# Astrophysical aspects of general relativistic mass twin stars

## TL;DR

This paper explores the astrophysical implications of mass twin stars, which are compact stars with similar masses but different radii due to phase transitions, using an effective polytropic EoS model.

## Contribution

It introduces a polytropic EoS model to study mass twins and analyzes their properties in both slow rotation and full general relativity frameworks.

## Key findings

- Mass twins can exist with a strong first-order phase transition.
- A third stable branch appears in the mass-radius diagram.
- Constraints on maximum mass from GW170817 are discussed.

## Abstract

In this chapter we will introduce an effective equation of state (EoS) model based on polytropes that serves to study the so called "mass twins" scenario, where two compact stars have approximately the same mass but (significant for observation) quite different radii. Stellar mass twin configurations are obtained if a strong first-order phase transition occurs in the interior of a compact star. In the mass-radius diagram of compact stars, this will lead to a third branch of gravitationally stable stars with features that are very distinctive from those of white dwarfs and neutron stars. We discuss rotating hybrid star sequences in the slow rotation approximation and in full general relativity and draw conclusions for an upper limit on the maximum mass of nonrotating compact stars that has recently be deduced from the observation of the merger event GW170817.