On how the optical depth tunes the effects of ISM neutral atom flow on debris disks

TL;DR

This paper investigates how the optical depth of debris disks influences the impact of interstellar medium neutral atom flow on disk morphology, revealing that low optical depth disks exhibit notable asymmetries while high optical depth disks are less affected due to collisional destruction of dust grains.

Contribution

It demonstrates that the optical depth determines the extent of ISM flow effects on debris disk shape and identifies the role of collisional lifetime and inward migration in disk evolution.

Findings

Low optical depth disks show asymmetric density patterns due to ISM flow.

High optical depth disks are less affected by ISM flow because of rapid collisional destruction.

Inward migration driven by drag forces limits the formation of large clumps and warps.

Abstract

The flux of ISM neutral atoms surrounding stars and their environment affects the motion of dust particles in debris disks, causing a significant dynamical evolution. Large values of eccentricity and inclination can be excited and strong correlations settle in among the orbital angles. This dynamical behaviour, in particular for bound dust grains, can potentially cause significant asymmetries in dusty disks around solar type stars which might be detected by observations. However, the amount of orbital changes due to this non--gravitational perturbation is strongly limited by the collisional lifetime of dust particles. We show that for large values of the disk's optical depth the influence of ISM flow on the disk shape is almost negligible because the grains are collisionally destroyed before they can accumulate enough orbital changes due to the ISM perturbations. On the other hand, for values smaller than $10^{-3}$, peculiar asymmetric patterns appear in the density profile of the disk when we consider 1-10 mum grains, just above the blow-out threshold. The extent and relevance of these asymmetries grow for lower values of the optical depth. An additional sink mechanism, which may prevent the formation of large clumps and warping in the disks is related to the fast inward migration due to the drag component of the forces. When a significant eccentricity is pumped up by the ISM perturbations, the drag forces (Poynting-Robertson and in particular ISM drag) drive the disk particles on fast migrating tracks leading them into the star on a short timescale. It is then expected that disks with small optical depth expand inside the parent body ring all the way towards the star while disks with large optical depth would not significantly extend inside.