The Double-White-Dwarf Merger Rate from ZTF

Using the Zwicky Transient Facility (ZTF), Burdge et al. (2020) discovered systems of eclipsing double white dwarfs (EDWDs) having orbital periods <1 hr. From the properties of 3 of the discovered systems, I estimate a merger rate of DWDs, per WD in the Galaxy, of $R_{\mathrm{m}\mathrm{e}\mathrm{r}\mathrm{g}\mathrm{e},\mathrm{W}\mathrm{D}}\approx 8\times {10}^{-12}$yr${}^{-1}$, or a rate per unit stellar mass in the Galaxy, of $R_{\mathrm{m}\mathrm{e}\mathrm{r}\mathrm{g}\mathrm{e},\mathrm{M}\ast }\approx 4.8\times {10}^{-13}$yr${}^{-1}$M${}_{\odot }^{-1}$.This likely somewhat underestimates the rate, because of several known effects that work against EDWD detection in ZTF. The derived merger rate is within the uncertainty range, $R_{\mathrm{m}\mathrm{e}\mathrm{r}\mathrm{g}\mathrm{e},\mathrm{M}\ast }=(4.6-5.8)\times {10}^{-13}$yr${}^{-1}$M${}_{\odot }^{-1}$, measured independently by Maoz et al. (2018) based on two samples of DWDs discovered via radial-velocity variations. Based on the expected period distribution of DWDs and their detectability, of order 100 additional eclipsing DWDs with periods >1 hr are discoverable in ZTF, with potential to significantly improve the merger-rate’s measurement precision.