ALMA’s Sharpest View of IRAS 08544–4431: Unveiling the Dust Distribution Asymmetries in the Circumbinary Disk

Abstract

The interactions between post–asymptotic giant branch (post-AGB) binaries and their circumbinary disks play a central role in shaping the evolution of these systems. Circumbinary disks around post-AGB binaries are stable, though relatively short-lived, structures (104−105 yr), and their physical properties and evolution remain poorly understood. In this study, we target IRAS 08544−4431, one of the best-studied post-AGB binaries. This system has well-constrained stellar and orbital parameters, exhibits well-defined characteristic refractory-depleted photospheric chemistry, and hosts a stable, sub-Keplerian circumbinary disk extending from a well-resolved sublimation rim at approximately 8 au to outer radii of approximately 1000 au. We probe the disk layers close to the midplane using Atacama Large Millimeter/submillimeter Array (ALMA) Band 7 continuum and 12CO J = 3−2 observations of IRAS 08544−4431 obtained with baselines up to 16.2 km. These data provide the finest angular resolution as yet achieved with ALMA for dust-continuum observations of post-AGB binaries with an (almost) face-on disk (15 au; 10 mas). The resulting map of dust continuum enabled resolving disk inner rim, a largely smooth radial profile, and a prominent azimuthal asymmetry at a radius of ∼45 mas. Notably, the location of this dust asymmetry coincides with the forward-scattering peak reported in SPHERE/VLT observations of IRAS 08544−4431. The 12CO emission is more extended than the dust continuum and confirms the disk rotation in this system. However, the global 12CO map shows indications of a spatial offset from the dust asymmetry, suggesting some degree of gas–dust decoupling. Together, these observations establish IRAS 08544−4431 as a key laboratory for studying disk and dust evolution in a parameter space distinct from that of young stars with protoplanetary disks.