Vortices as nurseries for planetesimal formation in protoplanetary discs
  Turbulent, two-dimensional, hydrodynamic flows are characterized by the
emergence of coherent, long-lived vortices without a need to invoke special
initial conditions. Vortices have the ability to sequester particles, with
typical radii from about 1 mm to 10 cm, that are slightly decoupled from the
gas. A generic feature of discs with surface density and effective temperature
profiles that are decreasing, power-law functions of radial distance is that
four vortex zones exist for a fixed particle size. In particular, two of the
zones form an annulus at intermediate radial distances within which small
particles reside. Particle capture by vortices occurs on a dynamical time scale
near and at the boundaries of this annulus. As the disc ages and the particles
grow via coagulation, the size of the annulus shrinks. Older discs prefer to
capture smaller particles because the gas surface density decreases with time,
a phenomenon we term "vortex aging". More viscous, more dust-opaque and/or less
massive discs can have vortices that age faster and trap a broader range of
particle sizes throughout the lifetime of the disc. Thus, how efficiently a
disc retains its mass in solids depends on the relative time scales between
coagulation and vortex aging. If vortices form in protoplanetary discs, they
are important in discs with typical masses and for particles that are likely to
condense out of the protostellar nebula. Particle capture also occurs at
distances relevant to planet formation. Future infrared, submillimetre and
centimetre observations of grain opacity as a function of radial distance will
test the hypothesis that vortices serve as nurseries for particle growth in
protoplanetary discs.
