Radial oscillations and gravitational wave echoes of strange stars with nonvanishing lambda

Abstract: We study the effect of the cosmological constant on radial oscillations and gravitational wave echoes (GWEs) of non-rotating strange stars. To depict strange star configurations we used two forms of equations of state (EoSs), viz., the MIT Bag model EoS and the linear EoS. By taking a range of positive and negative values of cosmological constant, the corresponding mass-radius relationships for these stars have been calculated. For this purpose, first we solved the Tolman-Oppenheimer-Volkoff (TOV) equations with a non-zero cosmological constant and then we solved the pressure and radial perturbation equations arising due to radial oscillations. The eigenfrequencies of the fundamental $f$-mode and first 22 pressure $p$-modes are calculated for each of these EoSs. Again considering the remnant of the GW170817 event as a strange star, the echo frequencies emitted by such stars in presence of the cosmological constant are computed. From these numerical calculations, we have inferred relations between cosmological constant and mode frequency, structural parameters, GWE frequencies of strange stars. Our results show that for strange stars, the effective range of cosmological constant is $10^{-15}\, \mbox{cm}^{-2} \le\,\Lambda\, \le 3\times 10^{-13}\, \mbox{cm}^{-2}$.