Discriminating hadronic and quark stars through gravitational waves of fluid pulsation modes

One of the key aspects in the comprehension of neutron star (NS) interiors is the identification of observables that may impose constraints on the equation of state (EoS). At present, limits are obtained mainly through the study of the mass–radius relationship, the maximum rotational frequency and the cooling behaviour. However, since gravitational wave (GW) observatories such as Advanced LIGO and Advanced VIRGO will open a new window of the observation of NSs in the very near future, identifying observables that may emerge from the analysis of the GW emission of NSs is crucial. To this end, we investigate non-radial oscillations of hadronic, hybrid and pure self-bound strange quark stars with maximum masses above the mass of the recently observed massive pulsars PSR J1614–2230 and PSR J0348–0432 with M ≈ 2 M⊙. For the hadronic EoS we employ different parametrizations of a relativistic mean-field model and the EoS of Akmal, Pandharipande and Ravenhall. For quark matter we use the Massachusetts Institute of Technology bag model, including the effect of strong interactions and colour superconductivity. We find that the first pressure mode for strange quark stars has a very different shape than for hadronic and hybrid stars. For strange quarks stars, the frequency of the p1 mode is larger than ∼ 7 kHz and diverges at small stellar masses, but for hadronic and hybrid stars it is in the range ∼ 4–7 kHz. This allows an observational identification of strange stars even if extra information such as the mass, the radius or the gravitational redshift of the object is unavailable or uncertain. Also, we find as in previous works that the frequency of the g-mode associated with the quark–hadron discontinuity in a hybrid star is in the range 0.4–1 kHz for all masses. Thus, compact objects emitting GWs above 7 kHz should be interpreted as strange quark stars, and those emitting a signal within 0.4–1 kHz should be interpreted as hybrid stars. (paper)