HST CALSPEC Flux Standards: Sirius (and Vega)

The Space Telescope Imaging Spectrograph (STIS) has measured the flux for Sirius from 0.17--1.01~$\mu$m on the \emph{HST} White Dwarf scale. Because of the cool debris disk around Vega, Sirius is commonly recommended as the primary IR flux standard. The measured STIS flux agrees well with predictions of a special Kurucz model atmosphere, adding confidence to the modeled IR flux predictions. The IR flux agrees to 2--3% with respect to the standard template of Cohen and to 2% with the MSX absolute flux measurements in the mid-IR. A weighted average of the independent visible and mid-IR absolute flux measures implies that the monochromatic flux at 5557.5~\AA\ (5556~\AA\ in air) for Sirius and Vega, respectively, is $1.35\times10^{-8}$ and $3.44\times10^{-9}$~erg cm$^{-2}$ s$^{-1}$ \AA$^{-1}$ with formal uncertainties of 0.5%. Contrary to previously published conclusions, the Hipparcos photometry offers no support for the variability of Vega. Pulse pileup severely affects the Hp photometry for the brightest stars.