Baryonic models of ultra-low-mass compact stars for the central compact object in HESS J1731-347

TL;DR

This paper investigates baryonic dense matter models to explain the unusually low mass and small radius of the central compact object in HESS J1731-347, considering nucleonic and heavy baryon compositions constrained by multi-messenger data.

Contribution

It demonstrates how different baryonic compositions and symmetry energy slopes can account for the low-mass compact star, extending universal relations to very low masses.

Findings

Purely nucleonic models require small symmetry energy slope L_sym.

Including heavy baryons allows higher L_sym with slightly reduced maximum mass.

Universal I-Love-Q relations hold for very low-mass configurations.

Abstract

The recent attempt on mass and radius inference of the central compact object within the supernova remnant HESS J1731-347 suggests for this object an unusually low mass of $M = 0.77^{+0.20}_{-0.17}\,M_{\odot}$ and a small radius of $R = 10.4^{+0.86}_{-0.78}$\,km. We explore the ways such a result can be accommodated within models of dense matter with heavy baryonic degrees of freedom which are constrained by the multi-messenger observations. We find that to do so using only purely nucleonic models, one needs to assume a rather small value of the slope of symmetry energy $L_{\rm sym}$. Once heavy baryons are included higher values of the slope $L_{\rm sym}$ become acceptable at the cost of a slightly reduced maximum mass of static configuration. These two scenarios are distinguished by the particle composition and will undergo different cooling scenarios. In addition, we show that the universalities of the $I$-Love-$Q$ relations for static configurations can be extended to very low masses without loss in their accuracy.