First Detection of Gas-Phase Methanol in a Protoplanetary Disk

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 (> 3σ), reaching a peak signal-to-noise of 5.5σ, with the kinematic pattern expected for TW Hya. Using an appropriate disk model, a fractional abundance of 3 × 10 −12 − 4 × 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 ≈ 30 AU reaching a column density ≈ 3−6×10 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.

Detecting Weak Spectral Lines in Interferometric Data through Matched Filtering

“…Alternatively, a more detailed filter kernel can be generated from an astrochemical model of the source, with emission calculated using a radiative transfer code such as RADMC-3D (Dullemond 2012) or LIME (Brinch & Hogerheijde 2010). An example is shown in the middle panels of Figure 2, generated from the parametric CH 3 OH abundance model in Walsh et al (2016). Details of the model are presented in Appendix D.…”

“…Top:a simple kernel based on Keplerian rotation. Middle:a kernel based on a parametric model of CH 3 OH from Walsh et al (2016). Bottom:a data-driven kernel generated from H 2 CO observations from Öberg et al (2017).…”

“…In Section 2, we provide an overview of matched filtering and detail the steps of the method. In Section 3, we apply the technique to ALMA Cycle 2 observations from Walsh et al (2016) of CH 3 OH in a protoplanetary disk. In Section 4, we discuss how much S/N boost might be expected for a given data set, compare the technique to other methods, and suggest applications where matched filtering may prove useful.…”

The Gas Disk: Evolution and Chemistry

“…The observationally derived average abundances of methanol are up to two orders or magnitude lower compared to model predictions. This observations probably indicate a surface chemistry dominated formation pathway of methanol, but existing models seem to overestimate the efficiency of methanol formation and/or the efficiency of various desorption processes (Walsh et al 2016).…”

“…One prominent example for this suggested formation process is methanol (Walsh et al 2010;Dutrey et al 2014). Gas phase methanol was recently detected in the disk of TW Hya (Walsh et al 2016). The observationally derived average abundances of methanol are up to two orders or magnitude lower compared to model predictions.…”