White Dwarf Binaries: Probes of Future Astrophysics

TL;DR

This paper argues that white dwarf binaries are powerful probes of binary evolution, gravitational wave astrophysics, SN Ia progenitors, and ISM enrichment. It outlines a 2040s vision in which a transformative facility enables phase-resolved spectroscopy of faint WD binaries identified by LSST, complemented by Gaia and other surveys to build comprehensive samples. The approach aims to constrain common-envelope efficiency, characterize the gravitational-wave foreground for space-based detectors, and clarify SN Ia pathways through precise mass and orbital measurements. The proposed capabilities would significantly advance our understanding of stellar and Galactic evolution and strengthen the cosmological utility of SN Ia standard candles.

Abstract

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.

Figures (2)

• Figure 1: Evolution of white dwarf binaries: in a main-sequence star binary (1), the more massive star evolves first and fills its Roche lobe (2), leading to the formation of a common envelope (3). The envelope ejection (4) leaves behind a PCEB (5). If the companion is more massive than the white dwarf, it fills its Roche lobe when it leaves the main sequence. The systems undergo a phase of high rate mass transfer, during which the binary is observed as a super-soft X-ray source (6), leading either to a CV with a nuclearly evolved donor (7), or a double white dwarf (9). The CV later evolves in an AM CVn (8). If the companion is less massive, angular-momentum losses shrink the orbit, bringing it in contact with its Roche lobe. The system can then become a CV (10), or enter a second common envelope phase (11). If, at its onset, the companion core is degenerate, a double white dwarf is formed (12), otherwise, a helium star–white dwarf binary is formed (13). Subsequent Roche-lobe overflow produces an AM CVn with either a white dwarf (14) or helium star (15) donor.
• Figure 2: Synthetic magnitudes for PCEBs (cyan) and double white dwarfs binaries (orange) within 3 kpc and brighter than $G \simeq 23$, computed using the Monte Carlo simulator from Santos-Garcia+2025. AWDs are accretion-powered, and their typical magnitudes (pink area) depend on the accretion rate. Gaia provides accurate magnitudes down to $G \simeq 20$ (dashed brown line), thus allowing obtaining volume-limited complete samples of PCEBs, double white dwarfs and AWDs out to $\simeq 100\,$pc (dashed blue line), $\simeq 150\,$pc (dashed orange line) and $\simeq 400\,$pc (dashed pink line), respectively. In the 2040s, we envisage a new facility capable of achieving $\mathit{SNR} \gtrsim 5$ down to $G \simeq 23$ (dashed black line), thus allowing to probe larger volumes, out to $\simeq 300\,$pc, $\simeq 350\,$pc and $\simeq 1600\,$pc, for PCEBs (dot-dashed blue line), double white dwarfs (dot-dashed orange line) and AWDs (dot-dashed pink line), respectively.