Paper Title: Wolf-Rayet – compact object binaries as progenitors of binary compact objects
Authors: Erika Korb, Michela, Mappeli, Giuliano Iorio, Guglielmo Costa
First Author’s Institution: Physics and Astronomy Department Galileo Galilei, University of Padova, Padova, Italy
Status: published in Astronomy & Astrophysics [open access]
This guest post was written by Shanshan Deng, a 2nd-year graduate student in Astronomy at Nanjing University. She is currently exploring the fascinating world of gravitational waves through numerical simulations in strong gravity regimes. Beyond gravitational waves, she also has a keen interest in the mysteries of dark energy and dark matter in the universe. Outside of astrophysics research, she often spends her time enjoying outdoor activities such as hiking, trekking, and archery.
The universe is a violent and dynamic place. Over the past decade, the LIGO-Virgo-KAGRA (LVK) Collaboration has opened a new window into its hidden face: gravitational waves (GWs). These ripples in spacetime, predicted by Einstein’s theory of general relativity, are generated by some of the most cataclysmic events in the cosmos, like the collisions of black holes and neutron stars, now commonly referred to as Binary Compact Object mergers (BCOs). But how do these BCOs form in the first place?
One leading hypothesis involves a special type of star called a Wolf-Rayet (WR) star. WR stars are massive, hot, and losing mass at an incredible rate through powerful stellar winds. They’re essentially nearing the end of their lives and are destined to become either a black hole or a neutron star. Now, imagine a WR star in a close partnership with a compact object; that is, either a black hole (BH) or a neutron star (NS). These systems, dubbed WR-COs, are the focus of intense research as potential BCO progenitors.
Think of it like this, you have a future black hole (the WR star) already paired up with a compact object. It’s halfway toward a compact object collision already! As the WR star continues to evolve, it too collapses, potentially leading to two compact objects tightly bound are ready to merge, unleashing powerful gravitational waves that ripple outwards across the vast universe.
But is this theoretical pathway actually relevant? Today’s study, using a powerful tool called the SEVN population synthesis code, sought to answer this question by simulating the lives and deaths of millions of binary star systems. These simulations, effectively stellar “experiments,” explore different scenarios and parameters to understand how BCOs form. The results are shedding new light on the role of WR-COs in creating the GW events we observe.
Simulating the Stellar Universe
The SEVN code is essentially a virtual universe where stellar evolution is accelerated. It takes into account various physical processes that govern the lives of stars, including mass transfer, stellar winds, supernova explosions, and even natal kicks (the momentum “kick” a compact object receives during its birth in a supernova).
The researchers ran a staggering number of simulations: 5 million binary star systems were evolved across 96 combinations of key parameters, including:
- Metallicity: The amount of elements heavier than hydrogen and helium in stars. Metallicity affects stellar winds and evolution.
- Common Envelope Efficiency (αCE): A parameter that describes how efficiently the envelope of a star is ejected during a phase of unstable mass transfer, known as a common envelope phase. This phase occurs when one star in a binary system expands dramatically and engulfs its companion star. The crucial question is: how much of the orbital energy of the binary system can be effectively transferred to the giant star’s envelope to expel it and prevent a merger? This efficiency is highly uncertain because the process is incredibly complex, involving turbulent gas dynamics, magnetic fields, and radiation, all acting on three-dimensional scales. Factors that likely influence αCE include the internal structure of the giant star, the mass ration of the binary system, and the details of how energy is transferred from the orbit to the stellar envelope. It’s a very uncertain process!
- Core-Collapse Supernova (CCSN) Models: The way in which a massive star dies as a supernova, heavily influencing its mass and whether it forms a neutron star or black hole.
- Natal Kick Distributions: The magnitude and frequency of kicks given to neutron stars and black holes from supernovae.
The Results: WR-COs Dominate BCO Formation
The simulations revealed a striking result: WR-COs are the primary progenitors of BCOs, especially at intermediate and high metallicities, as shown in Figure 1. In other words, most of the black hole and neutron star mergers we detect likely started their lives as a WR star orbiting a compact object! In fact, more than 99% of all BCOs formed via the WR-CO route at metallicities similar to our Sun! At lower metallicities, the contribution of WR-COs decreases slightly, but still remains dominant.
However, it’s not a one-way street. Despite their crucial role in BCO production, only a small fraction (5-30%) of WR-COs actually end their lives as merging compact objects. The rest might have wider orbits so the GW timescale is longer than the age of the Universe, or a supernova might kick one of them out of the binary entirely. The exact pathway is complex and dependent on the specific parameters of the system.
Cyg X-3: A Galactic Rosetta Stone?
The study also examined Cyg X-3, a unique system in our own Milky Way. Cyg X-3 is a confirmed WR-CO candidate, making it a vital test case for these theories. But it’s unclear whether the CO in the system is a black hole or a neutron star. As shown in Figure 2, the simulations revealed that Cyg X-3 is a promising BCO progenitor, particularly if it hosts a black hole! Between 70% and 100% of similar systems with a black hole in the model formed a BCO. This is much better odds than if it hosted a neutron star instead.
The authors conclude that future observations of WR-COs, similar to Cyg X-3, could be the “Rosetta stone” for understanding BCO formation. By studying these systems in detail, we can test and refine our models, ultimately deciphering the origins of the gravitational waves that are revolutionizing our understanding of the universe.
Looking Ahead
This research highlights the critical role of WR-COs in the formation of gravitational wave sources. By combining sophisticated simulations with observations of real systems like Cyg X-3, astronomers are piecing together the complex puzzle of binary star evolution.
As the LVK Collaboration continues to detect more and more GW events, the need to understand their origins becomes even more pressing. These studies provide a vital theoretical framework that may help us connect observations of BCOs with the long, winding road of stellar evolution. So next time you hear about two black holes colliding in a distant galaxy, remember that it might have all started with a Wolf-Rayet star.
Astrobite edited by Brandon Pries
Featured image credit: Wikipedia
