Grand Design Spiral Arms in the Compact, Embedded Protoplanetary Disk of Haro 6-13

TL;DR

This study uses high-resolution ALMA observations to reveal spiral arms in a small, embedded protoplanetary disk, suggesting potential gravitational instability and infall processes as mechanisms for spiral formation.

Contribution

First detailed ALMA imaging of spiral arms in a small, embedded protoplanetary disk, highlighting the role of optical thickness and infall in spiral arm development.

Findings

Detected low-contrast spiral arms at 10-35 au

Indications of gravitational instability due to optical thickness

Possible influence of infall from surrounding envelope

Abstract

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 ($R\lessapprox 50$ au) have not been well-characterized in comparison with large disks. We present ALMA 1.3 mm continuum observations of the disk around the T Tauri star Haro 6-13 at a resolution of $\sim0.04''$ ($\sim5$ au). A pair of low-contrast spiral arms are detected at disk radii from $\sim10-35$ au. They can be approximated as Archimedean spirals with pitch angles ranging from $\sim10-30^\circ$. The low value of the disk-averaged spectral index between 1.3 and 3 mm ($\alpha=2.1$) 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.