Formation channels of gravitationally resolvable double white dwarf binaries inside globular clusters

¹ Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences, ul. Bartycka 18, 00-716 Warsaw, Poland
² Faculty of Mathematics and Computer Science, A. Mickiewicz University, Uniwersytetu Poznańskiego 4, 61-614 Poznań, Poland
³ School of physics and astronomy, Beijing Normal University, 19 Xinjiekouwai St, Beijing 100875, PR China
⁴ National Astronomical Observatories, Chinese Academy of Sciences, 20A Datun Road, Beijing 100101, PR China
⁵ School of Astronomy and Space Sciences, University of Chinese Academy of Sciences, 19A Yuquan Road, Beijing 100049, PR China
⁶ Kavli Institute for Astronomy and Astrophysics at Peking University, 100871 Beijing, China

^★ Corresponding author: hellstrom@camk.edu.pl

Received: 16 June 2025
Accepted: 21 August 2025

Abstract

Current gravitational wave detectors are sensitive to coalescing black holes and neutron stars. However, double white dwarfs (DWDs) have long been recognized as promising sources of gravitational waves, and upcoming detectors such as LISA will be able to observe these systems in abundance. Double white dwarfs are expected to be the dominant gravitational wave (GW) sources in parts of the LISA frequency range, making it crucial to understand their formation for future detections. The Milky Way contains many white dwarfs (WDs) in both the field and star clusters, promising a rich population of DWDs for LISA. However, the large number of sources may make it difficult to resolve individual binaries, and DWDs in the field and clusters often have similar properties, complicating the identification of their origins from GW signals alone. In this work, we focus on eccentric and tight DWDs, which cannot form in the field, but require dynamical interactions in dense clusters to increase their eccentricity after circularization through mass transfer phases and common-envelope evolution during binary evolution. These binaries may also form in three- and four-body dynamical interactions, whereby a DWD binary may directly form with high eccentricity and low separation. Our results show that we should expect eccentric and tight DWDs in clusters that can provide a meaningful GW signal; however, the number is low, with an upper limit of 10-15 in the MW. These can be used to independently obtain the distances of their host clusters.