On how optical depth tunes the effects of the interstellar medium on debris discs

The flux of neutral atoms of the interstellar medium (ISM) surrounding stars and their environment affects the motion of dust particles in debris discs, 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 discs around solar-type stars, which might be detected by observations. However, the amount of orbital change as a result of this non-gravitational perturbation is strongly limited by the collisional lifetime of dust particles. We show that, for large values of the disc's optical depth, the influence of the ISM flow on the disc shape is almost negligible. This is because the grains are collisionally destroyed before they can accumulate enough orbital changes as a result of the ISM perturbations. However, for values smaller than 10(-3), peculiar asymmetric patterns appear in the density profile of the disc when we consider grains of 1-10 mu m 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 might prevent the formation of large clumps and warping in the discs, is related to the fast inward migration as a result of the drag component of the forces. When a significant eccentricity is enlarged by the ISM perturbations, the drag forces (Poynting-Robertson drag and, in particular, ISM drag) drive the disc particles on fast migrating tracks, leading them into the star on a short time-scale. It is then expected that discs with small optical depth expand inside the parent body ring all the way towards the star, while discs with large optical depth would not significantly extend inside.