Phases of Dense Matter in Compact Stars

Abstract

Formed in the aftermath of gravitational core-collapse supernova explosions, neutron stars are unique cosmic laboratories for probing the properties of matter under extreme conditions that cannot be reproduced in terrestrial laboratories. The interior of a neutron star, endowed with the highest magnetic fields known and with densities spanning about ten orders of magnitude from the surface to the centre, is predicted to exhibit various phases of dense strongly interacting matter, whose physics is reviewed in this chapter. The outer layers of a neutron star consist of a solid nuclear crust, permeated by a neutron ocean in its densest region, possibly on top of a nuclear "pasta" mantle. The properties of these layers and of the homogeneous isospin asymmetric nuclear matter beneath constituting the outer core may still be constrained by terrestrial experiments. The inner core of highly degenerate, strongly interacting matter poses a few puzzles and questions which are reviewed here together with perspectives for their resolution. Consequences of the dense-matter phases for observables such the neutron-star mass-radius relationship and the prospects to uncover their structure with modern observational programmes are touched upon.

Role of the crust in the tidal deformability of a neutron star within a unified treatment of dense matter

“…(with R the circumferential radius of the star, M its gravitational mass, and k 2 the second gravito-electric Love number, c the speed of light, G the gravitational constant), depends on its complex internal structure (see, e.g., Ref. [3] for a recent review about the different phases of dense matter in a NS). Beneath a very thin atmosphere and possibly a surface ocean, the interior of a NS consists of a solid crust on top of a liquid core.…”

Role of the symmetry energy and the neutron-matter stiffness on the tidal deformability of a neutron star with unified equations of state

“…The detection of the gravitational-wave signal GW170817 from the merger of two neutron stars (NSs) [1] and the subsequent observations of electromagnetic counterparts [2][3][4][5][6][7][8] offer new opportunities to probe the properties of matter under conditions so extreme that they cannot be experimentally reproduced (see, e.g., Ref. [9] for a recent review). Apart from estimates of the masses of the two inspiralling NSs, the analysis of this signal has also provided valuable information on their tidal deformations during the last orbits [10][11][12].…”

Maximum mass and universal relations of rotating relativistic hybrid hadron-quark stars

“…These stars are hybrid hadron-quark stars (or just hybrid stars), i.e., configurations with a quark core surrounded by a hadronic shell. 1 The third family has been studied for a long time [30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46], with several of these earlier works focusing on "twin stars", which are hybrid stars in the third family whose mass is in the same range as the mass of neutron stars, but have smaller radii. Interest in hybrid stars has recently increased due to GW170817, because GW170817 is also compatible with at least one component being a hybrid star as first pointed out in [47].…”