Astronomers have long puzzled over the origins of Type Ia supernovae, powerful cosmic explosions critical for understanding the universe’s expansion. These events are thought to arise from white dwarfs in binary systems, but the exact processes leading to their ignition remain unclear. In their study, Chickles et al. present the discovery of ATLAS J1138-5139, an ultracompact binary white dwarf system that could illuminate one pathway. This system's extreme orbital compactness makes it a detectable gravitational wave source and a promising Type Ia progenitor.

Discovery and Characteristics of ATLAS J1138-5139

ATLAS J1138-5139 was discovered through the ATLAS survey, which monitors sky brightness variations. The system consists of two white dwarfs in a binary orbit lasting only 28 minutes. Observations with telescopes like TESS and ULTRACAM confirmed significant phenomena, including tidal distortions and mass transfer between the stars.

The system’s primary star is a carbon-oxygen white dwarf, while the secondary is a helium-core white dwarf. The carbon-oxygen white dwarf is gaining mass from the helium donor, creating an accretion disk around it. The mass of the primary star, slightly above one solar mass, suggests it is near the threshold for potential Type Ia supernova ignition.

Mass Transfer and Evolution

In ATLAS J1138-5139, mass transfer is key. The helium-core donor star has expanded to fill its Roche lobe, spilling material onto its denser companion. This process, combined with the system's compact orbit, positions the primary white dwarf as a candidate for either a supernova or stable binary evolution. The exact outcome depends on the rate of accretion and the amount of helium accumulated on the primary star.

Stable accretion could transform the system into an AM CVn-type binary, with the donor’s hydrogen shell eventually stripped to expose helium. However, if the accretion rate is high, it could trigger a detonation, leading to a full Type Ia supernova.

Implications for Gravitational Wave Astronomy

ATLAS J1138-5139 provides a rare opportunity to study gravitational waves from compact binaries. Its orbital period generates detectable gravitational wave signals, making it a prime candidate for observation by upcoming instruments like the Laser Interferometer Space Antenna (LISA). The system’s high signal-to-noise ratio reinforces its potential to serve as a benchmark for future detections of similar systems.

Broader Impact on Supernova Studies

The discovery highlights a possible progenitor pathway for Type Ia supernovae, offering insights into their frequency and origins. Current models suggest these explosions may result from double-detonation events, where helium on the surface of the accretor ignites the core. Systems like ATLAS J1138-5139, with their extreme conditions, may account for a significant fraction of these events. Additionally, observations of hypervelocity stars—stars ejected by supernovae—align with the characteristics of the donor star in this binary. This connection strengthens the case for this system being a near-future supernova progenitor.

Future Observations and Models

To confirm the fate of ATLAS J1138-5139, continued monitoring is essential. Observations will track mass transfer rates, orbital decay from gravitational waves, and signs of helium accumulation. By comparing these data with theoretical models, astronomers can refine predictions for binary evolution and supernova triggers.

Conclusion

ATLAS J1138-5139 represents a breakthrough in understanding Type Ia supernova progenitors and their connections to gravitational waves. The system’s extreme compactness and active mass transfer provide a living laboratory for studying these phenomena. With instruments like LISA on the horizon, the future holds promise for unraveling the mysteries of these cosmic explosions and their role in the universe.

Source: Chickles