2018A&A...615A..83V

Context. Protoplanetary disks have been studied extensively, both physically and chemically, to understand the environment in which planets form. However, the first steps of planet formation are likely to occur already when the protostar and disk are still embedded in their natal envelope. The initial conditions for planet formation may thus be provided by these young embedded disks, of which the physical and chemical structure is poorly characterized.
Aims. We aim to constrain the midplane temperature structure, one of the critical unknowns, of the embedded disk around L1527. In particular, we set out to determine whether there is an extended cold outer region where CO is frozen out, as is the case for Class II disks. This will show whether young disks are indeed warmer than their more evolved counterparts, as is predicted by physical models.
Methods. We used archival ALMA data of ¹³CO J=2-1, C¹⁸O J=2-1 and N₂D⁺J=3-2 to directly observe the midplane of the near edge-on L1527 disk. The optically thick CO isotopologues allowed us to derive a radial temperature profile for the disk midplane, while N₂D⁺, which can only be abundant when CO is frozen out, provides an additional constraint on the temperature. Moreover, the effect of CO freeze-out on the ¹³CO, C¹⁸O and N₂D⁺ emission was investigated using 3D radiative transfer modeling.
Results. Optically thick ¹³CO and C¹⁸O emission is observed throughout the disk and inner envelope, while N₂D⁺ is not detected. Both CO isotopologues have brightness temperatures ≥25K along the midplane. Disk and envelope emission can be disentangled kinematically, because the largest velocities are reached in the disk. A power law radial temperature profile constructed using the highest midplane temperature at these velocities suggest that the temperature is above 20K out to at least 75au, and possibly throughout the entire 125au disk. The radiative transfer models show that a model without CO freeze-out in the disk matches the C¹⁸O observations better than a model with the CO snowline at ∼70au. In addition, there is no evidence for a large (order of magnitude) depletion of CO
Conclusions. The disk around L1527 is likely to be warm enough to have CO present in the gas phase throughout the disk, suggesting that young embedded disks can indeed be warmer than the more evolved Class II disks.