Cooling compact stars and phase transitions in dense QCD

TL;DR

This paper presents simulations of compact star cooling involving quark core phase transitions, explaining the Cas A data through a transition in dense QCD matter that affects neutrino emission and star temperature evolution.

Contribution

It introduces a model linking phase transitions in dense QCD matter to observed neutron star cooling, with updated fits to Cas A data and analysis of parameter sensitivities.

Findings

Cooling curves depend strongly on star mass.

Phase transition parameters critically influence fit quality.

Model successfully explains Cas A fast cooling data.

Abstract

We report new simulations of cooling of compact stars containing quark cores and updated fits to the Cas A fast cooling data. Our model is built on the assumption that the transient behaviour of the star in Cas A is due to a phase transition within the dense QCD matter in the core of the star. Specifically, the fast cooling is attributed to an enhancement in the neutrino emission triggered by a transition from a fully gapped, two-flavor, red-green color-superconducting quark condensate to a superconducting crystalline or an alternative gapless, color-superconducting phase. The blue colored condensate is modeled as a Bardeen-Cooper-Schrieffer (BCS)-type color superconductor with spin-one pairing order parameter. We study the sensitivity of the fits to the phase transition temperature, the pairing gap of blue quarks and the time-scale characterizing the phase transition (the latter modelled in terms of a width parameter). Relative variations in these parameter around their best fit values larger than $10^{-3}$ spoil the fit to the data. We confirm the previous finding that the cooling curves show significant variations as a function of compact star mass, which allows one to account for dispersion in the data on the surface temperatures of thermally emitting neutron stars.