The Absolute Flux Distribution of LDS749B

Abstract

Observations from the Space Telescope Imaging Spectrograph define the flux of the DBQ4 star LDS749B from 0.12-1.0 μm with an uncertainty of ~1% relative to the three pure hydrogen WD primary HST standards. With T_{eff}=13575 K, log g=8.05, and a trace of carbon at <1x10^{-6} of solar, a He model atmosphere fits the measured STIS fluxes within the observational noise, except in a few spectral lines with uncertain physics of the line broadening theory. Upper limit to the atmospheric hydrogen and oxygen fractions by number are 1x10^{-7} and 7x10^{-10}, respectively. The excellent agreement of the model flux distribution with the observations lends confidence to the accuracy of the modeled IR fluxes beyond the limits of the STIS spectrophotometry. The estimated precision of ~1% in the predicted IR absolute fluxes at 30 μm should be better than the model predictions for Vega and should be comparable to the absolute accuracy of the three primary WD models.