Ice Sublimation in the Dynamic HD 100453 Disk Reveals a Rich Reservoir of Inherited Complex Organics

Protoplanetary disks around luminous young A-type stars are prime observational laboratories to determine the abundances of complex organic molecules (COMs) present during planet formation. In contrast to their lower stellar mass counterparts, these warmer disks contain the sublimation fronts of complex molecules such as CH₃OH on spatial scales accessible with the Atacama Large Millimeter/submillimeter Array (ALMA). We present ALMA observations of the Herbig Ae disk HD 100453 that uncover a rich reservoir of COMs sublimating from the dust cavity edge. In addition to CH₃OH, we detect ¹³CH₃OH for the first time in a Class II disk, revealing a factor of three enhancement of ¹³C in the disk large organics. A tentative detection of CH₂DOH is also reported, resulting in a D/H of 1%–2%, which is consistent with the expected deuterium enhancement from the low-temperature CH₃OH formation in molecular clouds and with the deuteration of CH₃OH measured in comets. The detection of methyl-formate (CH₃OCHO), at only a few percent level of CH₃OH, is an order of magnitude lower compared to claims toward other organic-rich Herbig Ae disks but is more in line with organic abundance patterns toward the earlier stages of star formation. Together these data provide multiple lines of evidence that disks, and therefore the planet- and comet-forming materials, contain inherited interstellar ices and perhaps the strongest evidence to date that much of the interstellar organic ice composition survives the early stages of planet formation.