Mechanism for the large positive period derivative in ZTF J1858–2024

Determine the physical mechanism driving the large positive orbital period derivative measured in the AM Canum Venaticorum system ZTF J1858–2024, and assess whether finite-entropy donors can account for the discrepancy between observations and current double white dwarf evolutionary models.

Background

ZTF J1858–2024 shows a significant positive period derivative, indicating outspiral, that is difficult to reproduce using standard binary evolution models under reasonable parameter choices.

The authors suggest that donor finite entropy might accelerate evolution but note uncertainty about whether this effect is large enough to match the observed rate, leaving the mechanism behind the rapid outspiral unresolved.

References

Our Eq. 17 accommodates this by pushing towards unphysically small ξad < −1/3, but in reality, we are uncertain exactly how the system is evolving so rapidly. It is possible that finite-entropy donors can accelerate the orbital evolution (e.g. Wong & Bildsten 2021), though whether this is a sufficiently large effect to account for the observed discrepancy is unclear.

— Expanding the ultracompacts: gravitational wave-driven mass transfer in the shortest-period binaries with accretion disks (2411.12796 - Chakraborty et al., 19 Nov 2024) in Section 4.3 (Further constraints based on P˙)