Magnetic fields: the key to understanding the physics of stellar evolution and exoplanetary systems

Some of the most intense solar fireworks have directly affected modern life on Earth. Rapid increases in solar activity send fluxes of charged particles towards our planet, disrupting satellite communications and generating spectacular aurorae. The sublime coloured lights flickering across the night sky arise from solar wind particles trapped in the large-scale magnetic field of the Earth. We are fortunate that this magnetic field protects our atmosphere, and us, from the harmful effects of the highest energy solar radiation. Yet however spectacular the events on our Sun may seem today, our middle-aged star is comparatively quiet and hides a much more violent past.

A few billion years ago the Sun was a contracting ball of hot gas, although it was still too cool within its core to fuse hydrogen into helium and shine as it does today. Young solar analogues, called T Tauri stars, are about a million times too faint to be seen with the naked eye. Space satellite observations, however, have shown that they are typically 1000 times more active in X-rays than the Sun is presently. They are surrounded by large dusty planet-forming discs, which are bombarded by X-rays arising from violent stellar magnetic events.

T Tauri stars are new-born stars which are still shrinking down under gravity to their adult proportions. As such stars collapse they should start to spin faster and faster, just like an ice-skater who pulls in their arms closer to their body. However, such stars are observed to be spinning slowly, an effect that has been attributed to how their strong magnetic fields interact with the gas and dust within the surrounding discs. T Tauri stars are embedded in a web of magnetic field which binds them to their discs, the eventual birthplace of planetary systems. It is expected that this magnetic web acts as a brake, allowing the star to remain spinning slowly. This process, however, is poorly understood.

Using a technique called Zeeman-Doppler imaging, which is analogous to methods used in medicine to produce 3D images of the internal organs of the body, I have been able to determine the structure of their magnetic webs. This newly available data signals the beginning of a fresh era of research into the formation of solar-like stars. My research will exploit information gained from the largest telescopes on Earth and from space satellites in order to investigate how forming stars interact with, and disrupt, their environments. The significant gains in observational results require corresponding development of state-of-the-art numerical models. Ultimately my research will provide insight into the formation of our own Sun, and Solar System, at a time when the planets were just beginning to form.

Roughly 500 planets, exoplanets, have now been found orbiting other stars with potentially thousands more awaiting discovery by the currently operating Kepler satellite. Some of the exoplanet host stars twinkle on the same timescale as it takes their planets to complete an orbit. This effect may be caused by the planets twisting the strands of the star's magnetic web; this process, if it can be understood, may provide a method of determining the magnetic properties of the exoplanets themselves. I will investigate star-planet interaction in detail during the STFC Ernest Rutherford Fellowship.