Keywords

We present Atacama Large Millimeter/submillimeter Array (ALMA) observations of the dust continuum emission at 1.3 mm and ¹²CO $J=2\to 1$ line emission of the transitional disk around DM Tau. DM Tau's disk is thought to possess a dust-free inner cavity inside a few au, from the absence of near-infrared excess on its spectral energy distribution (SED). Previous submillimeter observations were, however, unable to detect the cavity; instead, a dust ring ∼20 au in radius was seen. The excellent angular resolution achieved in the new ALMA observations, 43 × 31 mas, allows discovery of a 4 au radius inner dust ring, confirming previous SED modeling results. This inner ring is symmetric in continuum emission, but asymmetric in ¹²CO emission. The known (outer) dust ring at ∼20 au is recovered and shows azimuthal asymmetry with a strong-weak side contrast of ∼1.3. The gap between these two rings is depleted by a factor of ∼40 in dust emission relative to the outer ring. An extended outer dust disk is revealed, separated from the outer ring by another gap. The location of the inner ring is comparable to that of the main asteroid belt in the solar system. As a disk with a "proto-asteroid belt," the DM Tau system offers valuable clues to disk evolution and planet formation in the terrestrial-planet-forming region.

We study the influence of transport processes on the chemical evolution of DM Tau-like protoplanetary disks. Turbulent transport of gases and ices is implicitly modeled in full two dimensions (2D), using the mixing-length approximation, along with the time-dependent chemistry. We find that turbulent transport enhances abundances and column densities of many gas-phase species and ices, particularly, complex ones. The influence of turbulent mixing on disk chemistry is more pronounced in the inner, planet-forming disk region where gradients of temperature and high-energy radiation intensities are steeper than in the outer region. The molecules that are unresponsive to transport include, e.g., C₂H, C⁺, CH₄, CN, CO, HCN, HNC, H₂CO, OH, as well as water and ammonia ice. Their column densities computed with the laminar and 2D mixing model differ by a factor of ≲ 2–5. Molecules whose vertical column densities in the laminar and dynamical models differ by up to two orders of magnitude include, e.g., C₂H₂, some carbon chains, CS, H₂CS, H₂O, HCO⁺, HCOOH, HNCO, N₂H⁺, NH₃, CO ice, H₂CO ice, CH₃OH ice, and electrons. Molecules whose column densities are altered by diffusion by more than two orders of magnitude include, e.g., C₂S, C₃S, C₆H₆, CO₂, O₂, SiO, SO, SO₂, long carbon chain ices, CH₃CHO ice, HCOOH ice, O₂ ice, and OCN ice. We indicate several observable or potentially detectable tracers of transport processes in protoplanetary disks and the solar nebula, such as heavy hydrocarbon ices, complex organics, CO₂, O₂, SO, SO₂, C₂S, C₃S compared to CO and water ice.