New Grids of Pure-Hydrogen White-Dwarf NLTE Model Atmospheres \newline and the HST/STIS Flux Calibration

TL;DR

This paper presents new grids of pure-hydrogen NLTE white dwarf atmosphere models that improve flux calibration accuracy across a broad wavelength range, enhancing the reliability of flux standards for the Hubble Space Telescope.

Contribution

The paper introduces updated NLTE model atmospheres for hot white dwarfs, achieving better agreement with observed fluxes from the far-ultraviolet to mid-infrared wavelengths.

Findings

Improved flux agreement to 1% from 1500 Å to 30 μm.

Enhanced consistency of white dwarf flux standards across a broad spectrum.

Refined models used for HST flux calibration and the CALSPEC database.

Abstract

Non-local Thermodynamic Equilibrium (NLTE) calculations of hot white dwarf (WD) model atmospheres are the cornerstone of modern flux calibrations for the Hubble Space Telescope (HST) and for the CALSPEC database. These theoretical spectral energy distributions (SEDs) provide the relative flux vs. wavelength, and only the absolute flux level remains to be set by reconciling the measured absolute flux of Vega in the visible with the Midcourse Space Experiment (MSX) values for Sirius in the mid-IR. The most recent SEDs calculated by the \textsc{tlusty} and \textsc{tmap} NLTE model atmosphere codes for the primary WDs G191-B2B, GD153, and GD71 show improved agreement to 1\% from 1500~\AA\ to 30~\micron, in comparison to the previous 1\% consistency only from 2000~\AA\ to 5~\micron. These new NLTE models of hot WDs now provide consistent flux standards from the FUV to the mid-IR.