Neutron stars constraints

Figure 8. Bag pressure Bs for different formfactors of the quark interaction in dependence on the baryon chemical potential for normal (s = N) and superconducting (s = S, G2 = G1) quark matter under neutron star constraints.

Abstract From the earliest measurements of the masses of binary pulsars, observations of neutron stars have placed interesting constraints on the properties of high-density matter. The last few years have seen a number of observational developments that could place strong new restrictions on the equilibrium state of cold matter at supranuclear densities. We review these astronomical constraints and their context, and speculate on future prospects. 

However, it is impossible to isolate the matter in the core of a neutron star for detailed study. It is thus necessary to identify observable aspects of neutron stars that can be, in some sense, mapped to the equation of state of high-density material. In this review we discuss various constraints on the equation of state from astronomical observations. We focus on observations of accreting binary systems. 

In just the last year, several observations have allowed new constraints on neutron star structure (1) a mass of M > 1.6 M (at >95% confidence) has been measured for a neutron star (Nice et al. 2003) (2) the first surface redshift, 2 = 0.35, has been detected from a neutron star (Cottam et al. 2002), and (3) the first non-sinusoidal light curve has been measured from an accreting millisecond neutron star (Strohmayer et al 2003). These observations, along with many previously available data, hold out good hope for strong constraints on high-density matter in the next few years. 

Abstract The hadronic equation of state for a neutron star is discussed with a particular emphasis on the symmetry energy. The results of several microscopic approaches are compared and also a new calculation in terms of the self-consistent Green function method is presented. In addition possible constraints on the symmetry energy coming from empirical information on the neutron skin of finite nuclei are considered. 

An accurate measure of the radius and the mass of an individual neutron star will be of fundamental importance to discriminate between different models for the equation of state of dense hadronic matter. Unfortunately such a crucial information is still not available. A decisive step in such a direction has been done thanks to the instruments on board of the last generation of X-ray satellites. These are providing a large amount of fresh and accurate observational data, which are giving us the possibility to extract very tight constraints on the radius and the mass for some compact stars. 

As Baron et al. pointed out there is a constraint on the EOS from the observations of neutron-star gravitational masses to be about 1.5 M0. There is a maximum for the neutron-star mass for a given EOS and this value increases monotonically with the stifness of the EOS. They summarized their results in the following formula ... 

Baryonic candidates were traditionally brown dwarfs, old white dwarfs, neutron stars, and stellar mass black holes. Both they and the new ones are subject to the big bang nucleosynthesis constraint above others might accrete matter, thus radiating, absorb at some wavelengths, or otherwise reveal themselves. 

The chemical analysis has revealed that rather low C/O ratios are found in metal-poor extragalactic carbon stars, as found for galactic carbon stars of the solar vicinity. Furthermore, the three analyzed stars show similar s-elements enhancements [ls/Fe]=0.8-1.3 and [hs/Fe]=l.l-1.7. This leads to new constraints for evolutionary models. For instance, the derived C/O and 13C/12C ratios are lower than model predictions at low metallicity. On the contrary, theoretical predictions of neutrons exposures for the production of the s-elements are compatible with observations (see Fig. 1). Finally, from their known distances, we have estimated the luminosities and masses of the three stars. It results that SMC-B30 and Sgr-C3 are most probably intrinsic carbon stars while Sgr-Cl could be extrinsic. 

The maximum 13C abundance and its distribution in the 3 M model is, at first, similar to that found due to diffusive convective overshooting by Herwig et al (1997). However, the rotational mixing spreads the 13C peak out before the neutrons are produced (cf. Fig. 13), which is not the case in the models of Herwig et al (1997). At the present time one can not discriminate which of these scenarios would agree better with empirical constraints. However, we want to stress that both mechanisms of 13C production, rotation and overshooting, do not exclude each other, and that it is possible that they act simultaneously in AGB stars. 

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