Dependence on the Equation of State in SPH Simulations of Proto-Uranian Disk Formation from a Giant Impact

TL;DR

This study investigates how different equations of state (EOS) influence the outcomes of SPH simulations of Uranus's giant impact, emphasizing the importance of EOS choice for disk composition predictions.

Contribution

The paper systematically compares three EOS models and two SPH schemes in impact simulations, highlighting their effects on disk properties and composition.

Findings

Disk mass and size are primarily determined by impact angular momentum.

EOS choice significantly affects the predicted rock fraction in the disk.

Macroscopic impact outcomes are robust across different EOS and SPH schemes.

Abstract

The $98^\circ$ obliquity of Uranus is widely attributed to a giant impact that ejected material and formed a debris disk, which subsequently coalesced into its regular satellites. Previous Smoothed Particle Hydrodynamics (SPH) studies have yielded inconsistent disk compositions, a discrepancy often linked to the variety of numerical and physical modeling assumptions. We address this by presenting SPH simulations that systematically test three distinct EOS models alongside two SPH schemes (standard SPH, and the enhanced density-independent SPH). We utilized a $3M_{\oplus}$ impactor and explored a range of impact parameters which are capable of reproducing Uranus's current spin state. Our primary finding is that for impacts capable of reproducing Uranus's current rotation, the choice of EOS or SPH scheme barely affects macroscopic features such as the post-impact rotation period, disk mass, or disk size; these properties are primarily controlled by the impact's angular momentum. In contrast, the disk's rock fraction is highly EOS-dependent. Our results clarify that while disk mass and size are robust outcomes, the final disk composition is highly model-dependent. Therefore, accurate EOS modeling, integrated with detailed disk evolution studies, is essential to definitively validate the giant impact scenario for Uranus.