Expected Evolution of the Binary System ATLAS J1138-5139

Abstract ATLAS J1138-5139 is a newly detected ultra-compact double white dwarf (DWD) system which is composed of a 1.02 M ⊙ carbon-oxygen white dwarf (CO WD) and a 0.24 M ⊙ helium (He) WD with an orbital period of about 27.68 minutes , making it one of the shortest-period DWD systems known. The future evolution and final fate of this system remain unexplored. In this work, we investigate the evolution of ATLAS J1138-5139 with the one-dimensional stellar evolution code Modules for Experiments in Stellar Astrophysics . We find that ATLAS J1138-5139 will evolve into an AM Canum Venaticorum system in about ∼6.3 Myr. Afterwards, the transferred material from the He WD companion start to build up to form a He shell near the surface of the CO WD. This accumulated He-shell masses can be up to approximately 0.12 M ⊙ , which is likely to trigger a double-detonation (DDet) explosion of the CO WD. We therefore expect that ATLAS J1138-5139 will likely explode as a type Ia supernova eventually through the DDet explosion mechanism. Moreover, our calculations show that ATLAS J1138-5139 will be a promising target for gravitational-wave detection by future detectors like LISA, TianQin and Taiji.