[Paper Review] Neutron Star Formation and Birth Properties

Hans‐Thomas Janka|arXiv (Cornell University)|Feb 9, 2004

Pulsars and Gravitational Waves Research4 citations

TL;DR

This paper reviews theoretical models of neutron star formation, focusing on birth properties such as mass, spin, magnetic fields, and space velocities. It uses hydrodynamic simulations with neutrino diffusion to show that convective motions and asymmetric neutrino emission can explain pulsar kicks, with simulations indicating up to 500 km/s recoil velocities from hydrodynamic instabilities in the post-bounce phase.

ABSTRACT

Our current knowledge of neutron star formation, progenitors, and natal masses, spins, magnetic fields, and space velocities is briefly reviewed from a theorist's perspective. More observational information is badly needed to constrain theoretical possibilities.

Motivation & Objective

• To synthesize current theoretical understanding of neutron star formation and birth properties from a theorist's perspective.
• To identify key observational gaps in progenitor stars, natal masses, spins, magnetic fields, and space velocities.
• To explore mechanisms behind pulsar kicks, particularly asymmetric supernova explosions and anisotropic neutrino emission.
• To assess the role of hydrodynamic instabilities and convective motions in shaping neutron star dynamics during early evolution.
• To evaluate the potential for future detection of neutrinos and gravitational waves from neutron star birth.

Proposed method

• Hydrodynamic simulations with neutrino diffusion are used to model the post-bounce evolution of nascent neutron stars.
• Two-dimensional simulations track convection, angular momentum transport, and lepton fraction gradients in rotating proto-neutron stars.
• The simulations include energy and momentum transfer from asymmetric shock expansion and neutrino emission to infer neutron star recoil velocities.
• Analytical models based on angular momentum conservation and neutrino emission are used to estimate spin-down and velocity evolution.
• Theoretical scaling relations are derived to link initial conditions to final neutron star properties, such as $ J_{\text{ns}}^{\text{f}}/J_{\text{ns}}^{\text{i}} = (M_{\text{ns}}^{\text{f}}/M_{\text{ns}}^{\text{i}})^{qf(\lambda)/\kappa_n} $.
• Simulations of shock evolution over one second post-bounce are used to correlate explosion asymmetries with pulsar kick velocities.

Experimental results

Research questions

• RQ1 What mechanisms can produce the observed pulsar space velocities of 200–500 km/s, with some exceeding 1000 km/s?
• RQ2 How do hydrodynamic instabilities in the post-bounce supernova shock contribute to asymmetric explosion and neutron star recoil?
• RQ3 To what extent can neutrino emission anisotropy account for pulsar kicks, and what physical conditions are required?
• RQ4 How do convective motions and angular momentum transport affect the spin and structure of nascent neutron stars?
• RQ5 What constraints do observed neutron star birth properties place on the equation of state and initial conditions of core-collapse supernovae?

Key findings

• Hydrodynamic instabilities in the neutrino-heating region can produce global asymmetries in supernova explosions, transferring impulses of over 500 km/s to the neutron star within one second of core bounce.
• Simulations show that asymmetric shock expansion leads to net recoil velocities of up to 380 km/s, consistent with observed pulsar speeds.
• Neutrino-driven convection in proto-neutron stars develops strongest at intermediate radii and is suppressed near the rotation axis due to steep gradients in specific angular momentum.
• Neutrino emission can remove at most 43% of the total angular momentum, reducing specific angular momentum by about 30% during cooling and contraction.
• Anisotropic neutrino emission with only 3% asymmetry could produce kicks of ~1000 km/s, but achieving such asymmetries requires extreme magnetic fields (~10^16 G) or non-standard neutrino properties.
• Theoretical models suggest that both explosion asymmetries and neutrino emission anisotropy may contribute to pulsar kicks, with potential observational signatures in neutrino and gravitational wave signals.