Gravitational wave sources from massive binary and triple stellar systems

Dorozsmai, Andras (Andris) (2024). Gravitational wave sources from massive binary and triple stellar systems. University of Birmingham. Ph.D.

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Abstract

In this thesis, I investigate the formation paths of merging stellar mass binary black holes originating from massive binary and massive triple systems.

Massive stars are short-lived objects and extremely rare, making up less than 1 percent of the total population of stars. Despite their rarity, they dominate and drive the evolution of galaxies due to their enormous energy output and are progenitors to some of the most energetic astrophysical phenomena in the Universe. Understanding their evolution is crucial for almost all branches of astronomy. However, due to their scarcity, it is remarkably difficult to directly study them. Therefore, we need to consider every possible indirect observational clues to constrain their evolution.

Gravitational wave (GW) astronomy offers a unique way to study massive stars. Most GW signals detected to date were emitted by merging binary black holes. The large number of such detections make it possible to infer the demographics of these objects. Since massive stars are progenitors of stellar mass black holes, the demographics of stellar mass black hole binaries are heavily influenced by currently poorly understood processes related to massive stars and their interactions, such as various mixing mechanisms in stellar interiors, mass transfer episodes and three-body dynamics.
Therefore, in principle, observations of GW sources can help us to better understand several aspects of massive star evolution. We have to understand how the properties of merging binary compact objects depend on uncertain stellar physics. This can help us to identify certain features in the demographics of GW sources that help us to learn about the poorly known evolutionary processes massive stars experience.

This is in practice, however, an extremely difficult task. Due to the large number highly uncertain evolutionary stages that massive stars undergo to become GW sources, at the present time, we cannot create completely robust methods to directly infer stellar physics from GW observations. However, it is still crucial to understand how different assumptions of poorly constrained physics change our predictions about the demographics of merging binary black holes. Such studies can guide us in understanding which are the most critical aspects of stellar evolutionary models that needs urgent attention for improvement. Furthermore, it can also help us determining the best way to combine GW data with other, electromagnetic observations of immediate evolutionary stages of massive stars to obtain the most robust tests for our evolutionary models.

In this thesis, I use a population synthesis approach. I sample a large number of systems composed of zero-age main sequence stars from distributions that are in agreement with recent observations of young, massive stars. Using stellar evolutionary codes, I evolve these systems and I predict the properties of their remnants and eventual GW sources they might form, by making different assumptions about the physics of key processes that define their evolution.

In Chapter 2, I study the the so-called classical isolated binary evolution channel. This formation channel is considered to be one of the most promising source of merging binary black holes. In the first part of Chapter 2, I explore how uncertainties related to the first phase of mass transfer can affect the properties of GW sources. Among many results, I show that models with different assumptions about the highly uncertain angular momentum loss during the first phase of mass transfer yield very similar binary black hole observables (in terms of merger rate, mass and mass ratio distributions), however, the dominant formation channel is entirely different. This highlights why inferring stellar physics alone from GW data should be done with extreme caution given our current uncertainties.

In the second part of Chapter 2, I investigate the impact of uncertainties of stellar winds on the sources from the isolated binary formation channel. It is well known that the mass-loss rates of line-driven winds are by uncertain about a factor of three, and this can have a significant effect on the mass spectrum of black holes formed from single star evolution. I show that the impact of lowered mass loss rates on the masses of merging binary black holes, however, significantly depends on the assumption of other, seemingly unrelated binary physics. This demonstrates that inferring mass loss rates based on the masses of merging binary black holes can be misleading, unless the implications of other, uncertain binary physics is well explored.

In the final part of Chapter 2 I show that a large fraction of GW sources, formed via two subsequent stable phases of mass transfer, never cross the Humphreys-Davidson limit during their evolution. Therefore, these systems are likely not affected by any related uncertainties. As we show, the predicted merger rate of this channel can be comparable to the inferred merger rate from GW observations. This implies that even, if the most massive stars never cross the Humpreys-Davidson limit (and therefore do not experience significant radial expansion), currently inferred merger rates are still compatible with the classical isolated binary channel.

In Chapter 3, I study the evolution of hierarchical triples, in which the stars in the inner binary experience chemically homogeneous evolution. My primary aim is to understand the implications of this evolutionary channel in the context of GW astronomy.

I show that in a large fraction of these sources, the tertiary eventually fills its Roche-lobe. This is unique to triples with chemically homogeneously evolving stars and occurs very rarely in classically evolving triples. More importantly, in about 30 per cent of the cases, this tertiary mass transfer occurs towards a short period binary black hole (of which a large fraction eventually forms GW sources). The impact of the tertiary mass transfer on the inner binary black hole can be significant, as the inner orbit can shrink, leading to shorter delay times associated with GW mergers.

I also show that three-body dynamics plays a significant role in the evolution of the inner binary in an appreciable fraction of these triples. In some case, von Zeipel-Lidov-Kozai oscillations can prompt double helium star mergers in the inner binary. Such a scenario would be extremely rare among massive stars.

Although, chemically homogeneous evolution is predicted to be rather uneventful, if it occurs with an isolated binary (stars remain fully detached and might form merging binary black holes), if these systems are part of a hierarchical triple, then the tertiary star can influence their evolution in various ways, leading to a number of different possible sub-channels with qualitatively different evolution, possibly accompanied by the formation of various electromagnetic transients.

In the final Chapter 4, I briefly summarise the main findings of this thesis, and discuss recommended future work.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):