First detection of gas-phase methanol in a protoplanetary disk

TL;DR

This paper reports the first detection of gas-phase methanol in a protoplanetary disk, revealing its potential as a marker for ice chemistry and organic molecule formation during planet formation.

Contribution

It provides the first unambiguous detection and spatial analysis of gas-phase methanol in a protoplanetary disk, linking ice chemistry to disk organic composition.

Findings

Methanol detected across six velocity channels with >3σ significance.

Methanol emission peaks at approximately 30 AU, suggesting a ring-like distribution.

Estimated methanol abundance ranges from 3×10⁻¹² to 4×10⁻¹¹ relative to H₂.

Abstract

The first detection of gas-phase methanol in a protoplanetary disk (TW Hya) is presented. In addition to being one of the largest molecules detected in disks to date, methanol is also the first disk organic molecule with an unambiguous ice chemistry origin. The stacked methanol emission, as observed with ALMA, is spectrally resolved and detected across six velocity channels ($>3 \sigma$), reaching a peak signal-to-noise of $5.5\sigma$, with the kinematic pattern expected for TW~Hya. Using an appropriate disk model, a fractional abundance of $3\times 10^{-12} - 4 \times 10^{-11}$ (with respect to H$_2$) reproduces the stacked line profile and channel maps, with the favoured abundance dependent upon the assumed vertical location (midplane versus molecular layer). The peak emission is offset from the source position suggesting that the methanol emission has a ring-like morphology: the analysis here suggests it peaks at $\approx 30$~AU reaching a column density $\approx 3-6\times10^{12}$~cm$^{-2}$. In the case of TW Hya, the larger (up to mm-sized) grains, residing in the inner 50~AU, may thus host the bulk of the disk ice reservoir. The successful detection of cold gas-phase methanol in a protoplanetary disk implies that the products of ice chemistry can be explored in disks, opening a window to studying complex organic chemistry during planetary system formation.