Gravitational radiation‐reaction driven instabilities in rotating neutron stars

LIN, ZIKUN; BRATTON, ERIC; FRIDOLIN WEBER; MILVA G. ORSARIA; IGNACIO FRANCISCO RANEA SANDOVAL

Conferencia; IWARA 2020 Video Conference; 2020

We investigate in this work two different types of instabilities that set limits on the rotation rates of neutron (compact) stars. The first one is that caused by rotation at the Kepler frequency, at which mass shedding at the star´s equator sets in. The second limit is set by instabilities driven by the growth of gravitational radiation-reaction (GRR) driven (Formula presented.) -modes of order (Formula presented.) (Formula presented.), which are moderated by shear and bulk viscosity. The calculations are performed for two relativistic models for the nuclear equation of state, DD2 and ACB4. The latter accounts for a phase transition that gives rise to the existence of so-called mass-twin compact stars. Our results confirm that the stable rotation periods of cold neutron stars are determined by the (Formula presented.) modes and that these modes are excited at rotation periods between 1 and 1.4 ms (20?30% above the Kepler periods of these stars). The situation is reversed in hot neutron stars where bulk viscosity damps the GRR modes, pushing the excitation period of the (Formula presented.) -mode instability to values below the Kepler period. For cold mass-twin compact stars, we find that the (Formula presented.) instability sets in at rotation periods between 0.8 and 1 ms (25?30% below the Kepler period). This feature may allow one to distinguish conventional neutron stars from their possibly existing mass-twin counterparts observationally, provided the (Formula presented.) -mode instability, which is expected to compete with the (Formula presented.) -mode instability, sets the limit on stable rotation of compact stars.