Methanol Emission Tracing Ice Chemistry and Dust Evolution in the TW Hya Protoplanetary Disk

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 −1 ) observations of TW Hya with the Atacama Large Millimeter/submillimeter Array 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 lukewarm molecular layer of the disk ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>35</mml:mn> <mml:mo>.</mml:mo> <mml:msubsup> <mml:mrow> <mml:mn>9</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>10.6</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>25.9</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> K) and with a disk-integrated column density of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>1</mml:mn> <mml:mo>.</mml:mo> <mml:msubsup> <mml:mrow> <mml:mn>8</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.5</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>1.3</mml:mn> </mml:mrow> </mml:msubsup> <mml:mo>×</mml:mo> <mml:mn>1</mml:mn> <mml:msup> <mml:mrow> <mml:mn>0</mml:mn> </mml:mrow> <mml:mrow> <mml:mn>12</mml:mn> </mml:mrow> </mml:msup> </mml:math> 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 (≲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.