Tracing the Origins of a Cosmic Pair: Unveiling the History of the Ultra-Compact Binary ZTF J2252−05

In their 2025 study, W. Yu and collaborators examine the AM Canum Venaticorum (AM CVn) system ZTF J2252−05, a pair of closely orbiting stars where a white dwarf pulls in helium-rich material from its small, hydrogen-poor companion. These rare systems are important for understanding the final stages of stellar evolution and may even lead to Type Ia supernovae, explosions used to measure cosmic distances. Using ultraviolet data from the Hubble Space Telescope (HST), Yu’s team determined the system’s composition and structure to uncover how it formed.

Three Possible Origins for AM CVn Systems

The researchers considered three potential formation routes for AM CVn binaries. In the white dwarf channel, both stars are white dwarfs that spiral together until one begins transferring helium; in the helium-star channel, the donor is a semi-degenerate helium star; and in the cataclysmic variable (CV) channel, the system starts with a hydrogen-rich star that later loses its outer layers. Each path produces unique chemical signatures, particularly in the nitrogen-to-carbon (N/C) ratio, which becomes the key diagnostic for tracing a system’s origin.

Observations with Hubble and Supporting Data

Yu’s team observed ZTF J2252−05 with Hubble’s Cosmic Origins Spectrograph, targeting ultraviolet wavelengths where crucial elements like nitrogen, silicon, and carbon leave distinct marks. Because AM CVn stars vary in brightness, the team coordinated with ground-based telescopes and citizen scientists to confirm the system was in a stable, faint state before observation. The resulting data showed that the white dwarf supplies about 84% of the ultraviolet light, while the bright spot, the impact point where gas hits the disk, provides the rest.

Modeling the Atmosphere of the White Dwarf

By fitting the HST spectrum with models of hydrogen-deficient white dwarfs, the researchers found that the accreting star has a temperature of 23,300 K, surface gravity log g = 8.4, and mass around 0.86 times the Sun’s. They detected nitrogen, silicon, aluminum, and iron in its atmosphere but almost no carbon or hydrogen, setting a nitrogen-to-carbon ratio above 153, one of the highest ever measured in an AM CVn system. These measurements were refined using advanced statistical fitting and corrections for interstellar dust.

Decoding the System’s Evolution

Comparing their results with stellar evolution models, Yu et al. determined that ZTF J2252−05 could not have formed through the helium-star channel, which would produce a lower N/C ratio. Instead, the system’s properties match either the white dwarf or cataclysmic variable channel, both of which yield similar nitrogen-rich material. The white dwarf’s temperature also proved significantly higher than earlier optical estimates, showing that ultraviolet spectroscopy provides far more accurate insight into these compact binaries.

A Step Toward a Broader Picture

ZTF J2252−05 serves as a benchmark system for developing detailed spectroscopic models of AM CVn binaries. Yu and colleagues’ work demonstrates that ultraviolet data can reveal key clues about the chemical and evolutionary history of these stars, paving the way for a larger survey of similar systems. Expanding this analysis to more binaries will help astronomers map how such ultra-compact pairs form, evolve, and contribute to the broader population of white dwarfs and potential supernova progenitors.

Source: Yu