Rapid cooling of the compact star in Cassiopea A as a phase transition in dense QCD

We present a model of the compact star in Cassiopea A that accounts for its unusually fast cooling behavior. This feature is interpreted as an enhancement in the neutrino emission triggered by a transition from a fully gapped, two-flavor, color-superconducting phase to a crystalline phase or an alternative gapless, color-superconducting phase. By fine-tuning a single parameter -- the temperature of this transition -- a specific cooling scenario can be selected that fits the Cas A data. Such a scenario requires a massive $M\sim 2M_{\odot}$ star and is, therefore, distinctive from models invoking canonical 1.4 $M_{\odot}$ mass star with nucleonic pairing alone.