. The prevalence of P. paneum in Belgium compared to results in other countries is of major importance due to the mycotoxigenic capacity of this species. Information on numerous aspects of silage making process and silo management showed that neither the crop rotation, the date and weather at harvest, the use of additives, the feeding rate, the type and dimension of silo, the covering used nor the forage chopping length at ensiling could be significantly correlated to the fungal species composition isolated in farm silages. There were also no significant relationships between the chemico-fermentative parameters tested (dry matter, pH, NH3 content, concentration of lactic, acetic and butyric acids) and the fungal species composition in silages. The prevalent fungi were tolerant of the wide ranges of conditions found in the farm silages with year-to-year variations. Survey of Fungal Diversity in Silages Supplied to Dairy Cattle in Belgium Over a Two-Year Period. Available from: https://www.researchgate.net/publication/313255127_Survey_of_Fungal_Diversity_in_Silages_Supplied_to_Dairy_Cattle_in_Belgium_Over_a_Two-Year_Period [accessed Jul 24, 2017]

Comparison of momentum transport models for numerical relativity

The main problems of nonvacuum numerical relativity, compact binary mergers and stellar collapse, involve hydromagnetic instabilities and turbulent flows, so that kinetic energy at small scales leads to mean effects at large scale that drive the secular evolution. Notable among these effects is momentum transport. We investigate two models of this transport effect, a relativistic Navier-Stokes system and a turbulent mean stress model, that are similar to all of the prescriptions that have been attempted to date for treating subgrid effects on binary neutron star mergers and their aftermath. Our investigation involves both stability analysis and numerical experimentation on star and disk systems. We also begin the investigation of the effects of particle and heat transport on postmerger simulations. We find that correct handling of turbulent heating is crucial for avoiding unphysical instabilities. Given such appropriate handling, the evolution of a differentially rotating star and the accretion rate of a disk are reassuringly insensitive to the choice of prescription. However, disk outflows can be sensitive to the choice of method, even for the same effective viscous strength. We also consider the effects of eddy diffusion in the evolution of an accretion disk and show that it can interestingly affect the composition of outflows

Effects of Neutron-Star Dynamic Tides on Gravitational Waveforms within the Effective-One-Body Approach

Extracting the unique information on ultradense nuclear matter from the gravitational waves emitted by merging neutron-star binaries requires robust theoretical models of the signal. We develop a novel effective-one-body waveform model that includes, for the first time, dynamic (instead of only adiabatic) tides of the neutron star as well as the merger signal for neutron-star–black-hole binaries. We demonstrate the importance of the dynamic tides by comparing our model against new numerical-relativity simulations of nonspinning neutron-star–black-hole binaries spanning more than 24 gravitational-wave cycles, and to other existing numerical simulations for double neutron-star systems. Furthermore, we derive an effective description that makes explicit the dependence of matter effects on two key parameters: tidal deformability and fundamental oscillation frequency

Inscription choragique d'Athènes

Foucart Paul-François. Inscription choragique d'Athènes. In: Bulletin de correspondance hellénique. Volume 2, 1878. pp. 391-397