Metal abundances in helium-rich white dwarf atmospheres.

Abstract

IUE observations of L745-46A, L119-34 and Ross 640 reveal the presence of Si, Fe, C, and very strong Mg II lines. These spectra, together with optical observations of white dwarfs with Ca II lines (type DZ) are analyzed within a network of model atmospheres with varying H/He, C/He and metals/He ratios. The influence of non-negligible H content (electron donor) is discussed. The final abundances are presented. Carbon is present in amounts comparable, perhaps somewhat smaller, than in white dwarfs of spectral type DQ (C₂ and UV C I lines). Its presence supports the carbon dredge-up hypothesis.