Dust production in debris discs: constraints on the smallest grains

LESIA-Observatoire de Paris, UPMC Univ. Paris 06, Univ. Paris-Diderot, 92195 Meudon France
e-mail: philippe.thebault@obspm.fr

Received: 23 October 2015
Accepted: 4 January 2016

Abstract

Context. The surface energy constraint puts a limit on the smallest fragment s_surf that can be produced after a collision. Based on analytical considerations, this mechanism has been recently identified as being having the potential to prevent the production of small dust grains in debris discs and to cut off their size distribution at sizes larger than the blow-out size.

Aims. We numerically investigate the importance of this effect to find out under which conditions it can leave a signature in the small-size end of a disc’s particle size distribution (PSD). An important part of this work is to map out, in a disc at steady-state, what is the most likely collisional origin for μm-sized dust grains, in terms of the sizes of their collisional progenitors.

Methods. For the first time, we implement the surface energy constraint into a collisional evolution code. We consider a typical debris disc extending from 50 to 100 au and two different stellar types: sun-like and A star. We also consider two levels of stirring in the disc: dynamically “hot” (⟨e⟩ = 0.075) and “cold” (⟨e⟩ = 0.01). In all cases, we derive s_surf maps as a function of target and projectile sizes, s_t and s_p, and compare them to equivalent maps for the dust-production rate. We then compute disc-integrated profiles of the PSD and estimate the imprint of the surface energy constraint.

Results. We find that the (s_p,s_t) regions of high s_surf values do not coincide with those of high dust production rates. As a consequence, the surface energy constraint generally has a weak effect on the system’s PSD. The maximum s_surf-induced depletion of μm-sized grains is ~30% and is obtained for a sun-like star and a dynamically “hot” case. For the e = 0.01 cases, the surface energy effect is negligible compared to the massive small grain depletion that is induced by another mechanism: the “natural” imbalance between dust production and destruction rates in low-stirring discs.