Gravitational waves from coalescing neutron stars encode information about nuclear matter at extreme densities, inaccessible by laboratory experiments. The late inspiral is influenced by the presence of tides, which depend on the neutron star equation of state. Neutron star mergers are expected to often produce rapidly rotating remnant neutron stars that emit gravitational waves. These will provide clues to the extremely hot post-merger environment. This signature of nuclear matter in gravitational waves contains most information in the 2–4 kHz frequency band, which is outside of the most sensitive band of current detectors. We present the design concept and science case for a Neutron Star Extreme Matter Observatory (NEMO): a gravitational-wave interferometer optimised to study nuclear physics with merging neutron stars. The concept uses high-circulating laser power, quantum squeezing, and a detector topology specifically designed to achieve the high-frequency sensitivity necessary to probe nuclear matter using gravitational waves. Above 1 kHz, the proposed strain sensitivity is comparable to full third-generation detectors at a fraction of the cost. Such sensitivity changes expected event rates for detection of post-merger remnants from approximately one per few decades with two A+ detectors to a few per year and potentially allow for the first gravitational-wave observations of supernovae, isolated neutron stars, and other exotica.

The Orion Nebula Cluster (ONC) is one of the nearest open clusters, which we can directly compare to numerical simulations. We performed a simulation of star cluster formation similar to the ONC using our new N-body/smoothed particle hydrodynamics code, ASURA+BRIDGE. We found that the hierarchical formation of star clusters via clump mergers can explain the observed three peaks in the stellar age distribution as well as the dynamically anisotropic structures of the ONC.

Abundances of heavy elements in dwarf galaxies reflect their early evolutionary histories. Recent astronomical observations have shown that there are star-to-star scatters in the abundances of r-process elements and the decreasing trend of Zn toward higher metallicity in extremely metal-poor stars. However, the enrichment of heavy elements is not well understood. Here we performed a series of high-resolution N-body/smoothed particle hydrodynamics simulations of dwarf galaxies. We find that neutron star mergers can explain ratios of r-process elements to iron in dwarf galaxies due to their suppressed star formation rates. We also find that stars with [Zn/Fe] ≳ 0.5 reflect the ejecta from electron-capture supernovae. Inhomogeneity of the metals in the interstellar medium causes the scatters of heavy elements. We estimate that the timescale of metal mixing is ≲ 40 Myr using heavy element abundances in metal-poor stars.

Neutron star mergers are one of the candidate astrophysical site(s) of r-process. Several chemical evolution studies however pointed out that the observed abundance of r-process is difficult to reproduce by neutron star mergers. In this study, we aim to clarify the enrichment of r-process elements in the Local Group dwarf galaxies. We carry out numerical simulations of galactic chemo-dynamical evolution using an N-body/smoothed particle hydrodynamics code, ASURA. We construct a chemo-dynamical evolution model for dwarf galaxies assuming that neutron star mergers are the major source of r-process elements. Our models reproduce the observed dispersion in [Eu/Fe] as a function of [Fe/H] with neutron star mergers with a merger time of 100 Myr. We find that star formation efficiency and metal mixing processes during the first ≲ 300 Myr of galaxy evolution are important to reproduce the observations. This study supports that neutron star mergers are a major site of r-process.

Single-phase barium-filled skutterudite compounds, BayFexCo4−xSb12 (x = 0 to 3.0, y = 0 to 0.7), were synthesized by a two-step solid-state reaction method. The maximum filling fraction of Ba (ymax) in BayFexCo4–xSb12 increased with increasing Fe content and was found to be rather greater than that of CeyFexCo4–xSb12. The ymax varied from 0.35 to near 1.0 when Fe content changed from 0 to 4.0. BayFexCo4–xSb12 showed p-type conduction at a composition range of x = 0 to 3.0, y = 0 to 0.7. Carrier concentration and electrical conductivity increased with increasing Fe content and decreased with increasing Ba filling fraction. The Seebeck coefficient increased with increasing Ba filling fraction and with decreasing Fe content. Lattice thermal conductivity decreased with increasing Ba filling fraction and reached a minimum at a certain Ba filling fraction (y = 0.3 to 0.4). The greatest ZT value of 0.9 was obtained at 750 K for p-type Ba0.27Fe0.98Co3.02Sb12. It is expected that further investigation on the optimization of filling fraction would result in a higher ZT value at the moderately low Fe content region.

Effects of Ba filling fraction and Ni content on the thermoelectric properties of n-type BayNixCo4−xSb12 (x = 0−0.1, y = 0−0.4) were investigated at temperature range of 300 to 900 K. Thermal conductivity decreased with increasing Ba filling fraction and temperature. When y was fixed at 0.3, thermal conductivity decreased with increasing Ni content and reached a minimum value at about x = 0.05. Lattice thermal conductivity decreased with increasing Ni content, monotonously (y ≤ 0.1). Electron concentration and electrical conductivity increased with increasing Ba filling fraction and Ni content. Seebeck coefficient increased with increasing temperature and decreased with increasing Ba filling fraction and Ni content. The maximum ZT value of 1.25 was obtained at about 900 K for n-type Ba0.3Ni0.05Co3.95Sb12.