Cooling of hypernuclear compact stars

TL;DR

This paper investigates the thermal evolution and cooling mechanisms of hypernuclear compact stars, emphasizing the role of hyperonic pairing and direct Urca processes in stars of various masses.

Contribution

It provides a detailed analysis of hyperonic pairing effects and identifies dominant neutrino cooling channels in hypernuclear stars with different masses.

Findings

Hyperonic pairing suppresses certain neutrino emission processes.

Intermediate-mass stars match observed surface temperatures.

Massive stars can cool rapidly via direct Urca processes.

Abstract

We study the thermal evolution of hypernuclear compact stars constructed from covariant density functional theory of hypernuclear matter and parameterizations which produce sequences of stars containing two-solar-mass objects. For the input in the simulations, we solve the Bardeen-Cooper-Schrieffer gap equations in the hyperonic sector and obtain the gaps in the spectra of $\Lambda$, $\Xi^0$ and $\Xi^-$ hyperons. For the models with masses $M/M_{\odot} \ge 1.5$ the neutrino cooling is dominated by hyperonic direct Urca processes in general. In the low-mass stars the $(\Lambda p)$ plus leptons channel is the dominant direct Urca process, whereas for more massive stars the purely hyperonic channels $(\Sigma^-\Lambda)$ and $(\Xi^-\Lambda)$ are dominant. Hyperonic pairing strongly suppresses the processes on $\Xi^-$s and to a lesser degree on $\Lambda$s. We find that intermediate-mass $1.5 \le M/M_{\odot} \le 1.8$ models have surface temperatures which lie within the range inferred from thermally emitting neutron stars, if the hyperonic pairing is taken into account. Most massive models with $M/M_{\odot} \simeq 2$ may cool very fast via the direct Urca process through the $(\Lambda p)$ channel because they develop inner cores where the $S$-wave pairing of $\Lambda$s and proton is absent.