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Supervisor: Dr J P Pye.
The X-ray emission from `normal' late-type (or cool) stars (ie. spectral types F-M) including our own Sun (a main-sequence star of spectral type G2 V), arises in the hot, outer layers of the stellar atmosphere, the corona - gas at around 1 to 10 million degrees Kelvin.
All late-type stars possess a turbulent convection zone below their visible surface (the photosphere). Interactions between the convective motions and the star's rotation produce magnetic fields which extend upwards into the star's atmosphere. This effect is known as the magnetic dynamo. The coronal gas is heated by energy from the convective motions or from the magnetic fields. Rapidly spinning stars have higher X-ray luminosities, thus a star's X-ray luminosity provides a measure of its activity. Although the general model of stellar activity driven by a magnetic dynamo is now well established, there are many effects that remain to be understood, such as chemical-element composition on activity, effects of pre-main-sequence environment, and the causes of the observed activity `saturation' and `super-saturation'.
Our own Sun is a fairly-slowly rotating, middle-aged main-sequence star, with a modest level of X-ray emission from its corona. This image to the top left was taken by the Yohkoh satellite, in October 1991, when the Sun was at the peak of its 11-year activity cycle (Image courtesy of ISAS/NASA.). In contrast, young, very active solar-like stars, like those in open star clusters emit X-rays at 50-100 (Hyades) and 500-1000 (Pleiades) times that of the Sun. The ages of the Pleiades, Hyades and Sun are about 70, 700 and 4500 million years respectively. Studies of other stars allow us to estimate the Sun's past and future.
Cool-star research in the X-ray Astronomy Group is currently focussed on the areas of:
Star clusters allow us to investigate the properties of a sample of stars all with approximately the same age and chemical-element composition. By observing a number of clusters we can compare stellar properties at different ages and for different compositions. The X-ray image to the right shows the central region of the Hyades star cluster.
Our programme is currently based around observations with ROSAT and ASCA. From 1999/2000, we look forward to extending our programme to include observations with XMM and AXAF. We have several on-going Leicester-based and international collaborative programmes using ROSAT, ASCA, and associated multiwavelength data.
Contact: John Pye (pye@star.le.ac.uk)
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