The Mystery of Neutron Star Binaries in Star Clusters

Neutron star (NS) binaries are pairs of neutron stars that orbit each other. These unique objects have puzzled astronomers for a long time, especially when they are found in the outskirts of star clusters. Normally, heavier stars, such as neutron stars, are expected to move toward the center of star clusters due to a process called dynamical friction. The cluster center is denser, making it a natural place for these heavy objects to gather. Yet, some neutron star binaries, particularly Millisecond Pulsars (MSPs), have been spotted far from the cluster centers, raising questions about how they got there.

For instance, the star cluster NGC 6752 contains two MSPs located far from the center. The first MSP, identified as PSR J1911-5958A, is known to be the farthest of its kind from the cluster center. It is found about 3.3 half-light radii away, which is quite a distance. This MSP has a companion star that is a helium white dwarf. The second MSP, PSR J1911-6000C, is also located far from the cluster center but doesn't have a companion.

#The Challenge of Explaining Their Locations

It is generally thought that the location of these MSPs in the outskirts of their clusters cannot simply be explained as a byproduct of their formation in those locations. The observed distribution of neutron stars and their companions in clusters does not align with what we would expect if they were primarily born in the outskirts. There are many other factors at play.

The common thought is that interactions involving massive black holes (BH) in the cluster may eject neutron star binaries into the outskirts. However, the evidence suggests that heavy black hole interactions might not be the main reason for the observed locations of these binaries.

#Alternative Mechanisms for Ejection

After examining this issue closely, several other potential explanations have been put forward for how these neutron star binaries might make their way to the outskirts of clusters:

1. Three-body Interactions: The most likely explanation for the observed neutron star binaries in the outskirts of clusters involves a three-body interaction with a regular star. This means that the neutron star binary interacts with another star in a way that could push it outwards.

2. Core Collapse: In some clusters, a process called core collapse can occur, which may also help in moving neutron stars outwards from the core. In a core collapse, the center of the cluster becomes very dense and can cause different dynamics that push some stars farther away.

3. Initial Formation: Another idea is that some neutron star binaries could have been formed in the outskirts of the cluster. However, considering how neutron stars behave, this scenario seems less likely for most cases.

#Understanding the Role of Black Holes

While it was initially believed that interactions with massive black hole binaries were the main cause for neutron star binaries being pushed to the outskirts, this idea is being challenged. It appears that the combination of three-body interactions and core collapse mechanisms may play a more significant role in these neutron star motions.

A black hole binary would need to be dynamically strong enough to keep a neutron star binary in the cluster, but only in specific situations. If the dynamics of the cluster allow, it is more likely that a neutron star gets ejected entirely from the cluster, rather than being placed just in the outskirts.

#Simulations and Observations

To better understand these scenarios, astronomers have conducted simulations to study how neutron stars behave in star clusters over time. These simulations aim to mimic the evolution of star clusters and the interactions that might occur within them.

Two clusters, NGC 6752 and 47 Tuc, have been studied in simulations to evaluate the behaviors of neutron binaries. The results from these simulations support the idea that regular stars, when interacting with neutron stars or their binaries, could likely lead to ejection scenarios.

#Common Ejection Mechanisms

From the simulations, it becomes apparent that several mechanisms contribute to placing neutron stars in the outskirts:

• Interactions with Regular Stars: Ordinary stars, such as main-sequence stars or white dwarfs, tend to be far more common in star clusters than black holes. Therefore, it makes sense that interactions with these stars could result in the ejection of neutron stars into the outskirts of clusters more frequently.

• Ejection through Binary Interactions: Neutron stars can also be ejected due to interactions with other binary star systems. For instance, a binary system could exchange one of its components with a neutron star, leading to the ejection of the neutron star binary from the cluster.

• Natal Kicks: When a neutron star forms, it may receive a kick or thrust due to asymmetries in the surrounding material during its creation. This kick can potentially push the neutron star or its binary into the outskirts during or after its formation.

#Observational Evidence

The study of various clusters has shown that many neutron star binaries found in their outskirts do not share common traits expected of those that would have formed there. For example, many MSPs are observed to be significantly more frequent in globular clusters compared to other areas in the Milky Way galaxy.

This suggests that the conditions that lead to the formation of neutron stars or MSPs must be heavily influenced by cluster dynamics. In many cases, these stars are interacting with other regular stars rather than just black holes, which don't form in abundance in all types of clusters.

#Conclusion

Neutron star binaries found in the outskirts of star clusters pose a fascinating question for astronomers. Through examining the dynamics of star clusters and conducting simulations, researchers are uncovering the possible mechanisms that lead to these neutron stars being pushed away from the denser cluster cores.

While heavy black-hole interactions might be a part of the story, they are more likely overshadowed by interactions with normal stars and the complex dynamics of star clusters. Overall, the ongoing study of star clusters promises to shed further light on these remarkable neutron star binaries and their journeys within the cosmos.