Astrophysics Research Overview

Searching for planetary transits is a demonstrated technique for the discovery of new planets beyond the solar system. The Wide Angle Search for Planets (WASP) is undertaking a comprehensive, wide-field sky survey to detect planetary transits in stars down to 18th magnitude. With greatly increased numbers of known extra-solar planets we will investigate the accretion processes that lead to planet formation, and thus better understand the origin of our own home. WASP will also provide a wealth of data on all classes of variable stars, allowing the systematic analysis of large samples and the discovery of new and rare variable types.

When a star like the Sun exhausts its fuel its internal structure undergoes major changes and it moves away from the main sequence of the Hertzsprung Russell diagram. During this phase of evolution it seeds the interstellar medium with dust and gas in the form of a wind, providing the raw materials for the formation of new stars. This research involves using infrared and sub-millimeter telescopes to probe the latter stages of stellar evolution, particularly those stars that evolve on the timescale of a human lifetime. These include not only the 'born-again' stars, which re-ignite some of their fuel as they head towards the white dwarf region of the H-R diagram, but also the explosions of novae, in which thermonuclear runaway occurs on the surface of a white dwarf in a close binary system.

Accretion is one of the most widespread and important phenomena in the universe. It is the dominant source of X-rays in the universe, powering X-ray sources from black-hole binaries to active galaxies. Accretion discs are also crucial in the formation of stars and contain the material out of which planets grow. Close binary stars provide the best opportunities for studying the physical processes of accretion. Keele's programme investigates accretion onto neutron stars and white-dwarf stars, using satellites such as XMM, Chandra and HST, complemented by ground-based telescopes. A particular strength is the understanding of magnetically channelled accretion, where the accretion process interacts with a strong magnetic field on the compact star.

Stars like the Sun or of even lower mass are born in clusters and associations. We search for young Suns, low-mass stars and brown dwarfs in star forming regions and clusters in order to find how common they are in a variety of environments and follow the temporal evolution of their discs, rotation rates, magnetic activity and chemical abundances. Our goals are to understand the way in which birth environment influences the development of low-mass stars (and their planetary systems) and to investigate the astrophysics, such as mixing, convection and magnetic fields, that are incorporated into pre main sequence evolutionary models.

Many stars are found in pairs (binaries) and orbit around each other with orbital periods of years, weeks, days or even every few hours. These stars have often interacted very strongly and may have exchanged mass or thrown their outer layers out of the binary system. This can produce some of the most dramatic objects in the sky, e.g., black hole X-ray binaries and Type-Ia supernovae. It is difficult to predict how stars behave when they interact strongly, so research at Keele uses surveys to find simpler examples of close binary stars which have interacted in the past, and may do again, but are currently not exchanging mass. We then study these binaries in more detail to measure their properties, e.g., the number in the galaxy, their distribution of orbital periods or the masses and sizes of the stars. This research gives information which is being used to understand the properties of many types of interacting binary stars.

The stellar atmospheric parameters of effective temperature and surface gravity are of fundamental astrophysical importance. They are the prerequisites to any detailed abundance analysis. As well as defining the physical conditions in the stellar atmosphere, these parameters are directly related to the physical properties of the star; mass, radius and luminosity. Model atmospheres are our analytical link between the physical properties and the observables - flux distributions and spectral line profiles. We can obtain effective temperature and surface gravity from suitable observations, assuming of course that the models we use are adequate and appropriate.

Raphael Hirschi studies massive stars and related topics like gamma-ray bursts (GRBs), supernovae (SNe) and the origin of elements by performing numerical simulations. Models are being computed at metallicities ranging from solar down to very low metallicities and from the main sequence until the pre-supernova stage. These models are able to predict properties of the star during its evolution (mass, surface composition and position in the HR diagram), long and soft GRB rates coming from single stars and the production of chemical elements in massive stars.

The nuclei of all galaxies, including our own Milkyway, are thought to contain super-massive black holes, ranging in mass from a few million to a few billion times that of our Sun. While the nuclei of some galaxies remain dormant, a substantial fraction radiate vast amounts of energy, greater than all the starlight in one whole galaxy combined. The intense emission from these so-called Active Galactic Nuclei (or AGN) can occur in a region smaller than the size of our own Solar System, via accretion of matter onto the supermassive black hole. The emission closest to the black hole event horizon is in the form of high energy radiation, such as X-rays. Research into the high energy emission from AGN is conducted with space-based observatories such as XMM-Newton, Chandra, Suzaku, RXTE and Integral. By studying the atomic X-ray line emission from elements such as iron, the properties of the very hot, innermost regions around super-massive black holes can be deduced. The iron line emission can be warped by relativistic effects near to the black hole and provides a direct probe of the black hole's strong gravitational pull.

Research is conducted into Gamma-ray bursts, otherwise known as GRBs. GRBs are thought to be the most powerful explosions in the known Universe, however until recently their origins were largely unknown. Recent research is showing that some types of GRB are linked with energetic supernovae, occuring in many distant galaxies throughout the Universe, associated with regions of intense star formation. A new development includes the detection of many high redshifts bursts, including one such burst at a redshift of z=6.3, one of the most distant known objects in the Universe. The study of distant high redshift bursts is important to our understanding of the early Universe shortly after the Big Bang and to how the first stars and galaxies formed.