The Double-white-dwarf Merger Rate from ZTF

Abstract Using the Zwicky Transient Facility (ZTF), Burdge et al. discovered systems of eclipsing double white dwarfs (EDWDs) having orbital periods &lt; 1 hr. From the properties of 3 of the discovered systems, I estimate a merger rate of DWDs, per unit stellar mass in the Galaxy, of R merge,M* ≈ 4.8 × 10 −13 yr <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msup> <mml:mrow/> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> <mml:mspace width="0.25em"/> <mml:msubsup> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>⊙</mml:mo> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> . 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 merge,M* = (4.6−5.8) × 10 −13 yr <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msup> <mml:mrow/> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> <mml:mspace width="0.25em"/> <mml:msubsup> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>⊙</mml:mo> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> , measured independently by Maoz et al. 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 EDWDs with periods &gt; 1 hr are discoverable in ZTF, with potential to significantly improve the merger-rate’s measurement precision.