A faint dust ring beyond the compact disk of Sz 76

Abstract

We report new ALMA Band 7 high-resolution ($\sim$0.05${}^{{}^{\prime \prime }}$) continuum and ${}^{12}\text{CO}(J=3\text{–}2)$ observations of the disk around the young low-mass star Sz 76. Visibility-plane modeling reveals a previously undetected, faint outer dust ring centered at $\sim$45 au surrounding the compact central disk. Including this low-surface-brightness feature increases the characteristic dust radius from $R_{\text{dust,68}}\approx 7$ au to $41$ au. Combined with an outer gas extent of $R_{\text{CO,68}}\approx 77$ au, the resulting gas-to-dust size ratio decreases from an extreme value of $\sim$11 to $\sim$2, placing Sz 76 in line with typical protoplanetary disks and contradicting a scenario of unimpeded inward radial drift. These results demonstrate that spatial filtering or limited surface-brightness sensitivity can severely underestimate disk sizes, highlighting the necessity of multi-scale observations to properly identify dust traps and accurately constrain disk evolution.