Neutron-star radii provide useful information on the equation of state of neutron rich matter. Particularly interesting is the density dependence of the equation of state (EOS). For example, the softening of the EOS at high density, where the pressure rises slower than anticipated, could signal a transition to an exotic phase. However, extracting the density dependence of the EOS requires measuring the radii of neutron stars for a broad range of masses. A "normal" 1.4M⊙ (M⊙=solar mass) neutron star has a central density of a few times nuclear-matter saturation density (ρ0). In contrast, low mass (≃ 0.5M⊙) neutron stars have central densities near ρ0 so its radius provides information on the EOS at low density. Unfortunately, low-mass stars are rare because they may be hard to form. Instead, a precision measurement of nuclear radii on atomic nuclei may contain similar information. Indeed, we find a strong correlation between the neutron radius of 208 Pb and the radius of a 0.5M⊙ neutron star. Thus, the radius of a 0.5M⊙ neutron star can be inferred from a measurement of the the neutron radius of 208 Pb. Comparing this value to the measured radius of a ≃ 1.4M⊙ neutron star should provide the strongest constraint to date on the density dependence of the equation of state.

Nonuniform neutron-rich matter present in both core-collapse supernovae and neutron-star crusts is described in terms of a semiclassical model that reproduces nuclear-matter properties and includes long-range Coulomb interactions. The neutron-neutron correlation function and the corresponding static structure factor are calculated from molecular dynamics simulations involving 40,000 to 100,000 nucleons. The static structure factor describes coherent neutrino scattering which is expected to dominate the neutrino opacity. At low momentum transfers the static structure factor is found to be small because of ion screening. In contrast, at intermediate momentum transfers the static structure factor displays a large peak due to coherent scattering from all the neutrons in a cluster. This peak moves to higher momentum transfers and decreases in amplitude as the density increases. A large static structure factor at zero momentum transfer, indicative of large density fluctuations during a first-order phase transition, may increase the neutrino opacity. However, no evidence of such an increase has been found. Therefore, it is unlikely that the system undergoes a simple first-order phase transition. Further, to compare our results to more conventional approaches, a cluster algorithm is introduced to determine the composition of the clusters in our simulations. Neutrino opacities are then calculated within a single heavy nucleus approximation as is done in most current supernova simulations. It is found that corrections to the single heavy nucleus approximation first appear at a density of the order of 10 13 g/cm 3 and increase rapidly with increasing density. Thus, neutrino opacities are overestimated in the single heavy nucleus approximation relative to the complete molecular dynamics simulations.

Articles you may be interested inCurrent response characteristics of microchannel plates xray detector using synchrotron radiation (0.6-2 keV and 5-20 keV) Rev. Sci. Instrum. 59, 252 (1988); 10.1063/1.1140236 Effect of rotational coupling on emission probabilities of Lyman and Werner band systems of the vacuum ultraviolet spectrum of H2We have measured relative cross sections as a function of impact energy for the emission of Werner bands C(2Ptr lII~)-(Isoll:g +) and Lyman-a radiation for 0.05-6-keV electrons on molecular hydrogen. The cross sections for the Werner bands are normalized on theoretical Born or Bethe cross sections, evaluated by means of the theoretical generalized oscillator strengths of Miller and Krauss and the theoretical optical oscillator strengths and transition probabilities of Allison and Dalgarno. By comparing the cross sections thus obtained and the relative intensity measurements, the sensitivity of our optical detection system can be determined between about 1030 and 1250 A. It is then also possible to bring the relative cross sections for Lyman-a radiation at 1215.7 A on an absolute scale. The latter cross sections appear to be 1.31 times smaller than those reported by Vroom and De Heer, which data were normalized on the so-called countable ultraviolet cross sections of Fite and Brackmann. In the latter normalization, the contribution of the Werner bands to the countable ultraviolet cross sections has been neglected.Nonempirical LCAO MO SCF calculations using Gaussian-type functions have been used to predict the geometries of acetylene, vinylidene carbene, 1I"-protonated acetylene, and the vinyl cation. The computed geometry and molecular ionization potential of acetylene give good agreement with experimental data. The vinyl cation is predicted to be more stable than 1I"-protonated acetylene by about 18 kcal/mole.

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In many experiments Lyman-α radiation has been detected by means of the iodine-vapor-filled ultraviolet photon counter of Brackmann, Fite, and Hagen in combination with a LiF–O2 filter. In the case of electrons incident on molecular hydrogen, Fite and Brackmann have used the detector for determination of the cross section for ``countable ultraviolet radiation'' (cuv) being chiefly composed of Lyman-α radiation and an unknown fraction of molecular radiation. In the present investigation the transmission properties of the LiF–O2 filter are investigated. At 100 eV impact energy it is found that 26% of the transmitted radiation is molecular radiation. Considering this fraction, it is possible to evaluate the cross sections for Lyman-α radiation from Fite and Brackmann's cuv cross sections. These cross sections agree very well (within about 6%) with those of de Heer and Carrière, obtained by a different procedure.