The Arc in the DX Cha Circumbinary System: Evidence For a Retrograde Circumbinary Disk
Observations of the binary system DX Cha (HD 104237) reveal a compact, asymmetric ring structure with a radius of 0.43\,au. This ring is just outside the binary orbit, which has semi-major axis $a_{\rm b} = 0.22$\,au and eccentricity $e_{\rm b} = 0.665$; placing the ring at $\approx 1.2$ times the binary apocenter distance. The inner regions of circumbinary disks, $\approx 2-3\,a_{\rm b}$, are typically evacuated by strong gravitational torques from the binary, resulting in a deep gap between the binary and the disk. Accordingly, previous numerical simulations of DX Cha have found an eccentric inner cavity with almost no material inside $\approx 1$\,au, and we find similar results when making the same assumption that the circumbinary disk orbits in the same direction as the binary. However, the disk can exist much closer to the binary if it is retrograde. For DX Cha we find that the inner edge of a retrograde disk occurs at $\approx 2a_{\rm b}$, and moreover takes the form of one or two arcs, in agreement with observations. We therefore suggest that the circumbinary disk in the DX Cha system could be orbiting retrograde to the binary star system in the center. We conclude that compact circumbinary disks observed in young stellar systems are important targets for future observations; if the disks are prograde then their properties are likely to be significantly different from current estimates, while if they are retrograde then this will have profound implications for our understanding of star and planet formation.