Disk formation and evolution during the Class 0/I phase: gravitational self-regulation

A main focus of my recent research is the formation and early evolution of protoplanetary disks. Studying the gas, dust, and magnetic field in disks with simulation, theory, and observation, I try to construct a clear physical picture of disk evolution and constrain the environment of planet formation.

As a first step, in my Ph.D. thesis (Xu & Kunz 2021a,b) I developed and performed radiation non-ideal MHD simulations to study the formation and early evolution of Class 0/I protoplanetary disks. (Class 0/I, or the main accretion phase, is when the star and the disk are still embedded in an infalling envelope.) Our main finding is that the disk is self-regulated by gravitational instability, which acts like a semi-local effective viscosity. More recently, I model multi-wavelength dust continuum observations from ALMA and VLA to demonstrate that this gravitationally self-regulated disk model not only fits well to but also makes correct predictions on observational data (Xu 2022, Xu et al. 2023).

Gravitational self-regulation in Class 0/I disks have many interesting implications. For observation, it highlights the possibility of a lot of disk mass being hided by high optical depth, and predicts abundant spiral substructures (unlike the rings and gaps seen in older disks) that can be discovered in the near future by observations with higher resolution and sensitivity (Xu et al. 2023). For planet formation, the large disk mass and the prospect of forming a family of large pebbles in Class 0/I may pave the way for planet formation as soon as the disk stops being stirred by gravitational instability (Xu & Armitage 2023).

A feedback loop of pressure-bump planet formation

In addition to young protoplanetary disks, I also study later stages of planet formation, especially how planet-disk interaction affects the architecture of planetary systems.

Recently, we discovered a feedback loop between planets exciting pressure bumps and pressure bumps facilitating the formation of new planets (Xu & Wang 2024). This feedback loop allows bursty planet formation, with many planets (or planet cores) forming at approximately the same time and location in the disk.

To test this feedback loop observationally, we look for its signature in the resulting planet architectures. We predict that depending on whether (and how) this feedback loop operates, planets can be divided into subclasses with significantly different levels of intra-system mass uniformity, and we confirm this "uniformity dichotomy" among the population of observed exoplanets.

A new theory for hot accretion flows

Beyond planet formation, I am generally interested in developing simple theoretical explanations for complex fluid behaviors. One such problem is the dynamics of hot (radiatively inefficient) accretion flows in systems with large scale separation; this includes, for example, accretion flows onto SMBHs like Sgr A* and M87* at sub-Bondi scales. A number of 3D simulations have found such flows to be highly turbulent and obey a \(\rho\propto r^{-1}\) scaling. I develop a new theory, simple convective accretion flow (SCAF), to explain these behaviors (Xu 2023). Using a combination of simulation and analytic theory, I demonstrate that the turbulence is driven by convective instability, and momentum balance in the flow sets an approximately -1 slope.