Double star formation
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Double Star Formation: Key Processes and Mechanisms
Binary Evolution and Double Neutron Star Formation
Double neutron stars (DNSs) are formed through complex binary evolution processes that involve multiple stages, including mass transfer, supernova explosions, and common envelope phases. The formation of DNSs requires both stars in a binary system to evolve into neutron stars, surviving two supernova events while remaining gravitationally bound. This process is influenced by several key parameters such as mass transfer efficiency, supernova type, common envelope efficiency, and neutron star kick velocities. Population synthesis models show that the observed DNS merger rates and properties can be explained by varying these parameters, with Bayesian analysis helping to identify the most likely scenarios that match observations 138.
Supernova Types and Natal Kicks in Double Neutron Star Systems
The type of supernova that forms the second neutron star in a DNS system plays a crucial role in determining the system's final properties. There is strong evidence for two distinct supernova mechanisms: electron-capture supernovae (ECS) and core-collapse supernovae (CC). ECS events typically involve low mass ejection and impart small kick velocities to the neutron star, resulting in DNSs with short orbital periods, low eccentricities, and low proper motions. In contrast, CC supernovae can produce higher mass ejection and larger kicks, leading to wider or more eccentric binaries. Most observed DNSs appear to have formed through ECS, with only a minority resulting from standard CC supernovae 4567+1 MORE.
Population Subtypes and Formation Channels
The growing sample of known DNSs in the Milky Way can be divided into three subpopulations based on their orbital characteristics: short-period, low-eccentricity binaries; wide binaries; and short-period, high-eccentricity binaries. These subpopulations also differ in spin period and spindown rates. Some DNSs, especially those with high eccentricity and short periods, may have formed in dense stellar environments like globular clusters and were later ejected, providing a pathway for DNS formation in regions without recent star formation .
Mass Transfer, Common Envelope, and Accretion
The evolution of DNS progenitors often involves a common envelope phase, where one star engulfs its companion, leading to significant orbital shrinkage. Stable mass transfer, especially from a stripped post-helium-burning secondary (case BB), is likely a key process in forming close DNS systems. The amount of mass accreted by the first-formed neutron star during these phases is generally limited, but it can affect the final spin and mass of the neutron star 38.
Orbital Dynamics and System Properties
The final properties of DNSs, such as orbital period, eccentricity, and system velocity, are sensitive to the details of the supernova explosions and the pre-explosion orbital configuration. DNSs formed via consecutive type II supernovae require fine-tuned initial parameters, and the resulting systems can have a wide range of eccentricities and center-of-mass velocities, typically less than 60 km/s. The majority of DNSs are expected to reside close to the galactic plane due to their low kick velocities, which has implications for the rates of DNS mergers detected by gravitational wave observatories 67.
Alternative Double Star Formation Scenarios
For wider double star systems (not limited to neutron stars), traditional formation at observed separations would lead to tidal instability. An alternative scenario suggests that such binaries may form through the disintegration of small stellar clusters, which imposes upper limits on their mass ratios and distinguishes their formation from that of planetary systems .
Conclusion
Double star formation, particularly for double neutron stars, is governed by a combination of binary evolution processes, supernova mechanisms, and environmental factors. The interplay between mass transfer, supernova type, and natal kicks shapes the observed diversity in DNS properties. Ongoing observations and advanced modeling continue to refine our understanding of these fascinating systems and their role in astrophysical phenomena such as gravitational wave events and gamma-ray bursts 1345+4 MORE.
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