Quark Matter Structure in Neutron Stars

For physically reasonable bulk and surface properties, quark matter has recently been found to coexist with nuclear matter inside neutron stars in a uniform background of electrons. The microstructure of this mixed phase starts out with a few quark matter droplets embedded in the nuclear matter but as the density of droplets increase towards the center of the neutron star they merge into rods, plates and other structures. The topology, size as well as Coulomb and surface energies of these structures depend sensitively on the quark/nuclear matter interface tension. A major fraction of the interior of neutron stars could consist of matter in this new phase, which would provide new mechanisms for glitches, cooling and supernovae. 1. The Structure of Quark Matter in Neutron Stars Over the past two decades many authors have considered the existence of quark matter in neutron stars. Assuming a first order phase transition one has, depending on the equation of states, found either complete strange quark matter stars or neutron stars with a core of quark matter surrounded by a mantle of nuclear matter and a crust on top. Recently, the possibility of a mixed phase of quark and nuclear matter was considered and found to be energetically favorable. Including surface and Coulomb energies this mixed phase was still found to be favored for reasonable bulk and interface properties. The structure of the mixed phase of quark matter embedded in nuclear matter with a uniform background of electrons was studied and resembles that in the neutron drip region in the crust. The resulting picture of a neutron stars is shown in Fig. 1. Starting from the outside, the crust consists of the outer layer, which is a dense solid of neutron rich nuclei, and the inner layer in which neutrons have dripped and form a neutron gas coexisting with the nuclei. The structure of the latter mixed phase has recently been calculated in detail 4 and is found to exhibit rod-, plateand bubble-like structures. At nuclear saturation density n0 ≃ 0.16fm −3 there is only one phase of uniform nuclear matter consisting of mainly neutrons, a small fraction of protons and the same amount of electrons to achieve charge neutrality. A mixed phase of quark matter (QM) and nuclear matter (NM) appear already around a few times nuclear saturation density lower than the phase transition in hybrid stars. In the beginning only few droplets of quark matter appear but at higher densities their number increase and they merge into: QM rods, QM plates, NM rods, NM bubbles, and finally pure QM at very high densities if the neutron stars have not become unstable towards gravitational collapse. Talk given at Intl. Symposium on Strangeness and Quark Matter, Chania, Greece, sept. 1-5 1994, org. A.D.Panagiotou and G.Vassiliadis, proc. to appear in World Scientific.