Formation of Dust Clumps with Sub-Jupiter Mass and Cold Shadowed Region in Gravitationally Unstable Disk around Class 0/I Protostar in L1527 IRS

Abstract

We have investigated the protostellar disk around a Class 0/I protostar, L1527 IRS, using multiwavelength observations of the dust continuum emission at λ = 0.87, 2.1, 3.3, and 6.8 mm, obtained by the Atacama Large Millimeter/submillimeter Array and the Jansky Very Large Array (VLA). Our observations achieved a spatial resolution of 3–13 au and revealed an edge-on disk structure with a size of ∼80–100 au. The emission at 0.87 and 2.1 mm is found to be optically thick, within a projected disk radius of r proj ≲ 50 au. The emission at 3.3 and 6.8 mm shows that the power-law index of the dust opacity (β) is β ∼ 1.7 around r proj ∼ 50 au, suggesting that grain growth has not yet begun. The dust temperature (T dust) shows a steep decrease with T dust ∝ r proj −2 outside the VLA clumps previously identified at r proj ∼ 20 au. Furthermore, the disk is gravitationally unstable at r proj ∼ 20 au, as indicated by a Toomre Q parameter value of Q ≲ 1.0. These results suggest that the VLA clumps are formed via gravitational instability, which creates a shadow on the outside of the substructure, resulting in the sudden drop in temperature. The derived dust masses for the VLA clumps are ≳0.1 M J. Thus, we suggest that Class 0/I disks can be massive enough to be gravitationally unstable, which may be the origin of gas giant planets in a 20 au radius. Furthermore, the protostellar disks could be cold due to shadowing.