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 that are composed of second-generation dust. To explore the transition between these phases, we searched for (CO)-C-12, (CO)-C-13, and (CO)-O-18 emission in seven dust-rich debris disks around young A-type stars, using the Atacama Large Millimeter/submillimeter Array (ALMA) in Band 6. We discovered molecular gas in three debris disks. In all of these disks, the (CO)-C-12 line was optically thick, highlighting the importance of less abundant molecules in reliable mass estimates. By supplementing our target list with literature data, we compiled a volumelimited 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 (CO)-C-12 J = 2-1 line luminosity threshold of similar to 1.4. x. 10(4) Jy. km. s(-1)pc(2) in the sample. This high incidence implies that the presence of CO gas in the bright debris disks around young A-type stars is more likely the rule than the exception. Interestingly, dust-rich debris disks around young FG-type stars exhibit, with the same detectability threshold as A-type stars, a significantly lower gas incidence. While the transition from the protoplanetary phase to the debris phase is associated with a drop in the dust content, our results exhibit a large spread in the CO mass in our debris sample, with peak values that are comparable to those in the protoplanetary Herbig Ae disks. In the particularly CO-rich debris systems, the gas may have a primordial origin, which is a characteristic of a hybrid disk.