Magnetic Fields

Substellar, Stellar and Post-stellar Magnetospheres - the astrophysics of their creation, maintenance & impact on their local environments

To use the wikipedia definition, a magnetosphere is "a region of space surrounding an astronomical object in which charged particles are manipulated or affected by that object’s magnetic field". Traditionally its been thought solely in terms of planets, but the discovery of active magnetospheres around both pulsars, and also substellar objects, most notably brown dwarfs, have stretched the definition to cover the full range of stellar objects. The group leader, Aaron Golden, started studying the magnetospheres of pulsars, was involved in the discovery of active magnetospheres around several nearby brown dwarfs, and is now working on the study of stellar magnetospheres, particularly those associated with flare stars.

Substellar Magnetospheres

It’s still mind boggling that certain failed stars - also known as brown dwarfs - turn out to have huge, powerful and incredibly stable magnetic field systems, with their associated magnetospheres’ supporting enormously powerful aurora that we can see through their Hα emission but also via intense coherent radio beams, that sweep past our line of sight - just like a lighthouse, and indeed, just like a pulsar (see below). How these fields originate, are sustained and how these aurora affect the dwarf atmospheres’ beneath are mysteries still be be fully explored  using radio and optical observatories around the world and in orbit.

Left: Modulated Hα emission from the aurora above the M8.5 dwarf LSR  J1835+3259, with the coherent radio emission observed simultaneously  below - from Hallinan et al. (2015). Right: schematic showing optical/radio auroral emission processes in the magnetosphere.

Stellar Magnetospheres

Conventional wisdom thinks in terms of the hot surface of a star (photosphere), the chromosphere and then the corona - the idea of a stellar ‘magnetosphere’ is an alternative way of thinking about the outer regions of the corona. It’s in this region that activity not-dissimilar to planetary/substellar auroral emission has been demonstrated to take place, at high radio frequencies for the chemically peculiar star CU Virginis, and low radio frequencies (~100 MHz) for dwarf flare stars such as UV Ceti (Lynch et al. 2017). Coincidentally, this is also the radio ‘window’ through which coronal mass ejections (CME) can be unambiguously identified. Of the thousands of exoplanets detected to date, the majority orbit cool dwarf stars that are magnetically active, whose flaring behaviour may be associated with coronal mass ejections (CME) - as CME’s represent one of the most lethal threats to habitability, their direct detection and characterisation using radio telescopes such as LOFAR and the Owens Valley LWA is an active area of study.

Top: The Owens Valley Long Wavelength Array under the Californian night sky, showing several of the 288 crossed broadband dipole antennas.  Bottom: Cartoon showing a flare and CME above a dwarf star, and a nearby exoplanet.

Post-stellar Magnetospheres

It all started with ‘post-stellar magnetospheres’, the post-stellar state in question being neutron stars, specifically isolated rotation-powered neutron stars, which are also known as pulsars. Just like the aurora, the electromagnetic emission observed from pulsars is beamed like a lighthouse above magnetic poles - however, the extreme physics involved (a collapsed star the size of Manhattan with a magnetic field billions of times more powerful than what we experience on Earth, spinning tens of times a second) require the use of both Einstein’s theories of Special and General relativity to even begin to understand what’s going on. In the past we've worked with the largest of ground-based telescopes combined with high speed cameras to study the optical emission from the magnetospheres of several nearby pulsars, but more recently we have started working with radio telescopes. The group has recently become involved as part of the European Pulsar Timing Array which allows us to incorporate our ongoing pulsar observations using the I-LOFAR radiotelescope in Birr into the hunt for nanohertz gravitational waves.

Left: The Crab Nebula in true colour, taken with the CHIMERA instrument with the 5m Hale telescope on Mount Palomar The pulsar is  the slightly dimmer of the two central stars. Right: A movie (with some  help from Michael Kramer - thanks again Michael!) of the Crab pulsar using data Aaron took with a  back-up Hubble Space Telescope STIS/MAMA camera as part of his Ph.D.  studies. The double-pulse you can see takes places every 30 millseconds.