Comparing population synthesis models of compact double white dwarfs to electromagnetic observations

¹ Department of Astrophysics/IMAPP, Radboud University, PO Box 9010, 6500 GL, Nijmegen, The Netherlands
² Leiden Observatory, Leiden University, Einsteinweg 55, 2333 CC, Leiden, The Netherlands
³ Department of Physics, University of Auckland, Private Bag 92019, Auckland, New Zealand
⁴ Anton Pannekoek Institute for Astronomy, University of Amsterdam, 1090 GE, Amsterdam, The Netherlands
⁵ SRON, Netherlands Institute for Space Research, Niels Bohrweg 4, 2333 CA Leiden, The Netherlands
⁶ Institute of Astronomy, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
⁷ Max Planck Institute for Astrophysics, Karl-Schwarzschild-Straße 1, 85748 Garching, Germany

^★ Corresponding author: wouter.vanzeist@astro.ru.nl

Received: 27 February 2025
Accepted: 9 May 2025

Abstract

Context. Studies of the Galactic population of double white dwarfs (DWDs) that would be detectable in gravitational waves by LISA have found differences in the number of predicted detectable DWDs of more than an order of magnitude, depending on the binary stellar evolution model used. Particularly, the binary population synthesis code BPASS predicts 20–40 times fewer detectable DWDs than the codes SEBA or BSE, which relates to differing treatments of mass transfer and common-envelope events (CEEs).

Aims. Our aim is to investigate which of these models is closer to reality by comparing their predictions to the DWDs known from electromagnetic observations.

Methods. We compared the DWDs predicted by a BPASS galaxy model and a SEBA galaxy model to a DWD catalogue and to the sample of DWDs observed by the Zwicky Transient Facility (ZTF), taking into account the observational limits and biases of the ZTF survey.

Results. We find that BPASS underpredicts the number of short-period DWDs by at least an order of magnitude compared to the observations, while the SEBA galaxy model is consistent with the observations for DWDs more distant than 500 pc. These results highlight how LISA’s observations of DWDs will provide invaluable information on aspects of stellar evolution such as mass transfer and CEEs, thereby allowing theoretical models to be better constrained.