The Birth Mass Function of Neutron Stars from Core-Collapse Supernova Simulations

Abstract

Neutron-star masses encode the physics of core-collapse supernovae and the sub- sequent binary evolution that may alter a neutron star after birth. In this work, I construct a theoretical neutron-star gravitational birth-mass function using core- collapse supernova simulation results and compare it to the observed neutron-star mass distribution. The model combines a progenitor-to-compactness mapping from two-dimensional FORNAX simulations with a compactness-to-gravitational-birth- mass mapping motivated by late-time three-dimensional FORNAX simulations. This produces a composite relation MZAMS → ξ1.75 → Mg,birth, which is then weighted by a Salpeter initial mass function to estimate the relative contribution of different progenitor intervals to the neutron-star birth distribution. I further examine the effects of removing black-hole-forming candidates and applying a simple period-dependent accretion correction to observed recycled systems. The resulting theoretical birth function is strongly concentrated in the low-mass neutron-star regime, with its largest contributions coming from the lowest-mass core- collapse progenitors, as expected from the steep Salpeter weighting. The model re- produces the qualitative structure of the low-mass observed populations, especially systems expected to remain close to their birth masses, but it underproduces the high-mass tail seen in some X-ray binaries, redbacks, and black widows. This dis- crepancy suggests that the observed high-mass regime cannot be interpreted as a direct, unmodified birth distribution. Instead, it likely reflects a combination of post-birth binary accretion, selection effects, black-hole formation boundaries, and potentially distinct evolutionary channels. These results support a picture in which the neutron-star birth-mass function is intrinsically weighted toward low masses, while the present-day observed distribution is broadened by subsequent binary evo- lution and compact-remnant survival effects.