The Cosmic Dance of Neutron Stars: GW190425

On April 25, 2019, scientists detected a significant event known as GW190425, which was the merging of two neutron stars. Neutron stars are extremely dense remnants of massive stars that have exploded in supernovae. GW190425 stands out because it had a higher total mass than typical neutron star pairs, making its origins a topic of much debate.

This event was linked to a process called Binary Evolution, where two stars orbit each other. In particular, it was thought that GW190425 came from a close pair of stars, one being a helium-rich star and the other a neutron star, just after they went through a phase known as the common envelope phase. Researchers are interested in understanding how such neutron star pairs form, especially in environments similar to our own solar system.

#Stellar Evolution and Mass Transfer

The study of binary stars often involves looking at how these stars evolve and interact over time. In the case of GW190425, a detailed approach using a computer code named MESA helps track how these stars change. Important factors include how they lose mass, rotate, and interact with each other.

In a binary system where mass transfer occurs, one star can pull material from its companion. This is especially true for a helium-rich star transferring material to a neutron star. There are different phases of mass transfer, each with distinct behaviors. For instance, Case BB and Case BC Mass Transfers happen during different stages of a star's life cycle.

In stable mass transfer scenarios, the helium-rich star can lose material gradually, which can reshape its structure. As it spirals closer to the neutron star, the two stars can exchange energy through gravitational interactions, which can influence their rotation and overall evolution.

#Formation Scenarios for GW190425

The immediate progenitors of GW190425 consist of a neutron star and a helium-rich star that have been interacting. For such a system to form, certain initial conditions regarding their masses and distances must be met. Modeling these initial conditions leads researchers to conclude that the systems need to start with specific properties to eventually evolve into a pair like GW190425.

The mass transferred from the helium-rich star to the neutron star can result in significant changes in the neutron star's characteristics. If the neutron star spins fast enough, it could potentially become what is known as a magnetar-a highly magnetic and rapidly rotating neutron star.

#Observations and Predictions

GW190425 was found during a third observing run of gravitational wave detectors, which are designed to pick up signals from cosmic events. One challenge in studying such events is that they often occur far from Earth, making them difficult to observe in other light spectra like visible light.

So far, heavier binary neutron star systems like GW190425 are believed to produce fainter aftereffects, such as kilonovae, which are explosions caused by neutron star mergers. These aftereffects can offer clues to understanding the event, but for GW190425, light signals were weak and hard to detect.

Researchers proposed that GW190425 likely formed through isolated binary evolution. This means that the neutron star and helium-rich star transitioned through predictable stages without significant external influences from other stars. The evolution led to one star transitioning into a neutron star after a Supernova, while the helium-rich star grew and changed throughout its life cycle.

#Mass Transfer Mechanics

During mass transfer, the helium-rich star expands significantly, especially after its carbon fuel is depleted. This expansion suggests that if the stars are close enough, the helium-rich star will spill material onto the neutron star. Different forms of mass transfer will produce various results.

Case BB mass transfer occurs when the donor star is burning helium in its outer layers, while Case BC mass transfer happens when carbon is igniting in the star's core. These processes influence how much mass is exchanged and the subsequent evolution of both stars.

This mass transfer can lead to complex interactions, which can lead to the formation of double neutron star systems. Gravitational waves will eventually cause these systems to merge, and this merger is what was detected during GW190425.

#The Role of Temperature and Composition

One critical factor affecting the evolution of the stars in these systems is metallicity, which refers to the abundance of elements heavier than hydrogen and helium. Low metallicity is thought to favor the formation of heavier neutron star systems. If the initial masses of the stars are adjusted or if conditions such as metallicity vary, it can significantly alter the final outcomes.

The initial mass and temperature of the progenitor stars help predict the types of events they will produce. By studying these stars, researchers can gain insights into how different conditions lead to the formation of systems like GW190425.

#The Magnetar Connection

One of the exciting aspects of these studies is the possibility that newly formed neutron stars could become Magnetars. These cosmic objects are not only incredibly dense but also possess strong magnetic fields that can significantly influence their behavior. If a neutron star accumulates enough mass from its companion, it may spin faster and develop a magnetic field strong enough to classify it as a magnetar.

This potential connection to magnetars adds a layer of complexity to the understanding of gravitational wave events. Such stars can lead to different types of supernova explosions that might accompany neutron star mergers.

#The Helium Envelope and Ejecta Mass

Before a supernova occurs, the remaining helium envelope mass can provide crucial information about what happens in the explosion. The mass left in the helium-rich star before it undergoes a supernova varies depending on its initial conditions. These remaining masses can hint at the characteristics of the resulting explosion.

The lighter the remaining envelope, the more likely it is to trigger certain types of supernovae, including type Ib and type Ic, based on their mass characteristics. The research suggests that GW190425 could be associated with such explosive events due to the mass transfer process and the structural changes in the helium-rich star.

#Conclusion

In summary, the formation of events like GW190425 revolves around the interactions between neutron stars and their helium-rich companions. These interactions cause significant changes throughout their life cycles, leading to fascinating outcomes, including the potential formation of magnetars and unique supernovae.

Research continues to uncover how different initial conditions and evolutionary paths contribute to these cosmic events. Understanding these processes not only helps explain GW190425 but also provides insights into the universe's broader behavior regarding massive star systems, neutron stars, and the gravitational waves they produce.