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

TL;DR

This paper identifies 18 new bright A-type stars as reliable optical spectrophotometric standards, providing scaled model fluxes and angular diameters with high accuracy, addressing the need for flux calibrators in high S/N spectroscopic studies of bright stars.

Contribution

It introduces a new set of trustworthy A-type flux standards for the optical range, expanding calibration options beyond white dwarfs, with validated fluxes and angular diameters.

Findings

18 new bright A-type stars identified as flux standards

Optical fluxes reliable within 3% accuracy

Angular diameters estimated with about 1% precision

Abstract

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%.