  Context: The sizes of dust in the interstellar medium follows a distribution
where most of the dust mass is in smaller grains. However, the re-distribution
from larger grains towards smaller sizes especially by means of rotational
disruption is poorly understood. Aims: We aim to study the dynamics of porous
grain aggregates under accelerated ration. Especially, we determine the
deformation of the grains and the maximal angular velocity up to the rotational
disruption event by caused by centrifugal forces. Methods: We pre-calculate
aggregates my means of ballistic aggregation analogous to the interstellar dust
as input for subsequent numerical simulations. In detail, we perform
three-dimensional N-body simulations mimicking the radiative torque spin-up
process up to the point where the grain aggregates become rotationally
disrupted. Results: Our simulations results are in agreement with theoretical
models predicting a characteristic angular velocity $\omega_{\mathrm{disr}}$ of
the order of ${ 10^8 - 10^9\ \mathrm{rad\ s^{-1}} }$, where grains become
rotationally disrupted. In contrast to theoretical predictions, we show that
for large porous aggregates ($< 300\ \mathrm{nm}$) $\omega_{\mathrm{disr}}$
reaches a lower asymptotic value. Hence, such grains can withstand an
accelerated ration more efficiently up to a factor of 10 because the
displacement of mass by centrifugal forces and the subsequent mechanical
deformation supports the buildup of new connections within the aggregate.
Furthermore, we report that the rapid rotation of grains deforms an ensemble
with initially 50:50 prolate and oblate shapes, respectively, preferentially
into oblate shapes. Finally, we present a best fit formula to predict the
average rotational disruption of an ensemble of porous dust aggregates
dependent on internal grain structure, total number of monomers, and applied
material properties.
