2025ApJ...982...62E

Methanol (CH₃OH) and formaldehyde (H₂CO) are chemically coupled organic molecules proposed to act as an intermediate step between simple molecules and more complex prebiotic compounds. Their abundance distributions across disks regulate the prebiotic potential of material at different disk radii. We present observations of multiple methanol and formaldehyde transitions toward the Herbig Ae disk HD 100546 obtained with the Atacama Large Millimeter/submillimeter Array, building upon the previous serendipitous detection of methanol in this source. We find that methanol has a higher rotational temperature (T_rot) than formaldehyde toward both the centrally concentrated emission component in the inner disk (0-110 au) and a radially separate dust ring farther out in the disk (180-260 au). T_rot decreases for methanol and formaldehyde from the inner ($15{2}_{-27}^{+35}$ K and $7{6}_{-8}^{+9}$ K) to the outer disk ($5{2}_{-6}^{+8}$ K and $3{1}_{-2}^{+2}$ K), suggesting that we are tracing two different chemical environments. T_rot for both species in the inner disk is consistent with thermal desorption as the origin, while the outer disk reservoir is driven by nonthermal desorption. The CH₃OH/H₂CO column density ratio decreases from ${14.6}_{-4.6}^{+5.2}$ in the inner disk to ${1.3}_{-0.2}^{+0.3}$ in the outer disk, consistent with modeling predictions. The CH₃OH/H₂CO column density ratio for the inner disk is consistent with the median value in the range of column density ratios compiled from solar system comets, which would have formed at a similar distance. This supports the notion that interstellar ice is inherited and preserved by protoplanetary disks around solar-mass and intermediate-mass stars as we are seeing “fresh” ice sublimation, as well as providing more evidence for the presence of prebiotic precursor molecules in planet-forming regions.