HST CALSPEC Flux Standards: Sirius (and Vega)

TL;DR

This paper presents precise flux measurements of Sirius and Vega using HST's STIS, confirming their suitability as IR flux standards and providing improved flux values with high accuracy.

Contribution

It offers new, highly accurate flux standards for Sirius and Vega based on HST measurements, validating their use as primary IR flux standards.

Findings

Flux of Sirius measured from 0.17 to 1.01 μm with high precision.

IR flux of Sirius agrees with model atmosphere predictions within 2-3%.

Vega shows no evidence of variability in Hipparcos photometry.

Abstract

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.