2020ApJ...901..166V

The chemical composition of gas and ice in disks around young stars sets the bulk composition of planets. In contrast to protoplanetary disks (Class II), young disks that are still embedded in their natal envelope (Class 0 and I) are predicted to be too warm for CO to freeze out, as has been confirmed observationally for L1527 IRS. To establish whether young disks are generally warmer than their more evolved counterparts, we observed five young (Class 0/I and I) disks in Taurus with the Atacama Large Millimeter/submillimeter Array, targeting C¹⁷O 2 - 1, H₂CO 3_1,2-2_1,1, HDO 3_1,2-2_2,1, and CH₃OH 5_K - 4_K transitions at 0.''48 x 0.''31 resolution. The different freeze-out temperatures of these species allow us to derive a global temperature structure. C¹⁷O and H₂CO are detected in all disks, with no signs of CO freeze-out in the inner ∼100 au and a CO abundance close to ∼10^–4. The H₂CO emission originates in the surface layers of the two edge-on disks, as witnessed by the especially beautiful V-shaped emission pattern in IRAS 04302+2247. HDO and CH₃OH are not detected, with column density upper limits more than 100 times lower than for hot cores. Young disks are thus found to be warmer than more evolved protoplanetary disks around solar analogs, with no CO freeze-out (or only in the outermost part of >=100 au disks) or processing. However, they are not as warm as hot cores or disks around outbursting sources and therefore do not have a large gas-phase reservoir of complex molecules.