According to the current paradigm of circumstellar disk evolution , gas-rich primordial disks evolve into gas-poor debris disks composed 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 } Jy km s ^ { -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 .