Early Planetesimal Formation from Combined Streaming and Gravitational Instabilities

A large amount of evidence accumulated over recent years has suggested that planet formation must start during the earliest phases of star formation, when the circumstellar disc is still embedded in its envelope, but how this occurs is still not clear. In the core accretion model, a critical step for planet formation is the formation of planetesimals. Yet, the dust growth to these km-sized solid aggregates is hindered by several growth barriers.

One of the most promising mechanisms to overcome these barriers is the streaming instability (SI), a drag instability caused by the aerodynamic coupling of gas and dust. However, the optimal conditions for strong dust clumping by SI are still debated. As an alternative, much effort has been devoted to the study of the formation of planetesimals via direct gravitational collapse in gravitationally unstable discs, with several works highlighting its potential efficiency in accumulating solids in dense aggregates.

The two aforementioned processes are usually investigated separately: in SI analyses, the self-gravity of the perturbations is neglected, while gravitational instability (GI) studies do not include some of the necessary ingredients for the onset of the SI. In this work, we aim to combine these phenomena to understand how the influence of the disc’s self-gravity modifies the classical SI. Since they occur on quite different scales, catching both processes in hydrodynamical simulations is very challenging. For this reason, we propose a local, linear stability analysis of a protoplanetary disc where gas and dust interact through both gravitational and drag forces. The background state is a radially drifting, differentially rotating flow, and the self-gravity of the perturbations is included. 

We will discuss how our analysis contributes to understanding whether these combined mechanisms (SI and GI) are efficient enough to explain planetesimal formation in young, embedded discs.