Stellar Astrophysics - Interacting binaries

Interacting binaries are pairs of stars orbiting around each other so closely that the two stars affect each other or `interact' in some way. Gravitational interaction can lead to one or both stars being distorted from their normal spherical shape and can lead to gas (and hence mass) being transferred from one star to the other. Typically the mass-gaining star is a compact object such as a white dwarf, and as gas accretes onto it, gravitational energy is released, leading to a brightening of the system. Since accretion can occur spasmodically, these cataclysmic variable stars are characterised by dramatic variations in their brightness in many wavelengths. In non-magnetic binaries, the gas forms an accretion disc about the compact object. The study of cataclysmic variables allows us to test theories of mass transfer between stars, and the structure and behaviour of accretion discs. The latter in particular has wide applications in astrophysics, including in active galactic nuclei.

The study of cataclysmic variables at the Center for Astrophysics uses multi-wavelength observations of eclipsing systems (where one star passes in front of the other). This allows us to constrain exactly where the emission is coming from. For example, when an eclipse is seen in the X-ray flux at the same time as in the optical lightcurve, it indicates that the X-ray emission is from the surface of the compact object.

Detailed modelling of the X-ray spectrum enables us to determine the temperature of the boundary layer between the accretion disc and the compact object. It is also possible to deduce how much gas lies above the accretion disc, partially obscuring the compact object and the inner disc. Simultaneous X-ray and ultraviolet observations have confirmed that the measured density of this so-called `iron curtain' depends on which wavelength is being considered. The CfA's involvement with XMM-Newton will open up new avenues in this area.

Lead researcher: Dr Barbara Hassall

Other researchers: Dr Stewart Eyres

Header Image : V838 Mon and Tadpole Galaxy : credit NASA, H. Ford (JHU), G. Illingworth (UCSC/LO), M.Clampin (STScI), G. Hartig (STScI), the ACS Science Team, and ESA ; and solar plume courtesy of SOHO /EIT consortium

Author: SPSEyres. Last Updated: Friday, 10 February, 2006 15:04