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Resolved molecular line observations reveal an inherited molecular layer in the young disk around TMC1A

Physical processes that govern the star and planet formation sequence influence the chemical composition and evolution of protoplanetary disks. To understand the chemical composition of protoplanets, we need to constrain the composition and structure of the disks from whence they are formed. We aim to determine the molecular abundance structure of the young disk around the TMC1A protostar on au scales in order to understand its chemical structure and any possible implications for disk formation. We present spatially resolved Atacama Large Millimeter/submillimeter Array observations of CO, $HCO^{+}$, HCN, DCN, and SO line emission, as well as dust continuum emission, in the vicinity of TMC1A. Molecular column densities are estimated both under the assumption of optically thin emission from molecules in LTE as well as through more detailed non-LTE radiative transfer calculations. Resolved dust continuum emission from the disk is detected between 220 and 260 GHz. Rotational transitions from HCO$^{+}$, HCN, and SO are also detected from the inner 100 au region. From the derived $HCO^{+}$ abundance, we estimate the ionization fraction of the disk surface and find values that imply that the accretion process is not driven by the magneto-rotational instability. The molecular abundances averaged over the TMC1A disk are similar to its protostellar envelope and other, older Class II disks. We meanwhile find a discrepancy between the young disk's molecular abundances relative to Solar System objects. Abundance comparisons between the disk and its surrounding envelope for several molecular species reveal that the bulk of planet-forming material enters the disk unaltered. Differences in HCN and $H_2 O$ molecular abundances between the disk around TMC1A, Class II disks, and Solar System objects trace the chemical evolution during disk and planet formation.