First detection of gas-phase methanol in a protoplanetary disk (1606.06492v1)

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.

Detection of Gas-Phase Methanol in a Protoplanetary Disk

The paper authored by Catherine Walsh and collaborators presents a significant advancement in astrochemical research by reporting the first detection of gas-phase methanol (\ce{CH3OH}) in the protoplanetary disk surrounding TW Hya. Using observations from the Atacama Large Millimeter/Submillimeter Array (ALMA), the paper provides comprehensive data indicating the presence of methanol as well as discussing its potential implications on our understanding of disk chemistry and planetary system formation.

Numerical Results and Analysis

The detection of \ce{CH3OH} was achieved through stacked emission across six velocity channels, reaching a peak signal-to-noise ratio of 5.5σ. The emission patterns align with the kinematics expected for TW Hya, confirming the presence of methanol in the disk. The analysis employs disk models to estimate a fractional abundance of \ce{CH3OH} ranging from $3\times 10^{-12}$ to $4\times 10^{-11}$ relative to \ce{H2}, dependent on its vertical location within the disk (midplane versus molecular layer). Additionally, the methanol emission exhibits a ring-like structure, peaking at approximately 30 AU and reaching a column density of $\approx 3-6\times10^{12}$ cm$^{-2}$.

Theoretical and Practical Implications

This detection underscores the role of ice chemistry in protoplanetary disks, showcasing that complex organic chemistry products can be studied during the stages of planetary system formation. Methanol, being a complex organic molecule, indicates active chemical processes within these environments and opens up a pathway for further exploration of larger COMs and the overall chemical composition diversity in disks.

The spatial distribution suggests that methanol is possibly concentrated around the CO snowline, highlighting the potential importance of snowlines in chemical evolution within disks. From a practical standpoint, the paper provides a foundation for utilizing ALMA's capability to enhance the detection of minor species through stacking multiple transitions to increase sensitivity.

Speculation on Future Developments

This detection prompts several interesting research directions. Future studies could focus on acquiring higher spatial resolution data to delineate the precise vertical and radial distribution of methanol, which could shed light on the physical processes leading to its desorption and distribution. Furthermore, experimental work investigating the efficiency of non-thermal desorption processes like photodesorption and reactive desorption in disk environments could refine the modeled abundance of methanol and other organic molecules.

The presence of methanol may also provide clues about the depletion mechanisms of \ce{CO} gas, suggesting possible chemical pathways transforming CO into more complex molecules. Understanding these pathways could clarify the comprehensive chemical network active during the formation of planets and comets.

Conclusion

The detection of gas-phase methanol in TW Hya offers critical insights into the chemical dynamics of protoplanetary disks, presenting a unique opportunity to probe the intricate processes of organic molecule formation and distribution during early planetary system development. This discovery, facilitated by ALMA’s pioneering observational capabilities, sets the stage for expanded exploration into the chemical complexity inherent in these astronomical environments.