Methanol emission tracing ice chemistry and dust evolution in the TW Hya protoplanetary disk

TL;DR

This paper reports the first high-sensitivity ALMA detection of multiple gas-phase methanol transitions in the TW Hya protoplanetary disk, constraining a disk-averaged rotational temperature around 36 K and a column density near 1.8 × 10^12 cm^-2. The emission is compact, peaking within the mm-sized dust disk (≲80 au), and shows diverse radial morphologies across transitions. Through a suite of gas-grain chemical models, the study identifies photodesorption as the dominant non-thermal desorption mechanism releasing methanol from ices, with grain-surface chemistry and fractal dust surfaces boosting the gas-phase abundance, though static models struggle to reproduce the observed magnitude and radial profile. The work also highlights the potential importance of vertical mixing and radial dust drift in transporting methanol-rich ices into the luke-warm molecular layer, linking ice reservoirs to the chemical inventory available for planet formation. Overall, the results demonstrate the intricate interplay between ice chemistry and dust evolution in setting the abundance of prebiotic molecules in planet-forming regions around Sun-like stars.

Abstract

Methanol (CH$_{3}$OH) ice is abundant in space and is a key feedstock for seeding chemical complexity in interstellar and circumstellar environments. Despite its ubiquity, gas-phase methanol has only been detected in one disk around a Solar-type star to date, TW Hya. Here we present new high sensitivity (~1 mJy/beam) observations of TW Hya with ALMA that detect four individual transitions of gas-phase methanol spanning upper level energies from 17 to 38 K. We confirm the presence of gas-phase methanol in the luke-warm molecular layer of the disk ($35.9^{+25.9}_{-10.6}$ K) and with a disk-integrated column density of $1.8^{+1.3}_{-0.5}\times 10^{12}$ cm$^{-2}$. A radially-resolved analysis suggests that the gas-phase methanol is centrally compact, peaking within the spatial extent of the mm-sized dust grains ($\lesssim 80$ au). Static gas-grain chemical disk models confirm photodesorption as an important mechanism releasing methanol into the gas phase, with the column density further boosted by the inclusion of grain-surface chemistry, reactive desorption, and an increase in dust-grain surface area assuming fractal grains. However, no model can fully reproduce the observed column density nor the radial distribution, and we suggest that the inclusion of dynamic processes such as vertical mixing and radial drift would be required to do so. Our results demonstrate that the abundance and distribution of the precursors for complex chemistry in the planet-forming regions around Solar-type stars is ultimately controlled by the interplay of grain surface chemistry coupled with the evolution of dust in their disks.

Figures (13)

• Figure 1: Morphology of the methanol (CH3OH) emission toward TW Hya. (a) 290 GHz continuum emission from Huang2018 overlaid with the dust disk extent derived from our observations in Ilee2022 (dashed ellipse). (b) Integrated intensity map of the $0\farcs7$ stacked methanol transitions. (c-f) As (b), but for the individually detected transitions.
• Figure 2: Radial profiles of emission for the detected methanol (CH3OH) transitions. The upper panels show the 04 observations, while the lower panels show the 07 observations. Transitions are labelled with $E_\mathrm{u}$, shaded regions indicate 1$\sigma$ uncertainties
• Figure 3: Rotational diagram calculated from the disk-integrated values of the 0$\farcs$7 observations. Upper limits are denoted with a downward triangle, random draws from the corresponding posterior probability distribution are shown in blue, and median values with their uncertainties are labelled.
• Figure 4: Radially resolved rotational diagram for the 0$\farcs$4 observations, where shaded regions indicate uncertainties obtained from the 16$^{\rm th}$--84$^{\rm th}$ percentile of the posterior distribution. Dashed lines show values derived from the disk integrated analysis (see Figure \ref{['fig:rotational']}). The vertical grey region indicates a radial extent of half the beam size.
• Figure 5: Physical conditions across a vertical slice of TW Hya at a radius of 30 au calahan21. Shown are the H$_\mathrm{nuc}$ density (top left), gas (dotted) and dust (dashed) temperature (top right), UV (dotted) and X-ray (dashed) flux (middle left), cosmic-ray (dotted) and X-ray (dashed) ionisation rate (middle right), dust grain fractional abundance (bottom left) and dust-to-gas mass ratio (bottom right), all as a function of $z/r$ (i.e., height divided by radius).