Upper limits on the observational effects of nuclear pasta in neutron stars

Abstract

The effects of the existence of exotic nuclear shapes at the bottom of the\nneutron star inner crust - nuclear `pasta' - on observational phenomena are\nestimated by comparing the limiting cases that those phases have a vanishing\nshear modulus and that they have the shear modulus of a crystalline solid . We\nestimate the effect on torsional crustal vibrations and on the maximum\nquadrupole ellipticity sustainable by the crust. The crust composition and\ntransition densities are calculated consistently with the global properties,\nusing a liquid drop model with a bulk nuclear equation of state (EoS) which\nallows a systematic variation of the nuclear symmetry energy. The symmetry\nenergy J and its density dependence L at nuclear saturation density are the\ndominant nuclear inputs which determine the thickness of the crust, the range\nof densities at which pasta might appear, as well as global properties such as\nthe radius and moment of inertia. We show the importance of calculating the\nglobal neutron star properties on the same footing as the crust EoS, and\ndemonstrate that in the range of experimentally acceptable values of L, the\npasta phase can alter the crust frequencies by up to a factor of three,\nexceeding the effects of superfluidity on the crust modes, and decrease the\nmaximum quadrupole ellipticity sustainable by the crust by up to an order of\nmagnitude. The signature of the pasta phases and the density dependence of the\nsymmetry energy on the potential observables highlights the possibility of\nconstraining the EoS of dense, neutron-rich matter and the properties of the\npasta phases using astrophysical observations.\n