Exotic stars

During the discussion of the HR diagram in Section 4.2 it was noted that 92 per cent of all observed stars fall on the main sequence 7.5 per cent are white dwarf 

Most spectroscopic binaries have periods ranging from days to months and are separated by distances of order 1 AU. A consequence of knowing the period of the star and separation, measured optically, is the determination of the mass of the stars. Assuming that the two stars are in circular orbit, for the sake of simplicity, then a centripetal force is required to keep them moving in orbit. Gravity provides this attraction and the two forces must be balanced. The complete solution of this problem is hard and only a combined mass can be derived without knowing some information other than the period of rotation. 

All is not what it seems, however, because most binary stars have a very faint neighbour and so both the red- and blue-shifted lines are not observed and the spectroscopic motion of just the bright star is enough to make the period measurement. The faint binary partner will pass in front of the bright star during the period of rotation if the plane of the binary orbit is along the line of sight from the Earth. 

The result is an eclipsing phase when the dull star is in front of the bright star and an apparent variation in the intensity of the bright star. 

Variable stars are so called because the luminosity of the star varies intrinsically and is not due to the passage of an eclipsing star. One such class of stars is the 

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Although the elemental composition of the solar system is roughly similar to that of many other stars, in particular with respect to the relative abundances of the nongaseous elements, there are, in detail, compositional differences among stars and there are, in addition, truly exotic stars that make the term cosmic abundances of elements questionable. We will therefore use the term solar system abundances of the elements in this chapter. 

Such a diverse collection of exotic stars places severe demands on the theory of stellar evolution. In the chapter by Karakas in this volumn, the evolution of low-mass stars up to the asymptotic-giant branch has been discussed more... 

Primary cosmic rays (cr) emitted by stars bombard the ISM, including planets such as Earth. The particles are 84 per cent protons and 14 per cent helium nuclei stripped of both electrons called alpha particles the remaining 2 per cent are electrons, heavier nuclei and more exotic particles. 

Equations of state involving only nucleonic matter are consistent with all available data. Some hints of evidence for very compact stars have been proposed (Li et al. 1999), which could indicate strange matter, but these are very model-dependent at the present. Even so, exotic states such as quark matter or strange matter are not excluded. 

The commonly accepted pulsar model is a neutron star of about one solar mass and a radius of the order of ten kilometers. A neutron star consists of a crust, which is about 1 km thick, and a high-density core. In the crust free neutrons and electrons coexist with a lattice of nuclei. The star s core consists mainly of neutrons and a few percents of protons and electrons. The central part of the core may contain some exotic states of matter, such as quark matter or a pion condensate. Inner parts of a neutron star cool up to temperatures 108iT in a few days after the star is formed. These temperatures are less than the critical temperatures Tc for the superfluid phase transitions of neutrons and protons. Thus, the neutrons in the star s crust and the core from a superfluid, while the protons in the core form a superconductor. The rotation of a neutron superfluid is achieved by means of an array of quantized vortices, each carrying a quantum of vorticity... 

Neutron stars (NSs) are perhaps the most interesting astronomical objects from the physical point of view. They are associated with a variety of extreme phenomena and matter states for example, magnetic fields beyond the QED vacuum pair-creation limit, supranuclear densities, superfluidity, superconductivity, exotic condensates and deconfined quark matter, etc. 

It is quite likely to find dense quark matter inside compact stars like neutron stars. However, when we study the quark matter in compact stars, we need to take into account not only the charge and color neutrality of compact stars and but also the mass of the strange quark, which is not negligible at the intermediate density. By the neutrality condition and the strange quark mass, the quarks with different quantum numbers in general have different chemical potentials and different Fermi momenta. When the difference in the chemical potential becomes too large the Cooper-pairs breaks or other exotic phases like kaon condensation or crystalline phase is more preferred to the BCS phase. 

Although might be anything at rs from the matching conditions, it will be zero for all general (not very exotic) systems. Namely, P = 0 is expected at some low particle number density ns on the surface, which is generally much below Po = h/Rc. So motions in the hth dimension cease already somewhere in the interior of the star. 

The value of the compactification radius, Rc In the present approach this radius was a free parameter. For demonstration we chose the radius Rc = 0.33 10 13 cm, when the strange A baryon could behave as the first excitation of a neutron. Such an extradimensional object can mimics a compact star with neutrons in the mantle and A s in the core. With smaller Rc the exotic component appears at larger densities - we may run into the unstable region of the hybrid star and the extra dimension remains undetectable. However, with larger Rc the mass gap becomes smaller and the transition happens at familiar neutron star densities. In this way, reliable observations could lead to an upper bound on Rc. 

In a simplistic and conservative picture the core of a neutron star is modeled as a uniform fluid of neutron rich nuclear matter in equilibrium with respect to the weak interaction (/3-stable nuclear matter). However, due to the large value of the stellar central density and to the rapid increase of the nucleon chemical potentials with density, hyperons (A, E, E°, E+, E and E° particles) are expected to appear in the inner core of the star. Other exotic phases of hadronic matter such as a Bose-Einstein condensate of negative pion (7r ) or negative kaon (K ) could be present in the inner part of the star. 

The influence of the appearance of such exotic states like quarks in stellar matter is topic of the study of quasi-stationary simulations of the evolution of isolated compact stars [15, 12, 7, 23] and accreting systems, where one companion is a superdense compact object [9,27], In this work we investigate the observability of the hadron-quark deconfinement phase transition in the dynamical evolution of a neutron star merger. 

Even after several decades of research [BUR57] into the mechanisms by which the elements are synthesized in stars, it is still often true that the degree to which an astrophysical environment can be understood is limited by the degree to which the underlying microscopic input nuclear physics data have been measured and understood. As new and more exotic high-temperature astronomical environments have been discovered and modeled (and as observations and models for more familiar objects have been refined) the needs for more and better data for nuclei away from stability have increased. In this brief overview, we discuss a few of the explosive astrophysical environments which are currently of interest and some of their required input nuclear data. 

A variational theory which includes all these different contributions was recently proposed and applied for completely stretched polyelectrolyte stars (so-called porcupines ) [203, 204]. As a result, the effective interaction V(r) was very soft, mainly dominated by the entropy of the counterions inside the coronae of the stars supporting on old idea of Pincus [205]. If this pair potential is used as an input in a calculation of a solution of many stars, a freezing transition was found with a variety of different stable crystal lattices including exotic open lattices [206]. The method of effective interactions has the advantage to be generalizable to more complicated complexes which are discussed in this contribution-such as oppositely charged polyelectrolytes and polyelectrolyte-surfactant complexes-but this has still to be worked out in detail. 

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