Comparison of White Dwarf Models with STIS Spectrophotometry

Computed flux distributions for four white dwarf (WD) stars with nearly pure hydrogen atmospheres are tied to the Vega flux scale with V-band Landolt photometry. With broadband photometric precision in the range 0.2%–0.4%, Space Telescope Imaging Spectrograph (STIS) low-dispersion spectrophotometry tightly constrains the relative continuum fluxes of the various model atmosphere computations. The set of non-LTE (NLTE) model continua has residuals that are slightly less than the residuals with respect to LTE models or with respect to a NLTE set that includes a metal line–blanketed model for G191B2B. The LTE models have Balmer line profiles that are matched to ground-based observations, while a consistent NLTE Balmer line analysis has not been published for all four stars. Residuals with respect to the LTE model continua are less than ~1% from 2000 to 9000 A. Total uncertainties in the adopted absolute flux distributions for the four WD standard stars decrease from 4% at 1300 A to 2% at 5000–10000 A and include estimates for all systematic effects. The STIS sensitivity in the broad hydrogen lines is uncertain with respect to the adjacent continuum by 2%–3% in the wings and by up to 5% in the line cores, because the flux in the wings differs among the various models and because the observed cores are contaminated by out-of-band light in the wide 52'' × 2'' calibration slit.