  Gas-rich primordial disks and tenuous gas-poor debris disks are usually
considered as two distinct evolutionary phases of the circumstellar matter.
Interestingly, the debris disk around the young main-sequence star 49 Ceti
possesses a substantial amount of molecular gas, and possibly represents the
missing link between the two phases. Motivated to understand the evolution of
the gas component in circumstellar disks via finding more 49 Ceti-like systems,
we carried out a CO J=3-2 survey with Atacama Pathfinder EXperiment, targeting
20 infrared-luminous debris disks. These systems fill the gap between
primordial and old tenuous debris disks in terms of fractional luminosity. Here
we report on the discovery of a second 49 Ceti-like disk around the 30 Myr old
A3-type star HD21997, a member of the Columba Association. This system was also
detected in the CO(2-1) transition, and the reliable age determination makes it
an even clearer example of an old gas-bearing disk than 49 Ceti. While the
fractional luminosities of HD21997 and 49 Ceti are not particularly high, these
objects seem to harbor the most extended disks within our sample. The
double-peaked profiles of HD21997 were reproduced by a Keplerian disk model
combined with the LIME radiative transfer code. Based on their similarities, 49
Ceti and HD21997 may be the first representatives of a so far undefined new
class of relatively old (>~8 Myr), gaseous dust disks. From our results,
neither primordial origin nor steady secondary production from icy
planetesimals can unequivocally explain the presence of CO gas in the disk of
HD21997.
