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          Institute: MPI für Astronomie     Collection: Publikationen_mpia     Display Documents

ID: 742514.0, MPI für Astronomie / Publikationen_mpia
On the methanol emission detection in the TW Hya disc: the role of grain surface chemistry and non-LTE excitation
Authors:Parfenov, S. Y.; Semenov, D. A.; Henning, T.; Shapovalova, A. S.; Sobolev, A. M.; Teague, R.
Date of Publication (YYYY-MM-DD):2017
Title of Journal:Monthly Notices of the Royal Astronomical Society
Start Page:2024
End Page:2031
Audience:Not Specified
Abstract / Description: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 × 1012 cm-2. A low CH3OH fractional abundance of ∼0.3-4 × 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.
Free Keywords:astrochemistry; line: formation; molecular processes; protoplanetary discs; stars: individual: TW Hya; submillimetre: planetary systems
External Publication Status:published
Document Type:Article
Communicated by:N. N.
Affiliations:MPI für Astronomie
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