Comparison of White Dwarf Models with STIS Spectrophotometry

Abstract

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.