Oscillations of superfluid hyperon stars: decoupling scheme and g-modes

Abstract

We analyse the oscillations of general relativistic superfluid hyperon stars, following the approach suggested by Gusakov & Kantor and Gusakov et al. and generalizing it to the nucleon-hyperon matter. We show that the equations governing the oscillations can be split into two weakly coupled systems with the coupling parameters s e , s µ , and s str . The approximation s e = s µ = s str = 0 (decoupling approximation) allows one to drastically simplify the calculations of stellar oscillation spectra. An efficiency of the presented scheme is illustrated by the calculation of sound speeds in the nucleonhyperon matter composed of neutrons (n), protons (p), electrons (e), muons (µ), as well as Λ, Ξ − , and Ξ 0 -hyperons. However, the gravity oscillation modes (g-modes) cannot be treated within this approach, and we discuss them separately. For the first time we study the composition g-modes in superfluid hyperon stars with the npeµΛ core and show that there are two types of g-modes ('muonic' and 'Λ-hyperonic') in such stars. We also calculate the g-mode spectrum and find out that the eigenfrequencies ν of the superfluid g-modes can be exceptionally large (up to ν ≈ 742 Hz for a considered stellar model).

Resonant tidal excitation of superfluid neutron stars in coalescing binaries

“…Our analysis did not account for hyperons, which are expected to appear at high core densities (∼ 7 × 10 14 g cm −3 ; see, e.g., Bednarek et al 2012;Weissenborn et al 2012;Gusakov et al 2014). As Dommes & Gusakov (2016) point out, gradients in the hyperon fraction might also be a source of buoyancy in superfluid NSs. While the direct Urca process involving hyperons (see review by Yakovlev et al 2001) may be fast enough compared to the g mode oscillation period to break the assumption of frozen composition, and/or the hyperons may be superfluid themselves (Takatsuka et al 2006;Wang & Shen 2010), the case studied by Dommes & Gusakov (2016) nonetheless shows that there can exist additional g modes in hyperonic NSs.…”

Gravitational Waves from Single Neutron Stars: An Advanced Detector Era Survey

“…The stationary phase approximation impliesh(f ) 2 = h(t) 2 |dt/df |, where the gravitational wave frequency evolution, df/dt, is derived directly from equation (1). Equation (20) assumes an optimal matched filter, which is most likely not feasible for the detection of such long-lived (i.e. 10 s) transient signals.…”

Dynamical tides in coalescing superfluid neutron star binaries with hyperon cores and their detectability with third-generation gravitational-wave detectors

“…The conditions for chemical equilibrium due to weak interactions are (Dommes & Gusakov 2016), Both models have a muonic contribution that peaks in the outer core. The HS has an additional contribution due to hyperons that peaks in the inner core.…”

“…We partially assess the impact of this hybrid approach by redoing the calculation of the g-modes using the GR oscillation equations. For this calculation, we ignore the gravitational perturbations (i.e., we adopt the Cowling approximation) and solve the superfluid GR oscillation equations (see, e.g., Dommes & Gusakov 2016;Passamonti et al 2016). We find that our hybrid approach overestimates the g-mode eigenfrequencies by 70% (see grey lines in Fig.…”

“…The conditions under which Λ-hyperons become superfluid in NS cores are uncertain(Takatsuka et al 2006;Wang & Shen 2010). We assume that they are normal fluid in this study, similar to the g-mode calculations inDommes & Gusakov (2016).…”