Exotic Stable Branches with Efficient TOV Sequences

TL;DR

This paper introduces a new method for modeling neutron stars that efficiently identifies all stable configurations, revealing complex behaviors like multiple stable branches and implications for astrophysics.

Contribution

It presents termination conditions and algorithms for constructing stable neutron star models, uncovering multiple stable branches and effects of phase transitions on star stability.

Findings

EoS with increasing sound speed can produce multiple stable branches.

Large phase transitions can create stable branches at various mass scales.

Stable stars with masses greater than 2 solar masses are supported despite phase transitions.

Abstract

Modern inference schemes for the neutron star equation of state (EoS) require large numbers of stellar models constructed with different EoS, and these stellar models must capture all the behavior of stable stars. I introduce termination conditions for sequences of stellar models for cold, non-rotating neutron stars that can identify all stable stellar configurations up to arbitrarily large central pressures along with an efficient algorithm to build accurate interpolators for macroscopic properties. I explore the behavior of stars with both high- and low-central pressures. Interestingly, I find that EoS with monotonically increasing sound-speed can produce multiple stable branches (twin stars) and that large phase transitions at high densities can produce stable branches at nearly any mass scale, including sub-solar masses, while still supporting stars with $M > 2\,M_\odot$. I conclude with some speculation about the astrophysical implications of this behavior.