Formation of Double Neutron Star Systems

Abstract: Double neutron star (DNS) systems represent extreme physical objects and the endpoint of an exotic journey of stellar evolution and binary interactions. Large numbers of DNS systems and their mergers are anticipated to be discovered using the Square-Kilometre-Array searching for radio pulsars and high-frequency gravitational wave detectors (LIGO/VIRGO), respectively. Here we discuss all key properties of DNS systems, as well as selection effects, and combine the latest observational data with new theoretical progress on various physical processes with the aim of advancing our knowledge on their formation. We examine key interactions of their progenitor systems and evaluate their accretion history during the high-mass X-ray binary stage, the common envelope phase and the subsequent Case BB mass transfer, and argue that the first-formed NSs have accreted at most $\sim 0.02\;M_{\odot}$. We investigate DNS masses, spins and velocities, and in particular correlations between spin period, orbital period and eccentricity. Numerous Monte Carlo simulations of the second supernova (SN) events are performed to extrapolate pre-SN stellar properties and probe the explosions. All known close-orbit DNS systems are consistent with ultra-stripped exploding stars. Although their resulting NS kicks are often small, we demonstrate a large spread in kick magnitudes which may, in general, depend on the past interaction history of the exploding star and thus correlate with the NS mass. We analyze and discuss NS kick directions based on our SN simulations. Finally, we discuss the terminal evolution of close-orbit DNS systems until they merge and possibly produce a short $\gamma$-ray burst.

• The paper presents a detailed model explaining how binary evolution and common envelope phases lead to the formation of double neutron star systems.
• It reveals that the first-born neutron star accretes less than 0.02 M⊙, resulting in a typical mass advantage of about 0.1 M⊙ over its companion.
• Monte Carlo simulations show that supernova kicks, generally under 50 km/s, play a key role in defining the final orbital and systemic properties of DNS systems.

Formation of Double Neutron Star Systems

The study of Double Neutron Star (DNS) systems is crucial for understanding a range of astrophysical phenomena, including supernova explosions, the formation of millisecond pulsars, and gravitational wave emission. This paper by Tauris et al. provides a comprehensive examination of the formation mechanisms and properties of DNS systems, integrating observational data with theoretical models to enhance our comprehension of their development.

Key Observations and Theoretical Considerations

The paper highlights that DNS systems primarily form through a complex sequence of binary stellar evolution processes. Initially, a massive binary system undergoes stages of mass transfer and common envelope (CE) evolution, wherein the hydrogen-rich envelope of a massive star is ejected. This process is critical for reducing the orbital separation to facilitate the eventual formation of a DNS.

DNS systems are often products of the evolution of HMXBs (High Mass X-ray Binaries), yet the transition from HMXBs to DNSs involves significant uncertainties, particularly concerning the CE phase's outcome. The study incorporates evidence from known observations indicating wide variations in the CE ejection efficiency, which heavily influences the final orbital configurations post-CE phase.

Acquired Mass and Neutron Star Mass Distribution

The research presented indicates that the first-formed neutron star (NS) in a DNS system typically accretes less than 0.02 $M_{\odot}$ during its lifetime. This modest accretion suggests that the observed NS masses in DNS systems closely reflect their birth masses. Notably, the data compiled in the paper shows a slight mass advantage for the first-born (recycled) NS over the second-born (young) NS, typically by around 0.1 $M_{\odot}$. This disparity is likely due to the deeper stripping of the progenitor star’s envelope in the second supernova (SN) explosion.

Kinematic Implications of Supernova Explosions

This work explores the kinematic effects of supernova explosions on DNS systems, emphasizing the kick velocities imparted on the newborn NSs. The paper illustrates that second SNe, often from ultra-stripped progenitors, usually result in smaller kick velocities (generally less than 50 km/s), albeit exceptions exist where large kicks have occurred, necessitating a more massive or less symmetric progenitor. Such kicks significantly affect the orbital parameters and systemic velocities of DNS systems, leading to various post-SN configurations.

Monte Carlo simulations play a crucial role in exploring the parameter space leading to observed DNS systems. These simulations assist in constraining pre-SN parameters, second SN characteristics, and resultant systemic velocities. An intriguing finding from these simulations is the apparent directional preference for supernova kicks, although this might be attributed to selection biases or input assumptions rather than an intrinsic asymmetry.

Future Implications and Observational Prospects

The future advancements in radio telescope arrays, particularly the SKA and FAST, are anticipated to increase the known population of DNS systems significantly. This expansion will refine our understanding of their formation pathways, spin properties, and kinematics. Moreover, impending developments in gravitational wave astronomy are expected to offer direct insights into DNS mergers, complementing optical and radio observations to provide a holistic view of DNS lifecycle and their role in enriching the interstellar medium with heavy elements like $r$-process elements.

In conclusion, this paper underscores the intricate nature of DNS formation, shaped by a tumultuous evolutionary path involving mass transfer events, supernova explosions, and accretion dynamics. The authors’ synthesis of theoretical and observational efforts sets a valuable foundation for forthcoming studies aimed at untangling the complexities inherent in the genesis and evolution of double neutron star systems.