Observations of edge-on protoplanetary disks with ALMA I. Results from
  continuum data
  We analyze a sample of 12 HST-selected edge-on protoplanetary disks for which
the vertical extent of the emission layers can be constrained directly. We
present ALMA high angular resolution continuum images (0.1arcsec) of these
disks at two wavelengths, 0.89mm and 2.06mm (respectively ALMA bands 7 and 4),
supplemented with archival band 6 data (1.33mm) where available. For most
sources, the millimeter continuum emission is more compact than the scattered
light, both in the vertical and radial directions. Six sources are resolved
along their minor axis in at least one millimeter band, providing direct
information on the vertical distribution of the millimeter grains. For the
second largest disk of the sample, the significant difference in vertical
extent between band 7 and band 4 suggests efficient size-selective vertical
settling of large grains. Furthermore, the only Class I object in our sample
shows evidence of flaring in the millimeter. Along the major axis, all disks
are well resolved. Four of them are larger in band 7 than in band 4 in the
radial direction, and three have a similar radial extent in all bands. For all
disks, we also derive the millimeter brightness temperature and spectral index
maps. We find that the disks are likely optically thick and that the dust
emission reveals low brightness temperatures in most cases (<10K). The
integrated spectral indices are similar to those of disks at lower inclination.
The comparison of a generic radiative transfer disk model with our data shows
that at least 3 disks are consistent with a small millimeter dust scale height,
of a few au (measured at r=100au). This is in contrast with the more classical
value of h_g\sim10au derived from scattered light images and from gas line
measurements. These results confirm, by direct observations, that large
(millimeter) grains are subject to significant vertical settling in
protoplanetary disks.
