Characterization of Ring Substructures in the Protoplanetary Disk of HD
  169142 from Multi-Wavelength ALMA Observations
  We present a detailed multi-wavelength characterization of the multi-ring
disk of HD 169142. We report new ALMA observations at 3 mm and analyze them
together with archival 0.89 and 1.3 mm data. Our observations resolve three out
of the four rings in the disk previously seen in high-resolution ALMA data. A
simple parametric model is used to estimate the radial profile of the dust
optical depth, temperature, density, and particle size distribution. We find
that the multiple ring features of the disk are produced by annular
accumulations of large particles, probably associated with gas pressure bumps.
Our model indicates that the maximum dust grain size in the rings is $\sim1$
cm, with slightly flatter power-law size distributions than the ISM-like size
distribution ($p\sim3.5$) found in the gaps. In particular, the inner ring
($\sim26$ au) is associated with a strong and narrow buildup of dust particles
that could harbor the necessary conditions to trigger the streaming
instability. According to our analysis, the snowlines of the most important
volatiles do not coincide with the observed substructures. We explore different
ring formation mechanisms and find that planet-disk interactions are the most
likely scenario to explain the main features of HD 169142. Overall, our
multi-wavelength analysis provides some of the first unambiguous evidence of
the presence of radial dust traps in the rings of HD 169142. A similar analysis
in a larger sample of disks could provide key insights on the impact that disk
substructures have on the dust evolution and planet formation processes.
