Ruling out nucleonic direct Urca cooling in low-mass neutron stars using nuclear data
Connecting nuclear experiments to neutron star composition requires controlling the extrapolation of experimental information beyond the densities probed in the laboratory. We combine a broad set of nuclear structure observables and INDRA-FAZIA isospin-transport data within a Bayesian inference of the dense matter equation of state (EoS). To limit uncontrolled extrapolation to high density, we employ a flexible asymptotically causal metamodel whose neutron star posteriors are comparable to those obtained with agnostic EoS models. The nuclear structure information is propagated through its full multidimensional distribution of correlated nuclear matter parameters, while the density-dependent INDRA-FAZIA likelihood is evaluated directly along each sampled metamodel symmetry energy curve. These two sources of laboratory information give compatible constraints on the symmetry energy, and their combination preserves agreement with finite nuclear observables. When combined with ab initio chiral effective field theory calculations and astrophysical observations, all these constraints substantially improve the determination of central proton fractions in low-mass neutron stars, while leaving larger uncertainties for massive stars. Within the adopted nucleonic metamodel, the reduced probability of large proton fractions strongly disfavors electronic direct-Urca onset at or below $1.4\,M_\odot$: its posterior probability falls from $\sim20\%$ with chiral and astrophysical constraints alone to below $1\%$ when the laboratory information is included.