  The latest observation of HL Tau by ALMA revealed spectacular concentric dust
rings in its circumstellar disk. We attempt to explain the multiple ring
structure as a consequence of aggregate sintering. Sintering is known to reduce
the sticking efficiency of dust aggregates and occurs at temperatures slightly
below the sublimation point of their constituent material. We here present a
dust growth model incorporating sintering and use it to simulate global dust
evolution due to sintering, coagulation, fragmentation, and radial inward drift
in a modeled HL Tau disk. We show that aggregates consisting of multiple
species of volatile ices experience sintering, collisionally disrupt, and pile
up at multiple locations slightly outside the snow lines of the volatiles. At
wavelengths of 0.87--1.3 mm, these sintering zones appear as bright, optically
thick rings with a spectral slope of $\approx 2$, whereas the non-sintering
zones as darker, optically thinner rings of a spectral slope of $\approx$
2.3--2.5. The observational features of the sintering and non-sintering zones
are consistent with those of the major bright and dark rings found in the HL
Tau disk, respectively. Radial pileup and vertical settling occur
simultaneously if disk turbulence is weak and if monomers constituting the
aggregates are $\sim 1~{\rm \mu m}$ in radius. For the radial gas temperature
profile of $T = 310(r/1~{\rm AU})^{-0.57}~{\rm K}$, our model perfectly
reproduces the brightness temperatures of the optically thick bright rings, and
reproduces their orbital distances to an accuracy of $\lesssim$ 30%.

Solar and Stellar Astrophysics;Earth and Planetary Astrophysics