Molecular gas in debris disks around young A-type stars
  According to the current paradigm of circumstellar disk evolution, gas-rich
primordial disks evolve into gas-poor debris disks compose of second-generation
dust. To explore the transition between these phases, we searched for
$^{12}$CO, $^{13}$CO, and C$^{18}$O emission in seven dust-rich debris disks
around young A-type stars, using ALMA in Band 6. We discovered molecular gas in
three debris disks. In all these disks, the $^{12}$CO line was optically thick,
highlighting the importance of less abundant molecules in reliable mass
estimates. Supplementing our target list by literature data, we compiled a
volume-limited sample of dust-rich debris disks around young A-type stars
within 150 pc. We obtained a CO detection rate of 11/16 above a $^{12}$CO
J=2$-$1 line luminosity threshold of $\sim 1.4 \times 10 ^4$ Jykms$^{-1}$pc$^2$
in the sample. This high incidence implies that the presence of CO gas in
bright debris disks around young A-type stars is likely more the rule than the
exception. Interestingly, dust-rich debris disks around young FG-type stars
exhibit, with the same detectability threshold as for A-type stars,
significantly lower gas incidence. While the transition from protoplanetary to
debris phase is associated with a drop of dust content, our results exhibit a
large spread in the CO mass in our debris sample, with peak values comparable
to those in protoplanetary Herbig Ae disks. In the particularly CO-rich debris
systems the gas may have primordial origin, characteristic of a hybrid disk.
