# The Sizes and Depletions of the Dust and Gas Cavities in the Transitional Disk J160421.7-213028

## TL;DR

This study uses ALMA observations to analyze the size, shape, and depletion levels of dust and gas cavities in the transitional disk J160421.7-213028, revealing complex structures and gradients that inform planet formation theories.

## Contribution

First detailed multi-component analysis of dust and gas cavities in J160421.7-213028, combining continuum and line emission data to characterize their sizes, shapes, and density profiles.

## Key findings

- Dust cavities are depleted by factors of at least 1000.
- Gas cavity is smaller, about 15 AU, with a smooth density gradient.
- Gas surface density does not sharply drop at 30 AU, contradicting some models.

## Abstract

We report ALMA Cycle 2 observations of 230 GHz (1.3 mm) dust continuum emission, and $^{12}$CO, $^{13}$CO, and C$^{18}$O J = 2-1 line emission, from the Upper Scorpius transitional disk [PZ99] J160421.7-213028, with an angular resolution of ~0".25 (35 AU). Armed with these data and existing H-band scattered light observations, we measure the size and depth of the disk's central cavity, and the sharpness of its outer edge, in three components: sub-$\mu$m-sized "small" dust traced by scattered light, millimeter-sized "big" dust traced by the millimeter continuum, and gas traced by line emission. Both dust populations feature a cavity of radius $\sim$70 AU that is depleted by factors of at least 1000 relative to the dust density just outside. The millimeter continuum data are well explained by a cavity with a sharp edge. Scattered light observations can be fitted with a cavity in small dust that has either a sharp edge at 60 AU, or an edge that transitions smoothly over an annular width of 10 AU near 60 AU. In gas, the data are consistent with a cavity that is smaller, about 15 AU in radius, and whose surface density at 15 AU is $10^{3\pm1}$ times smaller than the surface density at 70 AU; the gas density grades smoothly between these two radii. The CO isotopologue observations rule out a sharp drop in gas surface density at 30 AU or a double-drop model as found by previous modeling. Future observations are needed to assess the nature of these gas and dust cavities, e.g., whether they are opened by multiple as-yet-unseen planets or photoevaporation.