2017MNRAS.468.2024P

The recent detection of gas-phase methanol (CH₃OH) lines in the disc of TW Hya by Walsh et al. provided the first observational constraints on the complex O-bearing organic content in protoplanetary discs. The emission has a ring-like morphology, with a peak at ∼30-50 au and an inferred column density of ∼3-6 x 10¹² cm^–2. A low CH₃OH fractional abundance of ∼0.3-4 x 10^–11 (with respect to H₂) is derived, depending on the assumed vertical location of the CH₃OH molecular layer. In this study, we use a thermochemical model of the TW Hya disc, coupled with the ALCHEMIC gas-grain chemical model, assuming laboratory-motivated, fast diffusivities of the surface molecules to interpret the CH₃OH detection. Based on this disc model, we performed radiative transfer calculations with the LIME code and simulations of the observations with the CASA simulator. We found that our model allows us to reproduce the observations well. The CH₃OH emission in our model appears as a ring with radius of ∼60 au. Synthetic and observed line flux densities are equal within the rms noise level of observations. The synthetic CH₃OH spectra calculated assuming local thermodynamic equilibrium (LTE) can differ by up to a factor of 3.5 from the non-LTE spectra. For the strongest lines, the differences between LTE and non-LTE flux densities are very small and practically negligible. Variations in the diffusivity of the surface molecules can lead to variations of the CH₃OH abundance and, therefore, line flux densities by an order of magnitude.