New Faint Optical Spectrophotometric Standards. Hot White Dwarfs from the Sloan Digital Sky Survey

TL;DR

This paper identifies nine new hot white dwarf stars from SDSS data to serve as precise optical spectrophotometric standards, enhancing calibration accuracy for astronomical observations.

Contribution

It introduces a method to select and calibrate a network of DA white dwarfs as spectrophotometric standards using SDSS data and model atmospheres, improving calibration coverage.

Findings

Nine white dwarfs with <3% flux uncertainty identified

Four stars with <2% flux uncertainty suitable as standards

Good agreement with HST standards confirmed

Abstract

The spectral energy distributions for pure-hydrogen (DA) hot white dwarfs can be accurately predicted by model atmospheres. This makes it possible to define spectrophotometric calibrators by scaling the theoretical spectral shapes with broad-band photometric observations -- a strategy successfully exploited for the spectrographs onboard the Hubble Space Telescope (HST) using three primary DA standards. Absolute fluxes for non-DA secondary standards, introduced to increase the density of calibrators in the sky, need to be referred to the primary standards, but a far better solution would be to employ a network of DA stars scattered throughout the sky. We search for blue objects in the sixth data release of the Sloan Digital Sky Survey (SDSS) and fit DA model fluxes to identify suitable candidates. Reddening needs to be considered in the analysis of the hottest and therefore more distant stars. We propose a list of nine pure-hydrogen white dwarfs with absolute fluxes with estimated uncertainties below 3%, including four objects with estimated errors <2%, as candidates for spectrophotometric standards in the range 14<g<18, and provide model-based fluxes scaled to match the SDSS broad-band fluxes for each. We apply the same method to the three HST DA standards, linking the zero point of their absolute fluxes to ugr magnitudes transformed from photometry obtained with the USNO 1-m telescope. For these stars we estimate uncertainties of <1% in the optical, finding good consistency with the fluxes adopted for HST calibration.