Featureless stars: Flux Calibration for Extremely Large Telescopes

TL;DR

This work tackles the persistent challenge of sub-percent spectrophotometric flux calibration in large surveys by identifying a network of 29 bright featureless white dwarfs whose spectral energy distributions closely follow a blackbody from the optical to near-infrared. Through Gaia CMD-based candidate selection, Gaia BP/RP spectroscopy, and a broad multi-instrument spectroscopic campaign, the authors define a robust set of featureless standards and calibrate cross-survey photometry by deriving AB magnitude offsets for Gaia, SDSS, SMSS, PanSTARRS, DES, and 2MASS, while validating consistency with GALEX and WISE. They model the stars with Planck functions, determine per-filter systematic uncertainties, and provide a public framework (including PypeIt integration) to apply these corrections, enabling sub-percent flux calibration for future facilities like ELTs, Rubin, Euclid, and Roman. The study also reveals UV deviations in some stars, suggesting further ultraviolet spectroscopy is needed to fully understand their atmospheres, and proposes an updated Gaia CMD relation (with a quadratic term) to guide future target selection. Overall, the 29 bright featureless standards offer a practical, sky-covering calibration resource for next-generation astronomical surveys and cosmology experiments.

Abstract

The spectrophotometric flux calibration of recent spectroscopic surveys has reached a limiting systematic precision of approximately 1-3 percent, and is often biased near the wavelengths associated with H I Balmer absorption. As we prepare for the next generation of imaging and spectroscopic surveys, and high-precision cosmology experiments, we must find a way to address this systematic. Towards this goal, we have identified a global network of 29 bright (G < 17.5) featureless white dwarf stars that have a spectral energy distribution consistent with an almost pure blackbody form over the entire optical and near-infrared wavelength range. Based on this sample, we have computed the systematic uncertainty and AB magnitude offsets associated with Gaia, SDSS, SMSS, PanSTARRS, DES, and 2MASS, and we have also checked the consistency of our objects with both GALEX and WISE. The magnitude range of the featureless stars reported here are ideally suited to observations taken with the forthcoming generation of extremely large telescopes, as well as calibrating the survey data acquired by the Rubin, Euclid and Roman observatories. Finally, all of the high-precision spectrophotometric standard stars reported here have been included in the latest release of the PypeIt data reduction pipeline.

Figures (16)

• Figure 1: The SuzukiFukugita2018 blackbody star sample (black points with error bars) occupy a tight relationship on the Gaia colour-magnitude diagram. The solid line represents a linear fit to the data, where the dark and light shaded regions indicate the 68 and 95 percent confidence intervals, respectively.
• Figure 2: Both panels show the Gaia colour-magnitude diagram for white dwarfs, with our candidate selection box shown as the red polygon. The left panel displays a histogram of the Gaia white dwarfs within 220 pc of the Sun, colour-coded by the stellar density. The right panel shows the distribution of our candidate featureless stars (black symbols) after removing white dwarf stars that display absorption lines, based on Gaia BP/RP spectroscopy.
• Figure 3: Example spectra of our candidate featureless white dwarf stars, color-coded by the equivalent width of the absorption line feature (see colorbar). From top to bottom, we show the absorption features He i$\lambda5876$, the Ca ii$\lambda\lambda3933,3968$ doublet, the ${\rm C}_{2}$ Swan band, and H$\alpha$.
• Figure 4: Equivalent widths of the four diagnostic absorption lines (see panel labels) used in our study as a function of the Gaia BP/RP colour. Absorption lines that are detected at $>3\sigma$ confidence are shown by the blue/green/yellow symbols, colour-coded by the absolute magnitude. The red symbols show $3\sigma$ upper limits on features that are undetected. We adopt a $3\sigma$ limiting equivalent width of 400 mÅ for Ca ii$\lambda3933$, C$_{2}$ Swan, and H$\alpha$, while we use a $3\sigma$ limiting equivalent width of 300 mÅ for He i$\lambda5876$ (as indicated by the horizontal black dashed line in all panels). Given the strong trend of He i in the top panel, we also assume that all stars with $G_{\rm BP}-G_{\rm RP}\geq-0.20$ exhibit He i$\lambda5876$ absorption weaker than our 300 mÅ cutoff. The coloured curves shown in the top and bottom panel are based on the 1D DBA/DB/DC models of Cukanovaite2021, where the colour of the curve represents a surface gravity of $\log\,g=7.0,~7.5,~8.0,~8.5,~9.0$ (expressed in cgs units, from red to blue). The linestyle represents a He/H abundance of $10^{2}$, $10^{5}$, and $10^{8}$ (solid, dashed, dotted, respectively). Note that each curve represents a range of atmosphere temperatures; the model temperature is anti-correlated with the Gaia colour. In the top panel, we only display the ${\rm He/H}=10^{5}$ models, since all models are qualitatively similar.
• Figure 5: Sky distribution of our candidates (blue $\times$ symbols) and stars that are featureless down to a $3\sigma$ limiting equivalent width of 400 mÅ (red circles; see text for further details). We also plot the SuzukiFukugita2018 blackbody star sample as yellow circles. Note that one star (J1245$+$4238) is in common between our sample and SuzukiFukugita2018. The dark gray band marks the region within $\pm10^{\circ}$ of the Galactic plane, while the light gray region at $\delta>72^{\circ}$ marks the location that we could not observe candidates due to our telescope access.