THE TEMPERATURE SCALE OF HOT DA WHITE-DWARFS - TEMPERATURES FROM FAR-ULTRAVIOLET CONTINUUM FLUXES

Effective temperatures have been derived for all DA white dwarfs hotter than ∼25,000 K for which suitable spectra have been obtained with the International Ultraviolet Explorer (WE). The temperature determinations were made by a comparison of the observed far-ultraviolet (FUV) to visible flux ratios with the ratios predicted by models. This method gives temperatures that are comparable in accuracy with temperatures based on fitting hydrogen line profiles, when the nominal IUE fluxes have been suitably corrected. Three separate levels of correction are necessary. For reasonable photometric accuracy, the IUE fluxes must be adjusted to compensate for the time-dependent sensitivity degradation of the IUE cameras. The degradation is wavelength-dependent and exceeds 20% at some wavelengths at the current epoch. Compensation for this degradation can be made per the prescription of Bohlin (as reported by Bohlin and Grillmair in 1988). It is also necessary to correct for the temperature dependency of the camera sensitivity (a ≲2% effect). However, after using the above corrections to bring the IUE fluxes into accordance with the 1980 IUE absolute flux calibration, we found that the white dwarf fluxes were still inconsistent with model fluxes up to the 20% level. (Subsequent revisions to the IUE absolute flux calibration reported by Bohlin in 1986 and 1988 do not reduce this inconsistency.) Therefore, we used all the suitable IUE spectra of seven hot DA white dwarfs for which accurate temperatures based on hydrogen line profiles were available to derive a flux correction to the IUE absolute calibration. Application of the flux corrections to the IUE spectra resulted in derived temperatures that were consistent with the line profile temperatures. Given that the FUV fluxes of hot DA white dwarfs can be modeled much more accurately than for any other class of object, it is suggested that this "white dwarf" calibration should serve as the basic calibration for IUE, and by extension, for the Hubble Space Telescope.