A comparative study of optical and ultraviolet effective temperatures for DA white dwarfs from the IUE archive

We present a comparative study of effective temperatures determined from the hydrogen Balmer lines and from the UVenergy distribution for 140 DA white dwarfs drawn from the International Ultraviolet Explorer (IUE) archive. Our results indicate that the optical and UV temperatures of the majority of stars below T(eff) similar to 40; 000 K and within similar to 75 pc are in fairly good agreement given the uncertainties. At higher temperatures and/or larger distances, however, significant discrepancies are observed. Several mechanisms are investigated to account for these discrepancies, including the effect of interstellar reddening, the presence of metals in the photosphere, and the existence of unresolved binary white dwarfs. The results of our analysis reveal that wavelength-dependent extinction is the most natural explanation for the observed temperature differences. We also attempt to predict the differences in optical and UV temperatures expected from unresolved degenerate binaries by performing an exhaustive simulation of composite model spectra. In light of these simulations, we then discuss some known double degenerates and identify new binary candidates by restricting our analysis to stars located within 75 pc, where the effect of interstellar reddening is significantly reduced.