Cool white dwarfs as standards for infrared observations

Abstract

In the era of modern digital sky surveys, uncertainties in the flux of stellar standards are commonly the dominant systematic error in photometric calibration and can often affect the results of higher-level experiments. The Hubble Space Telescope (HST) spectrophotometry, which is based on computed model atmospheres for three hot (Teff > 30,000 K) pure-hydrogen (DA) white dwarfs, is currently considered the most reliable and internally consistent flux calibration. However many next generation facilities (e.g. Harmoni on E-ELT, Euclid and JWST) will focus on IR observations, a regime in which white dwarf calibration has not yet been robustly tested. Cool DA white dwarfs have energy distributions that peak close to the optical or near-IR, do not have shortcomings from UV metal line blanketing, and have a reasonably large sky density ($\simeq$ 4 deg${}^{-2}$ at G < 20), making them, potentially, excellent calibrators. Here we present a pilot study based on STIS+WFC3 observations of two bright DA white dwarfs to test whether targets cooler than current hot primary standards (Teff < 20,000 K) are consistent with the HST flux scale. We also test the robustness of white dwarf models in the IR regime from an X-shooter analysis of Paschen lines and by cross-matching our previously derived Gaia white dwarf catalogue with observations obtained with 2MASS, UKIDSS, VHS, and WISE.

12 pages, 8 figures, 2 tables. Accepted for publication in MNRAS. The best fitting models to the two white dwarfs analyzed are available from \<http://deneb.astro.warwick.ac.uk/phrhcc/WD_models.tar>

References in corpus (8)

• The UKIRT Infrared Deep Sky Survey (UKIDSS)
• Molecfit: A general tool for telluric absorption correction. I. Method and application to ESO instruments
• Techniques and Review of Absolute Flux Calibration from the Ultraviolet to the Mid-Infrared
• On the Measurement of Fundamental Parameters of White Dwarfs in the Gaia Era
• The Euclid mission design
• A New Generation of Cool White Dwarf Atmosphere Models. IV. Revisiting the Spectral Evolution of Cool White Dwarfs
• The Gaia DR1 Mass-Radius Relation for White Dwarfs
• A possible solution to the Lyman/Balmer line problem in hot DA white dwarfs

Cited by in corpus (2)

• $Gaia$ white dwarfs within 40 pc II: the volume-limited northern hemisphere sample
• $\text{Gaia}$ white dwarfs within 40 pc I: spectroscopic observations of new candidates