An Origin of Multiple Ring Structure and Hidden Planets in HL Tau: A Unified Picture by Secular Gravitational Instability

TL;DR

This paper reanalyzes the secular gravitational instability in HL Tau's protoplanetary disk, showing it can explain observed rings and potentially indicate ongoing planet formation, with inner regions forming planets and outer regions in earlier growth phases.

Contribution

It demonstrates that secular gravitational instability can account for HL Tau's multiple rings and suggests a unified scenario of planet formation stages within the disk.

Findings

Secular GI can produce the observed ring separations in HL Tau.

Inner disk regions may form planetary mass objects via gravitational fragmentation.

Outer regions are in the early growth phase of secular GI.

Abstract

Recent ALMA observation has revealed multiple ring structures formed in a protoplanetary disk around HL Tau. Prior to the ALMA observation of HL Tau, theoretical analysis of secular gravitational instability (GI) described a possible formation of multiple ring structures with separations of 13 AU around a radius of 100 AU in protoplanetary disks under certain conditions. In this article, we reanalyze the viability of secular GI by adopting the physical values inferred from the observations. We derive the radial distributions of the most unstable wavelength and the growth timescale of secular GI and verify that secular GI can form the ring structures observed in HL Tau. When a turbulent viscosity coefficient $\alpha$ remains small in inner region of the disk, secular GI grows in the whole disk. Thus, the formation of planetary mass objects should occur first in the inner region as a result of gravitational fragmentation after the non-linear growth of secular GI. In this case, resulting objects are expected to create the gaps at $r$ ~ 10 AU and ~ 30 AU. As a result, all ring structures in HL Tau can be created by secular GI. If this scenario is realized in HL Tau, the outer region corresponds to the earlier growth phase of the most unstable mode of secular GI, and the inner region corresponds to the outcome of the non-linear growth of secular GI. Therefore, this interpretation suggests that we are possibly witnessing both the beginning and end of planet formation in HL Tau.