The origin of the negative torque density in disk-satellite interaction

TL;DR

This paper investigates the negative torque density phenomenon in disk-satellite interactions, confirming its reality through analytical and numerical methods, and attributes it to overlapping Lindblad resonances near the perturber's orbit.

Contribution

It provides a linear analysis that explains the origin of negative torque density, highlighting the importance of resonance overlap, which was not considered in previous studies.

Findings

Negative torque density is confirmed to be a real effect.

Overlap of Lindblad resonances causes the negative torque density.

Results suggest more realistic torque prescriptions are needed for disk evolution models.

Abstract

Tidal interaction between a gaseous disk and a massive orbiting perturber is known to result in angular momentum exchange between them. Understanding astrophysical manifestations of this coupling such as gap opening by planets in protoplanetary disks or clearing of gas by binary supermassive black holes (SMBHs) embedded in accretion disks requires knowledge of the spatial distribution of the torque exerted on the disk by a perturber. Recent hydrodynamical simulations by Dong et al (2011) have shown evidence for the tidal torque density produced in a uniform disk to change sign at the radial separation of $\approx 3.2$ scale heights from the perturber's orbit, in clear conflict with the previous studies. To clarify this issue we carry out a linear calculation of the disk-satellite interaction putting special emphasis on understanding the behavior of the perturbed fluid variables in physical space. Using analytical as well as numerical methods we confirm the reality of the negative torque density phenomenon and trace its origin to the overlap of Lindblad resonances in the vicinity of the perturber's orbit - an effect not accounted for in previous studies. These results suggest that calculations of the gap and cavity opening in disks by planets and binary SMBHs should rely on more realistic torque density prescriptions than the ones used at present.