The recent detection of gas-phase methanol (CH3βOH) 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 βΌ30β50 au and an inferred column density of
βΌ3β6Γ1012 cmβ2. A low CH3βOH fractional abundance of βΌ0.3β4Γ10β11 (with respect to H2β) is derived, depending on the
assumed vertical location of the CH3βOH molecular layer. In this study, we
use a thermo-chemical 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 CH3βOH 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 to reproduce the observations well. The CH3βOH emission in our
model appears as a ring with radius of βΌ60 au. Synthetic and observed line
flux densities are equal within the rms noise level of observations. The
synthetic CH3βOH 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 CH3βOH abundance and,
therefore, line flux densities by an order of magnitude.Comment: Accepted for publication in MNRAS, 8 pages, 8 figure