2012MNRAS.426.2819S

The spectrophotometric variability of the classical magnetic chemically peculiar star a Cen in the ultraviolet spectral region from 1150 to 3100Å is investigated. This study is based on archival International Ultraviolet Explorer data obtained at various phases of the rotational cycle. The light variations in wavelengths shorter than lambda\mn 1616\mspaceÅ are mainly in antiphase to the light variations in the longer-wavelength region. The brightness of the star in the spectral region lambdalambda\mn 1616\mtext –\mn 1640\mspaceÅ is constant over the period of rotation, which means that a so-called `null wavelength region' exists over these wavelengths. There is also a second `null wavelength region' in the spectrum of a Cen. According to our investigation, the semi-amplitude of the light variations is practically zero over the period of rotation in the spectral region lambdalambda\mn 1244\mtext –\mn 1343\mspace Å. The exception is the light variation in the depressions at lambdalambda\mn 1265 and 1300 Å, which are formed by resonance lines of silicon. However, the fluxes are constant at the cores of some features and in small depressions. An explanation for the light variations of a Cen is flux redistribution through bound–free and bound–bound silicon transitions combined with an inhomogeneous surface distribution of silicon on the stellar surface. The influence of other elements on the flux redistribution is discussed.