First Detection of Equatorial Dark Dust Lane in a Protostellar Disk at
  Submillimeter Wavelength
  In the earliest (so-called "Class 0") phase of sunlike (low-mass) star
formation, circumstellar disks are expected to form, feeding the protostars.
However, such disks are difficult to resolve spatially because of their small
sizes. Moreover, there are theoretical difficulties in producing such disks in
the earliest phase, due to the retarding effects of magnetic fields on the
rotating, collapsing material (so-called "magnetic braking"). With the Atacama
Large Millimeter/submillimeter Array (ALMA), it becomes possible to uncover
such disks and study them in detail. HH 212 is a very young protostellar
system. With ALMA, we not only detect but also spatially resolve its disk in
dust emission at submillimeter wavelength. The disk is nearly edge-on and has a
radius of ~ 60 AU. Interestingly, it shows a prominent equatorial dark lane
sandwiched between two brighter features, due to relatively low temperature and
high optical depth near the disk midplane. For the first time, this dark lane
is seen at submillimeter wavelength, producing a "hamburger"-shaped appearance
that is reminiscent of the scattered-light image of an edge-on disk in optical
and near infrared. Our observations open up an exciting possibility of directly
detecting and characterizing small disks around the youngest protostars through
high-resolution imaging with ALMA, which provides strong constraints on
theories of disk formation.
