  High dust density in the midplane of protoplanetary disks is favorable for
efficient grain growth and can allow fast formation of planetesimals and
planets, before disks dissipate. Vertical settling and dust trapping in
pressure maxima are two mechanisms allowing dust to concentrate in
geometrically thin and high density regions. In this work, we aim to study
these mechanisms in the highly inclined protoplanetary disk SSTC2D
J163131.2-242627 (Oph163131, i~84deg). We present new high angular resolution
continuum and 12CO ALMA observations of Oph163131. The gas emission appears
significantly more extended in the vertical and radial direction compared to
the dust emission, consistent with vertical settling and possibly radial drift.
In addition, the new continuum observations reveal two clear rings. The outer
ring, located at ~100 au, is well resolved in the observations, which allows us
to put stringent constraints on the vertical extent of millimeter dust
particles. We model the disk using radiative transfer and find that the scale
height of millimeter sized grains is 0.5au or less at 100au from the central
star. This value is about one order of magnitude smaller than the scale height
of smaller micron-sized dust grains constrained by previous modeling, which
implies that efficient settling of the large grains is occurring in the disk.
When adopting a parametric dust settling prescription, we find that the
observations are consistent with a turbulent viscosity coefficient of about
alpha<=10^-5 at 100au. Finally, we find that the thin dust scale height
measured in Oph163131 is favorable for planetary growth by pebble accretion: a
10 M_Earth planet may grow within less than 10 Myr, even in orbits exceeding
50au.
