Light clusters as a possible source of crustal impurities: A quasi-particle approach

Context. The presence of impurities in the neutron star crust is known to affect, in an important way, the thermal and electrical conductivity of the star. Aims. In this work we explore the possibility that such impurities arise from the simultaneous presence of heavy ions and hydrogen and helium isotopes formed during the process of (proto-)neutron-star formation. Methods. We considered an equilibrium population of such light particles at temperatures close to the crystallization of the crust within an effective quasi-particle approach. We included in-medium binding energy shifts and used different versions of the relativistic mean-field approach for the crustal modeling. Thermal effects were also consistently included in the dominant ion species that is present in each crustal layer and that we described in the compressible liquid drop approximation. Results. We find that the impurity factor associated with light clusters is very small and can be neglected in transport calculations, even if some model dependence is observed.