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The presence of silicon at the surface of the two Hyades DA white dwarfs WD0421+162 and WD 0431+126 requires mechanisms that counteract the effects of thedownward diffusion. Radiative levitation calculations indicate that the siliconabundance observed in WD 0431+126 corresponds to the abundance supported byradiative levitation. Detailed time-dependent diffusion calculations that takeinto account radiative levitation and accretion indicate that accretion withrates of dM/dt(Si) < 1.00E4 g/s could also be present without disrupting theabundance supported by radiative levitation. In the case of WD 0421+162,accretion with a rate of dM/dt(Si) = 1.25E5 g/s must be invoked, because itsobserved silicon abundance is larger than the abundance supported by radiativelevitation. This accretion rate is lower than the accretion rate given by theaccretion-diffusion model, because the radiative levitation slows down thedownward diffusion of silicon. The silicon abundances observed in the twoHyades white dwarfs cannot be interpreted solely in terms of accretion. Theinterpretation of the silicon abundances must take into account the interplaybetween radiative levitation and accretion.

Radiative Levitation of Silicon in the Atmospheres of Two Hyades DA White Dwarfs