PhD ceremony Timothy Yiu

On Tuesday 19 May 2026, Timothy Yiu defends his thesis called
A radio study of extrasolar magnetospheres.

Summary of his thesis:
Radio observations allow us to study how stars and planets accelerate charged particles. Sun-like stars accelerate charges in flares on their surfaces, while planets such as Jupiter accelerate charges in their magnetospheres away from the surface.

In his thesis, Timothy Yiu investigates the Sun-to-Jupiter transition by studying the radio emission from intermediate objects — M-dwarf stars and brown dwarfs — that are much smaller than the Sun but more massive than Jupiter. Yiu finds that the transition likely happens when objects develop large-scale magnetic fields, similar to that seen on Jupiter. This suggests that large-scale fields lead to higher radio emission efficiencies.

Next, Yiu delves deeper into the radio emission from large-scale fields by studying a brown dwarf system, J1019+65. Yiu shows that the object's radio emission is periodic — repeating at its rotation period — which establishes its commonality with Jupiter-like radio emission.

Yiu also shows that J1019+65's radio emission disappears above around 200 MHz. This leads to a direct measurement of its magnetic field strength and provides a rare window into the object’’s interior where the field is generated. Yiu shows that in J1019+65, it is the balance between the magnetic force and Coriolis force that sets the field strength. These results open a path towards testing our theories of magnetic field generation in stars, brown dwarfs, and even exoplanets.

Link to Timothy's thesis