2000A&A...357..587K

The standard explanation for the presence of photospheric metals in white dwarfs of spectral type DZ and related types is a combination of episodic accretion from the interstellar medium and diffusion of heavy elements downwards in the atmosphere on time scales of typically 10⁶ years. Much of the empirical data on atmospheric parameters and element abundances available in the literature is based on International Ultraviolet Explorer spectra with low signal-to-noise and on atmospheric modeling, which does not include recent advances in input physics. We have therefore decided to address this problem again, using modern UV observations of the Hubble Space Telescope (HST), if available, and a new generation of theoretical models. In this paper we start with an analysis of the DZA white dwarfs L745-46A and Ross 640. These are key objects since they contain both hydrogen and metals in their atmospheres. Excellent UV spectra obtained with the Faint Object Spectrograph (FOS) onboard HST make them good test cases for the methods to be applied in future work on other objects and for the accuracy, which can be achieved. We have determined the atmospheric parameters T_eff and logg as well as hydrogen abundances and abundances of several heavy elements. The results are in good agreement with the theoretical predictions of Dupuis and collaborators. Particular emphasis is placed on a discussion of the remaining problems in the input physics and the corresponding uncertainties of the final results.