Hybrid stars with sequential phase transitions: the emergence of the g$_2$ mode

Abstract

Neutron stars are the densest objects in the Universe, with M ∼ 1.4M and R ∼ 12 km, and the equation of state associated to their internal composition is still unknown. The extreme conditions to which matter is subjected inside neutron stars could lead to a phase transition in their inner cores, giving rise to a hybrid compact object. The observation of 2M binary pulsars (PSR J1614-2230, PSR J0343+0432 and PSR J0740+6620) strongly constraints theoretical models of the equation of state. Moreover, the detection of gravitational waves emitted during the binary neutron star merger, GW170817, and its electromagnetic counterpart, GRB170817A, impose additional constraints on the tidal deformability. In this work, we investigate hybrid stars with sequential phase transitions hadron-quark-quark in their cores. For the outer core, we use modern hadronic equations of state. For the inner core we employ the constant speed of sound parametrization for quark matter. We analyze more than 3000 hybrid equations of state, taking into account the recent observational constraints from neutron stars. The effects of hadron-quark-quark phase transitions on the normal oscillation modes f and g, are studied under the Cowling relativistic approximation. Our results show that a second quark-quark phase transition produces a noticeable change on the oscillation frequency for both f and g modes and, also, giving rise to a new g 2 mode. We discuss the observational implications of our results associated to the gravitational waves detection and the possibility of detecting hints of sequential phase transitions and the associated g 2 mode.

Investigating the I-Love-Q and w-mode Universal Relations Using Piecewise Polytropes

“…We also have 9,484 stars in our second-branch set. Although our methodology does not explicitly consider changes in composition, second branches arise in EoS that have combinations of polytropic indices that mimic phase transitions, that is, those having Γ i ≈ 0 in some density segment i (see [49,50] for the implications of multiple phase transitions).…”