Looking for answers in the stars

13 August 2020

In terms of scale, even the smaller aspects of astrophysics play out over vast dimensions. “I work on things about the size of our solar system,” Philip Kwok Ching Leung (Croucher Scholarship 2017) said.

More specifically, Leung, a PhD student supervised by Professor Gordon Ogilvie at the Department of Applied Mathematics and Theoretical Physics, University of Cambridge, investigates protoplanetary disc systems, a generic early phase in the formation of a star system and its planets.

“I study them because there is still much to be understood about the link between the disc at this stage and the planets at a much later stage.”

To Leung, it is the pursuit of new understanding that sparked his initial interest in the stars. From a very young age, his father would show him countless documentaries on space, in which astrophysicists such as Stephen Hawking shared their knowledge of how the universe, including its solar systems, were formed.

Leung grew up in Hong Kong but completed his secondary education at Harrow School in London. It was during this time he furthered his interest in science and maths by competing in international competitions, representing the UK in mathematics and chemistry contests, including as a member of the UK team that competed in the International Chemistry Olympiad in Moscow.

At Harrow, he was also part of the school’s Christian Union. He explained how this, and his Christian upbringing, helped motivate him to discover more, through science. As a Christian, he sees the world, and the universe beyond, as God’s creation. But instead of contradicting science, he believes this works in tandem with it. Scientific methods and research provide him with the platform to discover the intricacies of the universe, he said.

His PhD research, which is due to be completed next summer, focuses on the magnetic fields thought to be threaded vertically through the giant discs of gas and dust present during the birth of a star – fields that may lead to further activities such as planet formation.

The discs, known as accretion discs, are similar to Saturn's rings, but on a much larger scale, Leung explained. They can be seen using microwave antennas that can observe systems as far as 300 light-years away.

Leung works on the theoretical side of astrophysics and uses the images and data provided by observations from these antennas to test theories and models. “Astrophysics is very much a dialogue between theory and observation. They always go together,” he said.

In order to build theoretical models that can be used to explain observations, astrophysicists start with simple models, and generally some big assumptions about a star, for example, whether it is a perfect sphere, liquid, or fluid.

“[From these simple models,] we start trying to derive some properties and scaling relations,” he said. When more observations are available from the star, this new data can be used to test how well the model reacts and if it still reproduces the effects observed. “Once an effect has been discovered and found to be robust mathematically, [the next step is] to explain it in physical terms to help people gain understanding, starting [with] how it actually works.”

Looking at images of protoplanetary disc systems, which have a star at the centre and are surrounded by rings of gas and dust, certain structures can be seen in the discs. Some are distinct gaps but others are gigantic jet-like outflows emitting vertically from the disc.

Since the 1980s, astrophysicists have proposed mechanisms that could be responsible for these jets, concluding that they require magnetic fields to be present.

Together with Ogilvie, Leung published a paper last year in the Monthly Notices of the Royal Astronomical Society on magnetic flux in protoplanetary discs, which investigated the evolution of large-scale poloidal magnetic fields in accretion discs.

Magnetic flux is the density of magnetic field lines passing through a given area. “We know that there is intrinsic magnetic flux in the universe. There are some theories on how that came about,” Leung said. “When we look at the sky, we see the light that comes to us is often polarised and often some of it may be indications of magnetic structures.”

Additionally, the rate at which large magnetic clouds have been observed to collapse is much slower than calculations that do not take into account additional forces, such as the presence of magnetic fields, would predict. This suggests that the fields are intrinsic to these molecular clouds, from which the star is formed, exerting pressure on the gases and preventing them from collapsing quickly.

In his most recent paper (again with Ogilvie), Leung has continued his investigations on magnetic flux in accretion discs, looking more specifically into the interaction between wind and turbulence found within them. By running numerical simulations and testing equations they were able to explore “the processes within the disc itself, particularly if the disc is weakly ionised,” Leung explained.

It is often assumed that the gas and dust in space are ionised because of the amount of radiation present, which in turn means they could conduct electricity and hence have magnetic fields embedded within them.

An example of such a conductive fluid is the Van Allen belts that surround the earth, which produce the polar auroras. These belts are made up of ionised particles that are above the earth's atmosphere but are held in alignment with its magnetic field.

Unlike the auroras around our planet, the coupling of ionised matter with large-scale magnetic fields around a new star may lead to the ejection of gas and dust that form into the gigantic jets. Alternatively, they could cause turbulence, through magnetic instabilities, which may lead to particles clumping and potentially forming planets, he said.

Leung has been investigating the symmetry related to the magnetic fields, and the shape they would take threading through the disc. “Traditionally, people have assumed they will take a sort of hourglass shape. Recently, some simulations have shown that the field can also take a slanted symmetry, where it's actually at a slant across a mid-part of the disc. That in turn will affect the wind launch properties and the turbulence properties significantly,” he said.

A higher disc magnetisation leads to a greater tendency towards, and more rapid settling into, the slanted symmetry steady wind, which may have important implications for mass and flux transport processes in protoplanetary discs.

After his PhD, Leung hopes to find a postdoctoral position in the UK so that he can continue his research. Recent observations show why these discs continue to fascinate him. These observations have revealed structural features, such as spirals, rings, and gaps, which as yet still require a physical explanation, proving there is still much to be explored in these protoplanetary disc systems light-years away.

Philip Kwong Ching Leung completed his BA with MSc in Natural Sciences (Experimental and Theoretical Physics) at the University of Cambridge in 2017. Since then he has been working with Professor Gordon Ogilvie in the Department of Applied Mathematics and Theoretical Physics at Cambridge, investigating the effect of magnetic fields in astrophysical accretion discs as part of his PhD programme. He received a Croucher Cambridge International Scholarship in 2017.

To view Philip Leung’s Croucher profile, please click here.