Spiral-Arm Sub-Structures in the Asymmetrical Dust Rings of the Circumstellar Disk MWC 758

TL;DR

This study analyzes asymmetrical dust rings in the MWC 758 circumstellar disk using ALMA data, revealing three spiral-like structures and estimating potential planet locations through spiral density wave modeling.

Contribution

It introduces a novel azimuthal segmentation and Gaussian fitting method to analyze disk structures and applies spiral density wave theory to infer disk properties and possible embedded planets.

Findings

Identified three spiral-like arm structures in the disk.

Estimated disk aspect ratios with two possible solutions (~0.03 and ~0.2).

Suggested planet locations consistent with previous NIR observations.

Abstract

Asymmetrical features in disks provide indirect evidences of embedded objects, such as planets. Observed with the Atacama Large Millimeter/submillimeter Array (ALMA), the circumstellar disk in MWC 758 traced with thermal dust continuum emission at wavelengths of 0.9 mm with an angular resolution up to 0.1" (15 au) exhibits an asymmetrical dust ring with additional features. In order to analyze the structures azimuthally and radially, we split the dust ring into small segments in azimuth. For each segment, we fit two-Gaussian functions to the radial intensity profile. The obtained best-fit parameters as a function of azimuth are analyzed. Three spiral-like arm structures are identified. When fitting the 0.9 mm features with the spiral density wave theory using the WKB approximation, two sets of disk aspect ratios are found: one solution gives relatively low values (~0.03) while the other solution is at the upper bound of the free parameter (~0.2). The planet locations suggested by the upper-bound result are similar to the ones determined by Benisty et al. (2015) for the NIR polarized intensity image. Comparing the reported spiral-like structures with the higher angular-resolution (0.04") ALMA image in Dong et al. (2018), we identify different structures in the West of the disk due to differences in the adopted analysis methods and the respective resolutions of the images.