$f$ and $p$ mode oscillation of proto-neutron stars with systematic variation of the nucleon effective mass

TL;DR

This study models proto-neutron stars with varying entropy and nucleon effective mass to understand how these factors influence oscillation frequencies, aiding gravitational wave detection and stellar evolution insights.

Contribution

It systematically investigates the impact of nucleon effective mass and entropy on PNS equations of state and oscillation modes using relativistic formalism, extending understanding of stellar evolution stages.

Findings

Thermal effects lower oscillation frequencies, easing GW detection.

Lower effective mass further reduces oscillation frequencies.

Universal relations between frequencies, compactness, and deformability are maintained across parameters.

Abstract

We develop equation of state (EoS) of proto-neutron stars (PNSs) at various stages of evolution by varying entropy per baryon $S$, using the Korea-IBS-Daegu-SKKU density functional model. With finite values for both temperature and density, we systematically investigate the influence of nucleon effective mass on EoS of PNSs, for different values of isoscalar effective mass $\mu_S^*$. For high entropy values, we aim to replicate conditions of failed core-collapse supernovae forming black holes. At each stage of evolution, structural and non-radial oscillation (fundamental $f$-mode and first pressure $p_1$-mode) properties are computed under isentropic conditions by varying $\mu_S^*$. We focus on the effects of $S$ and $\mu_S^*$ on oscillation frequencies $f_f$ and $f_{p_1}$ adopting complete general relativistic formalism and Cowling approximation. Thermal effects reduce the values of $f_f$ and $f_{p_1}$ of PNSs compared to those of cold NSs, consequently detection of the former gets facilitated. For high-mass PNSs, this reduction is more pronounced for $f_{p_1}$ than $f_f$. Moreover, lower values of $\mu_S^*$ reduce $f_f$ and $f_{p_1}$ further. Universality of mass-scaled angular frequency ($\omega_fM$) with compactness ($C$) and tidal deformability ($\Lambda$) are obtained as non-linear fits that shift upwards (downwards) in $\omega_fM-C$ ($\omega_fM-\Lambda$) plane for increasing values of $S$. For fixed $S$, the universality is also retained for variation of $\mu_S^*$. $S$ shows stronger correlation than $\mu_S^*$ with structural and oscillation properties of (P)NSs. Strength of correlation of $S$ is more prominent with $f_{p_1}$ than $f_f$ while the trend is opposite for $\mu_S^*$. These findings suggest that detection of oscillation frequencies by upcoming GW detectors, could potentially indicate the evolutionary stage of a star during its transition from supernova to cold NS.