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High energy stellar physics

The term "high-energy astrophysics" generally conjures thoughts of supermassive black holes at the centres of active galaxies and quasars, neutron stars accreting material from close companions, or the shocked surroundings and remains of supernova explosions. Yet our own Sun provides the closest example of a source of both high-energy particles and electromagnetic radiation. 

In so-called "normal" stars, like the Sun, high energy processes are mediated by magnetic fields, in which the kinetic energy of convection and differential rotation in the stellar interior is stored through magnetic dynamo processes that are still not thoroughly understood. Magnetic field at the stellar surface is most prominently observed in visible light indirectly in the form of sunspots that mark the footpoints of magnetic loops that extend up to 100,000 km above the solar surface. These loops are observed directly in extreme ultraviolet and X-ray light, where they entrain plasma heated to more than a million degrees.  The exact heating mechanism, or mechanisms, have not yet been firmly identified, but are likely to be dominated by the dissipation of Alfven wave energy excited by convection, and by magnetic reconnection.

Since the dynamo process depends on rotation, faster rotating stars are more magnetically active. Like people, stars calm down as they get older. At about 4.6 billion years old, the Sun is a middle-aged star, with an equatorial rotation period of about 25 days (34 days near the poles).  When it was born as a T Tauri star, it would have had a rotation period of between 5-10 days. As it contracted to the main sequence, it would have spun up to have a period of anything from about half a day to two days or so - see the posting on the young solar-like stars in the beta Pictoris moving group - and would have been at its most magnetically active.  Throughout these early times, its X-ray output would have been 1,000-10,000 times that of the present day average. Giant flares and coronal mass ejections would have been common.

Over time, the magnetic activity of stars is responsible for its own demise. In addition to powering X-ray emitting coronae, Sun-like stars also power a magnetically-threaded wind that carries away angular momentum, leading to a gradual slowing of the rotation rate and commensurate diminution of the magnetic activity that drives it. 

In addition to controlling the angular momentum evolution of single stars, magnetic activity also controls the evolution of interacting close binary stars with sun-like components. When such binary stars get really close, tidal forces lead to their rotation periods becoming synchronised with their orbital periods. Loss of angular momentum through winds leads to loss of orbital angular momentum - and gradual shrinkage of the orbit, bringing the stars closer together. For binaries in which one component is a white dwarf, this eventually leads to "Roche Lobe overflow", in which the very outer part of a star facing the white dwarf falls under the influence of the white dwarf's gravity.  Material is lost to the white dwarf, forming an accretion disk and what is known as a cataclysmic variable star that can undergo nova explosions - see postings on the pre-cataclysmic binary QS Vir and on the novae U Scorpii and V407 Cygni.

High energy stellar physics encompasses all aspects of this non-thermal evolution of stars:
  • Using the conspicuous X-ray emission properties of young low-mass stars to study young stellar populations and probe regions of star and planet formation.
  • The X-ray and magnetic activity of the early T Tauri phase, when the star is surrounded by a disk of gas that harbours nascent planets. The energetic radiation of the star is crucial for understanding the evolution of the gas disk and is likely a vital ingredient in planet formation. Accreting T Tauri stars can also drive polar jets of plasma at hundreds of km/s into the ambient medium.
  • Rotational evolution through wind-driven spin-down, and the behaviour of magnetic activity through time.
  • The influence of magnetic activity and the interplanetary radiation environment on planetary systems and the evolution of planetary atmospheres that experience erosion and mass loss through stellar wind impact.
  • The behaviour  of cataclysmic variables and novae whose evolution is largely controlled by the magnetic activity of the secondary, unevolved star.
  • Understanding the processes at work in stellar outer atmospheres that drives the spectacular magnetic behaviour we observed from UV to X-rays, and that we infer in stellar winds and coronal mass ejections.