# Compact stars with sequential QCD phase transitions

## TL;DR

This paper investigates models of compact stars with two sequential first-order QCD phase transitions, revealing the possibility of multiple stable star branches, including twin and triplet configurations with identical masses but different radii.

## Contribution

It introduces a novel model of compact stars with two distinct first-order phase transitions, leading to multiple stable star families and potential twin or triplet configurations.

## Key findings

- Existence of two separate hybrid star branches due to sequential phase transitions.
- Potential for twin stars with identical masses but different radii.
- Discovery of triplet star configurations with distinct internal phases.

## Abstract

Compact stars may contain quark matter in their interiors at densities exceeding several times the nuclear saturation density. We explore models of such compact stars where there are two first-order phase transitions: the first from nuclear matter to a quark-matter phase, followed at higher density by another first-order transition to a different quark matter phase [e.g., from the two-flavor color superconducting (2SC) to the color-flavor-locked (CFL) phase). We show that this can give rise to two separate branches of hybrid stars, separated from each other and from the nuclear branch by instability regions and, therefore, to a new family of compact stars, denser than the ordinary hybrid stars. In a range of parameters, one may obtain twin hybrid stars (hybrid stars with the same masses but different radii) and even triplets where three stars, with inner cores of nuclear matter, 2SC matter, and CFL matter, respectively, all have the same mass but different radii.