Formation of the double white dwarf binary PTF J0533+0209 through stable mass transfer?

TL;DR

This paper models the formation of the double white dwarf binary PTF J0533+0209 via stable mass transfer, challenging the common-envelope scenario, and explores its future evolution into a gravitational wave source detectable by space observatories.

Contribution

It introduces a new formation model for PTF J0533+0209 using magnetic braking and stable mass transfer, providing insights into DWD formation pathways.

Findings

Model reproduces observed WD masses and orbital period

Discrepancy in effective temperature and hydrogen abundance

Future evolution into an AM CVn binary for GW detection

Abstract

Double white dwarf (DWD) binaries are important for studies of common-envelope (CE) evolution, Type Ia supernova progenitors and Galactic sources of low-frequency gravitational waves (GWs). PTF J0533+0209 is a DWD system with a short orbital period of Porb ~ 20 min and thus a so-called LISA verification source. The formation of this system and other DWDs is still under debate. In this paper, we discuss the possible formation scenarios of this binary and argue that it is not likely to have formed through CE evolution. Applying a new magnetic braking prescription, we use the MESA code to model the formation of this system through stable mass transfer. We find a model which can well reproduce the observed WD masses and orbital period but not the effective temperature and hydrogen abundance of the low-mass He WD component. We discuss the possibility of using H flashes to mitigate this discrepancy. Finally, we discuss the future evolution of this system into a AM CVn binary such as those that will be detected by space-borne GW observatories like LISA, TianQin and Taiji.