IUE Echelle observations of the photospheres and circumstellar environments of hot white dwarfs

Abstract

For well over a decade, narrow, interstellar-like features have been observed in the high resolution ME spectra of many hot white dwarfs. It is now clear that these features originate in at least two different environments. In many of the hottest white dwarfs these features are formed in the stellar photosphere by trace amounts of key ions of C, N, O, Si, Fe and Ni. In these stars it is possible to establish photospheric abundances which can be directly compared with the theoretical predictions of radiative leviation. Such abundances are also critical to the interpretation the strong EUV and soft X-ray opacities observed in many white dwarfs. In other, primarily cooler, white dwarfs the narrow absorption features appear to be formed in circumstellar shells of unknown origin. Current ME observational programs together with archive spectra have yielded important new insights into the occurrence of heavy element absorption features in white dwarfs spectra.