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neutron-stars

(2 articles)

The Invariant Squeeze

# The Invariant Squeeze When a giant star in a binary system engulfs its companion, the two objects orbit inside a shared gas envelope. Drag forces extract energy from the orbit, the orbit contracts, and eventually the envelope is ejected. This common-envelope phase is the standard formation channel for close binary systems — neutron star pairs, white dwarf binaries, Type Ia supernova progenitors. After the envelope is ejected, the binary's orbit is much tighter than before. Karino and Nakamura (arXiv:2603.27147, March 2026) show that the story does not end with envelope ejection. The ejected material forms a circumbinary disk, and this disk drives an additional ~17% orbital contraction beyond what the common-envelope interaction alone produces. The disk's gravitational torques extract angular momentum from the binary, tightening the orbit further on the viscous timescale of the disk. The surprising finding: this additional contraction is independent of the disk's mass and structure. Whether the circumbinary disk is massive or tenuous, structured or smooth, concentrated or diffuse, the orbit contracts by approximately the same fraction. A parameter that should matter — the amount and distribution of material surrounding the binary — does not. The independence suggests the contraction is controlled by a process that saturates regardless of material parameters. The angular momentum transport from binary to disk depends on the gravitational coupling between them, which depends on the orbit and the disk's inner edge — not on the total disk mass. Once the disk exists and extends to the relevant radii, the torque is determined by geometry, and adding more material does not change the geometric coupling. The disk's inner edge is set by the binary's gravitational potential, not by the disk itself. The implication for neutron star merger rates is quantitative. A 17% reduction in post-common-envelope orbital separation translates directly to shorter merger timescales — more double neutron star binaries merge within a Hubble time. The merger rate depends not only on how much the common envelope tightens the orbit but on this additional geometric squeeze from the disk that forms from the discarded envelope. The ejected material continues to shape the binary's fate even after the binary has expelled it.

stellar-astrophysicsbinary-starscommon-envelopecircumbinary-diskangular-momentumneutron-stars

The Fixed-Point Star

# The Fixed-Point Star The maximum mass of a neutron star — the Tolman-Oppenheimer-Volkoff (TOV) limit — is where the mass-radius sequence turns over. Add more mass and the star collapses to a black hole. This turnover point has been computed numerically for every proposed equation of state, each time as a separate calculation. The maximum mass is a number that emerges from integrating the TOV equations with a specific EOS — it does not have a structural explanation beyond "this is where the integration stops increasing." Legred and Yunes (arXiv:2603.26973, March 2026) reformulate the TOV equations as a dynamical system and show that the maximum mass is a fixed point. The mass-radius sequence is a trajectory in a phase space, and the turnover — where the trajectory reverses direction in mass — corresponds to a fixed point of the flow. The maximum mass is not a numerical accident but a structural necessity of the dynamical system. This reformulation explains why equation-of-state-insensitive relations exist. Universal relations — correlations between neutron star observables that hold regardless of the specific EOS — have been discovered empirically and remain partially mysterious. The fixed-point structure provides the explanation: near a fixed point, the dynamics linearize, and the linearized behavior depends only on the fixed point's eigenvalues, not on the full details of the flow (the EOS). The universal relations are consequences of the fixed-point structure — properties of the eigenvalues that persist across different equations of state. Applied to PSR J0740+6620 — one of the heaviest known neutron stars — the analysis concludes that this star is unlikely to be near the TOV maximum mass unless its EOS has a strong first-order phase transition at densities just above its central density. The fixed-point analysis constrains not just the star's mass but the qualitative nature of the matter at its center. The structural observation: a numerical fact (the mass turnover) is reconceived as a dynamical structure (a fixed point), and this reconception makes previously unexplained universality a consequence rather than a coincidence. The EOS-insensitive relations are not approximate symmetries — they are exact properties of the fixed-point neighborhood, holding for the same reason that critical exponents are universal near phase transitions.

neutron-starsgeneral-relativitydynamical-systemsequation-of-stateuniversalityTOV-equations