On the Formation of the Ultrashort Period Binary SDSS J0651

With the advent of new low-frequency gravitational wave detectors such as LISA, it is expected that many short-period binaries (on the order of minutes) will be discovered because they are emitters of copious fluxes of gravitational radiation. One such recently discovered binary is SDSS J065133.338+284423.37 which was discovered by the ZTF and has an orbital period of 6.9 minutes (Burdge et al. 2019). In contrast to the discovery paper, we propose an alternate evolutionary pathway that can lead to the formation of such ultra-short orbital period binaries. Starting with progenitors that have masses of approximately 1.3 Msun, it is possible to reproduce most of the salient properties of J0651 without invoking tidal heating. We discuss the strengths and weaknesses of this ‘CV-channel’ type evolution in forming ultra-low period compact binaries.