The Arc in the DX Cha Circumbinary System: Evidence For a Retrograde Circumbinary Disk

Abstract

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.

Figures (8)

• Figure 1: Column density profiles for prograde (left) and retrograde (right) circumbinary disks once they have each reached a quasi-steady state. In each panel, the left cyan point represents the primary and the right cyan point represents the secondary. In the prograde case we see a large inner cavity has been evacuated which is punctuated by a stream that feeds the circumprimary disk. The strongest density enhancement in this case occurs at a radius of around $4-5a_{\rm b}$. In the retrograde case the disk extends much closer to the binary with a ring-like structure occurring at around $2a_{\rm b}$ with a thickness of around $0.5a_{\rm b}$. The retrograde disk structure is in better agreement with the observations for DX Cha.
• Figure 2: Series of snapshots showing the precession of the density enhancement at the apocentre of the eccentric inner disk edge. The snapshots are spaced by around $100T_{\rm b}$, and the timescale for a full precession of the enhancement is $\approx 500\, T_{\rm b}$ for these parameters. The inner disk structure is not fixed, but varies through the binary orbit; this is shown in more detail in Fig. \ref{['fig:prograde-series']}.
• Figure 3: Series of snapshots showing the disk structural changes on short timescales as the binary completes an orbit. Each panel is separated in time by an interval of $0.1 T_{\rm b}$. The two cyan dashed circles indicate the location and FWHM of the azimuthal asymmetric ring modeled in Juhasz2025. While the spiral streams can cross the location of the observed ring, the strong density enhancement associated with the inner edge of the circumbinary disk is at a significantly larger radius than the observed ring.
• Figure 4: Same as Fig. \ref{['fig:prograde']} except for the retrograde, rather than prograde, case.
• Figure 5: Same as Fig. \ref{['fig:prograde-series']} except for the retrograde, rather than prograde, case.