ALMA observation of the protoplanetary disk around WW Cha: faint double-peaked ring and asymmetric structure

TL;DR

This study uses ALMA observations to reveal a faint, double-peaked dust ring with asymmetric structures in the protoplanetary disk around WW Cha, providing insights into disk morphology and physical conditions.

Contribution

First detailed ALMA imaging and modeling of the faint dust ring and asymmetries in WW Cha's disk, highlighting complex structures and temperature conditions.

Findings

Faint double-peaked dust ring from 40 to 70 au without gaps.

Asymmetric structures confirmed by visibility analysis.

Disk temperature near 30 K at outer peak, close to CO freezeout.

Abstract

We present Atacama Large Millimeter/submillimeter Array (ALMA) Band 6 observations of dust continuum emission of the disk around WW Cha. The dust continuum image shows a smooth disk structure with a faint (low-contrast) dust ring, extending from $\sim 40$ au to $\sim 70$ au, not accompanied by any gap. We constructed the simple model to fit the visibility of the observed data by using MCMC method and found that the bump (we call the ring without the gap the bump) has two peaks at $40$ au and $70$ au. The residual map between the model and observation indicates asymmetric structures at the center and the outer region of the disk. These asymmetric structures are also confirmed by model-independent analysis of the imaginary part of the visibility. The asymmetric structure at the outer region is consistent with a spiral observed by SPHERE. To constrain physical quantities of the disk (dust density and temperature), we carried out radiative transfer simulations. We found that the midplane temperature around the outer peak is close to the freezeout temperature of CO on water ice ($\sim 30$ K). The temperature around the inner peak is about $50$ K, which is close to the freezeout temperature of H$_2$S and also close to the sintering temperature of several species. We also discuss the size distribution of the dust grains using the spectral index map obtained within the Band 6 data.