ALMA Observations of the Inner Cavity in the Protoplanetary Disk around Sz 84

Abstract

We present Atacama Large Millimeter/submillimeter Array observations of a protoplanetary disk around the T Tauri star Sz 84 and analyses of the structures of the inner cavity in the central region of the dust disk. Sz 84's spectral energy distribution (SED) has been known to exhibit negligible infrared excess at λ   ≲ 10  μ m due to the disk’s cavity structure. Analyses of the observed visibilities of dust continuum at 1.3 mm and the SED indicate that the size of the cavity in the disk of large (millimeter size) dust grains is 8 au in radius and that in the disk of small (submicron size) dust grains is 60 au in radius. Furthermore, from the SED analyses, we estimate that the upper limit mass of small dust grains at r < 60 au is less than ∼10 −3   M ⊕ , which is ≲0.01% of the total (small + large) dust mass at r < 60 au. These results suggest that large dust grains are dominant at r < 60 au, implying that dust grains efficiently grow with less efficient fragmentation in this region, potentially due to weak turbulence and/or stickier dust grains. The balance of grain growth and dust fragmentation is an important factor for determining the size of large dust grains in protoplanetary disks, and thus Sz 84 could serve as a good testbed for investigations of grain growth in such disks.