  Circumstellar disks around stars older than 10 Myr are expected to be
gas-poor. There are, however, two examples of old (30-40 Myr) debris-like disks
containing a detectable amount of cold CO gas. Here we present ALMA and
Herschel Space Observatory observations of one of these disks, around HD 21997,
and study the distribution and origin of the dust and its connection to the
gas. Our ALMA continuum images at 886um clearly resolve a broad ring of
emission within a diameter of ~4.5 arcsec, adding HD 21997 to the dozen debris
disks resolved at (sub)millimeter wavelengths. Modeling the morphology of the
ALMA image with a radiative transfer code suggests inner and outer radii of ~55
and ~150 AU, and a dust mass of 0.09 M_Earth. Our data and modeling hints at an
extended cold outskirt of the ring. Comparison with the morphology of the CO
gas in the disk reveals an inner dust-free hole where gas nevertheless can be
detected. Based on dust grain lifetimes, we propose that the dust content of
this gaseous disk is of secondary origin produced by planetesimals. Since the
gas component is probably primordial, HD 21997 is one of the first known
examples of a hybrid circumstellar disk, a so-far little studied late phase of
circumstellar disk evolution.
