White Dwarf Binaries: Probes of Future Astrophysics

White dwarf binaries are fundamental astrophysical probes. They represent ideal laboratories to test the models of binary evolution, which also apply to the sources of gravitational waves, whose detection led to the award of the 2017 Nobel Prize in Physics. Moreover, their final fate is intimately linked to Type Ia Supernovae (SNe Ia), i.e. the thermonuclear explosion of a white dwarf following the interaction with a companion star, which have become the fundamental yardsticks on cosmological distance scales and led to the discovery of dark energy and the award of the 2011 Nobel Prize in Physics. Finally, white dwarf binaries play a crucial role in influencing star formation and chemical evolution of the Galaxy by injecting energy into, and enriching, the interstellar medium with material ejected during nova eruptions and SN Ia explosions. In the next decade, the advent of the Large Synoptic Survey Telescope (LSST) at the Vera Rubin Observatory will lead to the discovery of hundreds of thousands of white dwarf binaries. Nonetheless, the intrinsic faintness of the majority of these systems will prevent their spectroscopic characterisation with the instruments available in the 2030s. Hence ESO’s Expanding Horizons call is timely for planning a future transformative facility, capable of delivering phase-resolved spectroscopic observations of faint white dwarf binaries, which are key to advancing our understanding of stellar and Galactic evolution and cosmology.

💡 Research Summary

White dwarf binaries (WDBs) are uniquely powerful laboratories for a broad range of fundamental astrophysical problems. They trace binary evolution, serve as the dominant low‑frequency gravitational‑wave (GW) foreground in the Milky Way, act as progenitors of Type Ia supernovae (SNe Ia), and inject chemically enriched material and kinetic energy into the interstellar medium (ISM) through nova eruptions and supernova explosions. The authors argue that the next decade will see an explosion of WDB detections thanks to Gaia Data Releases 4 and 5, the Large Synoptic Survey Telescope (LSST) at the Vera Rubin Observatory, and wide‑field multi‑object spectroscopic facilities such as 4MOST, WEAVE, SDSS‑V, and DESI. However, the bulk of the LSST‑identified population will be intrinsically faint (G ≈ 23) and lie beyond the reach of current high‑resolution spectrographs, which are limited to G ≈ 16–20. Consequently, crucial parameters—orbital periods, component masses, effective temperatures, abundances, and rotation rates—will remain inaccessible for the majority of systems, especially the compact sub‑populations (cataclysmic variables with brown‑dwarf donors, AM CVn stars, and double white dwarfs).

The paper identifies four key science drivers for the 2040s: (1) constraining the efficiency of the common‑envelope (CE) phase and magnetic wind braking by obtaining statistically complete, volume‑limited samples of WDBs with well‑measured orbital and stellar parameters; (2) characterising the Galactic GW foreground by measuring masses and periods of millions of double white dwarfs, thereby improving foreground subtraction for space‑based detectors such as LISA, Tian‑Qin, and the proposed Lunar GW Antenna; (3) discriminating among the single‑degenerate, double‑detonation, and double‑degenerate channels of SNe Ia by building a large, homogeneous set of precise white dwarf masses across evolutionary stages; and (4) quantifying the contribution of novae and SNe Ia to ISM enrichment, which is essential for models of star formation and chemical evolution in galaxies.

To achieve these goals, the authors propose a transformative, next‑generation spectroscopic facility operational in the 2040s. The instrument must deliver phase‑resolved spectroscopy for all LSST‑identified WDBs down to G ≈ 23, with signal‑to‑noise ≥ 5 and a resolving power R > 20 000, capable of sampling orbital periods from 5 minutes to 1 day. Such capability would enable (i) direct measurement of CE ejection efficiencies via space‑density comparisons of post‑CE binaries, accreting systems, and double white dwarfs; (ii) precise mass and period distributions for the millions of double white dwarfs that dominate the GW foreground; (iii) a statistical census of white dwarf masses that can be linked to SNe Ia rates and delay‑time distributions; and (iv) robust estimates of nova and supernova yields feeding back into ISM enrichment models.

The authors stress that while upcoming facilities (e.g., ELT‑class ANDES) will push high‑resolution spectroscopy to G ≈ 18–20, they will still be limited to a few hundred objects, far short of the required sample size. Therefore, a dedicated 8–10 m class telescope equipped with a multiplexed, high‑resolution, time‑domain spectrograph (potentially fiber‑fed integral‑field units) is essential. By providing comprehensive, phase‑resolved spectroscopic data for the full WDB population, this facility would close the current observational gap, transform our understanding of binary evolution physics, refine predictions for future GW missions, clarify the progenitor pathways of SNe Ia, and improve models of Galactic chemical evolution—making it a cornerstone of 2040s astrophysics.