Formation Channels of Gravitationally Resolvable Double White Dwarf Binaries Inside Globular Clusters

TL;DR

This paper investigates the formation of eccentric and tight double white dwarf binaries in globular clusters, which are promising gravitational wave sources detectable by future observatories like LISA.

Contribution

It identifies dynamical interactions in dense clusters as a key formation channel for eccentric DWDs, a process not prevalent in the field.

Findings

Eccentric and tight DWDs in clusters are rare, with an upper limit of 10-15 in the Milky Way.

Such binaries can help determine distances to their host clusters independently.

Dynamical interactions are crucial for forming these binaries with high eccentricity.

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 like LISA will be able to observe these systems in abundance. DWDs 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 where 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 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 distances of their host cluster.