Free Astronomy Magazine

STELLAR EVOLUTION in the spectra of some white dwarfs, it was thought that those signals originated from circumstellar material interposed between the star and the observer. A few years later, the soundness of this explanation was chal- lenged as a result of more recent observa- tions by another satellite, the EXOSAT (Eu- ropean X-ray Observatory Satellite) which al- lowed to determine that the metals were in the dwarfs’ atmospheres and not in orbit around them. To explain this new unexpected scenario, re- searchers began to consider more favourably the hypothesis according to which the de- finitive sinking of the metals may have been hindered by the radiation pressure genera- ted inside those very hot de- generate stars – a mechanism already proposed in the late ‘70s and known as “radiative levitation”. Overall, though, the sample of white dwarfs examined at the end of the ‘80s was still too small to draw general conclusions. The broad picture began to emerge with some clarity in the ‘90s, when the satellite for high energy astronomy ROSAT (Röentgen Satellite), the afore- said IUE and the Hubble Space Telescope allowed to investi- gate with sufficient accuracy the ultraviolet and X-ray re- gions of the spectrum of near- ly a hundred white dwarfs. For about a quarter of them the atmospheric composition is well defined and includes several metals, among which nitrogen, car- bon, iron, nickel, oxy- gen and silicon. But the measured abun- dances do not at all correspond to those theoretically predic- ted, and this regard- less of the tempera- ture range used and excluding any prefer- ences introduced by the methods used to study individual stars. The metals could there- fore largely derive from external sources. The temperature is a key parameter in radia- tive levitation, since the stronger the heat flux rising from the centre of white dwarfs (where it can reach more than 20 million kelvin), and more abundant, in terms of type and quantity, are the metals that in rising to the surface leave their “fingerprint” in the stars’ spectra. For “cooler” white dwarfs, those with surface temperatures below 20,000 kelvin, radiative levitation has little or no significant effect and any possi- ble metal contamination of external origin should disappear in a few days as predicted A nimation of the scenario depicted in the opening illustra- tion. [NASA, ESA, STScI, and G. Bacon (STScI)] Below: a plane- tary system de- stroyed by the evolution of a solar-type star, which, through the red giant phase has reached the white dwarf stage. Only plan- ets outside the Roche limit re- main intact. [NASA/JPL- Cal- tech/T. Pyle]