Spiral-arm Substructures in the Asymmetrical Dust Rings of the Circumstellar Disk MWC 758

Abstract

Asymmetrical features in disks provide indirect evidence 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 01 (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. for the NIR polarized intensity image. Comparing the reported spiral-like structures with the higher angular resolution (004) ALMA image in Dong et al., 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.