Nuclear and Hybrid Equations of State in Light of the Low-Mass Compact Star in HESS J1731-347

Abstract: We sample over $500$ relativistic mean-field theories constrained by chiral effective field theory and properties of isospin-symmetric nuclear matter and test them against known stellar structure constraints. This includes a recent mass and radius measurement of a compact object in supernova remnant HESS J1731-347, with an unusually low mass of $M=0.77^{+0.20}{-0.17}\,M{\odot}$ and a compact radius of $R=10.4^{+0.86}_{-0.78}$ km. We show that none of the sampled nuclear models meet all constraints at the $68\,\%$ credibility level, but that hybrid equations of state with a quark matter inner core and nuclear outer core easily can. This indicates a tension between astrophysical constraints and low-energy nuclear theory.