  Protoplanetary disks often appear as multiple concentric rings in dust
continuum emission maps and scattered light images. These features are often
associated with possible young planets in these disks. Many non-planetary
explanations have also been suggested, including snow lines, dead zones and
secular gravitational instabilities in the dust. In this paper we suggest
another potential origin. The presence of copious amounts of dust tends to
strongly reduce the conductivity of the gas, thereby inhibiting the
magneto-rotational instability, and thus reducing the turbulence in the disk.
From viscous disk theory it is known that a disk tends to increase its surface
density in regions where the viscosity (i.e. turbulence) is low. Local maxima
in the gas pressure tend to attract dust through radial drift, increasing the
dust content even more. We investigate mathematically if this could potentially
lead to a feedback loop in which a perturbation in the dust surface density
could perturb the gas surface density, leading to increased dust drift and thus
amplification of the dust perturbation and, as a consequence, the gas
perturbation. We find that this is indeed possible, even for moderately small
dust grain sizes, which drift less efficiently, but which are more likely to
affect the gas ionization degree. We speculate that this instability could be
triggered by the small dust population initially, and when the local pressure
maxima are strong enough, the larger dust grains get trapped and lead to the
familiar ring-like shapes. We also discuss the many uncertainties and
limitations of this model.
