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

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 $M=0.77_{-0.17}^{+0.20}{M}_{\odot }$ and a compact radius of $R=10.4_{-0.78}^{+0.86}$ km. We show that none of the sampled nuclear models meet all constraints at the $68\mathrm{\%}$ 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.