2025ApJ...988..106H

Millimeter continuum spiral arms have so far only been detected in a handful of protoplanetary disks, and thus we have a limited understanding of the circumstances in which they can form. In particular, substructures in small disks (R ⪅ 50 au) have not been well characterized in comparison with large disks. We present Atacama Large Millimeter/submillimeter Array 1.3 mm continuum observations of the disk around the T Tauri star Haro 6-13 at a resolution of $\sim 0\mathop{.}\limits^{^{\prime\prime} }04$ (∼5 au). A pair of low-contrast spiral arms are detected at disk radii from ∼10 to 35 au. They can be approximated as Archimedean spirals with pitch angles ranging from ∼10° to 30°. The low value of the disk-averaged spectral index between 1.3 and 3 mm (α = 2.1) and the high brightness temperatures suggest that the millimeter continuum is likely optically thick and thus may hide sufficient mass for the disk to become gravitationally unstable and form spiral arms. CO observations have shown that Haro 6-13 is surrounded by an envelope, raising the possibility that infall is facilitating spiral arm formation.