New bright optical spectrophotometric standards: A-type stars from the STIS Next Generation Spectral Library

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Exoplanets have sparked interest in extremely high signal-to-noise ratio spectroscopic observations of very bright stars, in a regime where flux calibrators, in particular DA white dwarfs, are not available. We argue that A-type stars offer a useful alternative and reliable space-based spectrophotometry is now available for a number of bright ones in the range 3<V<8 mag. By means of comparing observed spectrophotometry and model fluxes, we identify 18 new very-bright trustworthy A-type flux standards for the optical range (400-800 nm), and provide scaled model fluxes for them. Our tests suggest that the absolute fluxes for these stars in the optical are reliable to within 3%. We limit the spectral range to 400-800 nm, since our models have difficulties to reproduce the observed fluxes in the near-infrared and, especially, in the near-UV, where the discrepancies rise up to 10%. Based on our model fits, we derive angular diameters with an estimated accuracy of about 1%.