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