Baryon superfluidity and neutrino emissivity of neutron stars

For neutron stars with hyperon-mixed core, neutrino emissivity is studied under the equation of state, obtained by introducing repulsive three-body force universal for all baryons so as to assure the maximum mass compatible with the observation. By paying attention to the density-dependence of critical temperatures of baryon superfluids, which reflect the nature of baryon-baryon interaction and control neutron star cooling, we show what neutrino emission processes are efficient in the regions with and without hyperon mixing and remark its implications related to neutron star cooling.Comment: 5pages, 4 figure

Confronting Neutron Star Cooling Theories with New Observations

With the successful launch of Chandra and XMM/Newton X-ray space missions combined with the lower-energy band observations, we are in the position where careful comparison of neutron star cooling theories with observations will make it possible to distinguish among various competing theories. For instance, the latest theoretical and observational developments already exclude both nucleon and kaon direct URCA cooling. In this way we can now have realistic hope for determining various important properties, such as the composition, degree of superfluidity, the equation of state and steller radius. These developments should help us obtain better insight into the properties of dense matter.Comment: 11 pages, 1 figur

Neutrino emission due to Cooper pairing of protons in cooling neutron stars: Collective effects

The process of neutrino-pair radiation due to formation and breaking of Cooper pairs of protons in superconducting cores of neutron stars is considered with taking into account of the electromagnetic coupling of protons to ambient electrons. It is shown that plasma polarization strongly modifies the effective vector weak current of protons. Collective response of ambient electrons to the proton quantum transition contributes coherently to the complete interaction with the neutrino field and enhances the rate of neutrino-pair production by two orders of magnitude.Comment: 11 pages, 2 figure

3P2^3P_2-3F2^3F_2 Pairing in Dense Neutron Matter: The Spectrum of Solutions

The $^3P_2$-$^3F_2$ pairing model is generally considered to provide an adequate description of the superfluid states of neutron matter at densities some 2-3 times that of saturated symmetrical nuclear matter. The problem of solving the system of BCS gap equations expressing the $^3P_2$-$^3F_2$ model is attacked with the aid of the separation approach. This method, developed originally for quantitative study of S-wave pairing in the presence of strong short-range repulsions, serves effectively to reduce the coupled, singular, nonlinear BCS integral equations to a set of coupled algebraic equations. For the first time, sufficient precision becomes accessible to resolve small energy splittings between the different pairing states. Adopting a perturbative strategy, we are able to identify and characterize the full repertoire of real solutions of the $^3P_2$-$^3F_2$ pairing model, in the limiting regime of small tensor-coupling strength. The P-F channel coupling is seen to lift the striking parametric degeneracies revealed by a earlier separation treatment of the pure, uncoupled $^3P_2$ pairing problem. Remarkably, incisive and robust results are obtained solely on the basis of analytic arguments. Unlike the traditional Ginzburg-Landau approach, the analysis is not restricted to the immediate vicinity of the critical temperature, but is equally reliable at zero temperature. Interesting connections and contrasts are drawn between triplet pairing in dense neutron matter and triplet pairing in liquid $^3$He.Comment: 23 pages, 1 figur

Massive Hybrid Stars with Strangeness

How massive the hybrid stars could be is discussed by a "3-window model" proposed from a new strategy to construct the equation of state with hadron-quark transition. It is found that hybrid stars have a strong potentiality to generate a large mass compatible with two-solar-mass neutron star observations.Comment: 4 pages,1 figur

Hyperon Puzzle, Hadron-Quark Crossover and Massive Neutron Stars

Bulk properties of cold and hot neutron stars (NSs) are studied on the basis of the hadron-quark crossover picture where a smooth transition from the hadronic phase to the quark phase takes place at finite baryon density. By using a phenomenological equation of state (EOS) "CRover" which interpolates the two phases at around 3 times the nuclear matter density, it is found that the cold NSs with the gravitational mass larger than 2-solarmass can be sustained. This is in sharp contrast to the case of the first-order hadron-quark transition. The radii of the cold NSs with the CRover EOS are in the narrow range which is insensitive to the NS masses. Due to the stiffening of the EOS induced by the hadron-quark crossover, the central density of the NSs is at most 4 times the nuclear matter density and the hyperon-mixing barely occurs inside the NS core. This constitutes a solution of the long-standing hyperon puzzle. The effect of color superconductivity (CSC) on the NS structures is also examined with the hadron-quark crossover. For the typical strength of the diquark attraction, a slight softening of the EOS due to two-flavor CSC (2SC) takes place and the maximum mass is reduced by about 0.2-solarmass. The CRover EOS is generalized to the supernova matter at finite temperature to describe the hot NSs at birth. The hadron-quark crossover is found to decrease the central temperature of the hot NSs under isentropic condition. The gravitational energy release and the spin-up rate during the contraction from the hot NS to the cold NS are also estimated.Comment: 15 pages, 13 figures; prepared for the 2015 EPJA Topical Issue on "Exotic Matter in Neutron Stars

The importance of neighbourhood size in self organising systems

In recent times, the analysis of SOM (self-organising map) performance has concentrated on optimising the gain decay, rather than the size, form and decay of the neighbourhood function. We propose that the size, form and decay of region size plays a much more significant role in the learning, and especially in the development, of topographic feature maps. In this paper, a biologically-derived SOM model is presented. This model is able to select a single winning neuron and to form Gaussian outputs about this winner, without the need for a meta-level decision-making structure to artificially select a winner and fit a Gaussian output to that winner. Using this model, some fundamental characteristics of the relationship between neighbourhood size and SOM output states are demonstrated.<br /