Self-gravity in thin protoplanetary discs: 2. Numerical convergence solved and revealing the overestimation in mass of formed planets with softening

Abstract

The Gravitational Instability (GI) is a leading theory for explaining early planet formation in massive discs. In the early 2010s, 3D SPH simulations of GI failed to converge, initially attributed to resolution-dependent viscosity but later appearing in 2D SPH and grid-based simulations, suggesting a numerical artifact inherent to the 2D approximation of gravity. Recently, we derived from first principles a much improved prescription for gravity in 2D discs (via a Bessel kernel). This prescription introduces a characteristic length below which gravity smoothly transitions from a 3D to a 2D scaling. This cannot be captured by standard smoothing length approaches, widely used in 2D simulations. We employ this new prescription to resolve the convergence issue of GI in 2D, and compare the outcomes of GI in runs using the Bessel kernel with those obtained using softening prescriptions at high resolution. We conducted numerical simulations with the FargoCPT code, where the Bessel prescription was implemented. The 2D Bessel formalism of gravity effectively resolves the convergence issues encountered in 2D simulations. When compared to simulations employing softened or unsoftened potentials, I observe that a small softening parameter tends to overestimate gravitational effects. This results in an artificially high number of fragments, leading to final fragment masses that are overestimated by a factor of 2-3. Conversely, employing large softening parameters inhibits gravitational effects. Although our analysis initially suggests that a softening parameter of 0.6 H might offer the best compromise, in reality, the resulting fragments fail to remain gravitationally bound-a behavior not observed when using the Bessel kernel. Our findings strongly suggest that the Bessel prescription should be adopted to ensure a consistent and accurate treatment of gravity in thin discs.