THE ULTRAVIOLET CALIBRATION OF THE HUBBLE SPACE TELESCOPE .4. ABSOLUTE IUE FLUXES OF HUBBLE SPACE TELESCOPE STANDARD STARS

Absolute UV fluxes are presented for 37 stars selected as calibration standards for the Hubble Space Telescope (HST). In order to provide data of maximum precision, new absolute calibrations have been derived for the International Ultraviolet Explorer (IUE) SWP and LWR cameras for point sources in the large apertures, while the sensitivities of the trail and small aperture observing modes relative to the large apertures are updated for all three IUE cameras. More than 2700 individual spectra are used to define the absolute flux distributions of the 37 HST standard stars in the wavelength range 1150-3300 Å. A comparison with white dwarf model atmosphere calculations suggests that the systematic external errors in the fluxes are less than 15%, while comparison with ANS flux measurements demonstrates an internal consistency of the IUE spectrophotometry of 2%, over a dynamic range of 104, in bandpasses of 100 Å or greater. The IUE fluxes at the longest wavelengths agree with ground-based data to an accuracy of 3%.