Discovery of carbon monoxide emission from five debris disks around young A-type stars

Abstract: Over the past fifteen years, surveys mainly at millimeter wavelengths have led to the discovery of $\sim$20 gas-bearing debris disks, most of them surrounding young intermediate-mass stars. Exploring the properties and origin of this gas could be fundamental to better understanding the transition between the protoplanetary and debris disk phases, the evolution of icy planetesimal belts, and the formation of planetary atmospheres. To expand the list of known CO-bearing debris disks and to improve our knowledge of the environmental conditions under which they can form, we targeted twelve dust-rich debris disks around young ($<$50 Myr) intermediate-mass stars. Using the ALMA 12-m Array we performed millimeter continuum and CO line observations to search for dust and gas and to measure their quantity and spatial distribution. We discovered CO gas in five disks. Two of them have a low CO content of a few times 10$^{-5}$ M$\oplus$, similar to that of $\beta$ Pic. The other three disks, however, are CO-rich with $M\mathrm{CO}>10^{-3}$ M$\oplus$. By combining our results with those of other studies we concluded, in agreement with previous findings, that the detection rate of CO gas is significantly higher for disks around stars with $6.5~L\odot<L_<21.9~L_\odot$ ($\sim$A8$-$A0 spectral type) than for disks around less luminous stars ($0.18~L_\odot<L_<6.4~L_\odot$, K7$-$A9). A comparison of the measured CO masses and the estimated mass loss rates of solids in disks with low CO content ($<$10$^{-4}$ M$_\oplus$) suggests that collisions may play a role in CO gas production in such systems. Interestingly, however, the estimated mass loss rates of CO-rich debris disks are not higher than those of systems with low CO content. In light of this finding, we speculate on what could lead to the formation of CO-rich debris disks.