2018ApJ...863..106K

We present a suite of Atacama Large Millimeter Array (ALMA) interferometric molecular line and continuum images that elucidate, on linear size scales of ∼30-40 au, the chemical structure of the nearby, evolved, protoplanetary disk orbiting the close binary system V4046 Sgr. The observations were undertaken in the 1.1-1.4 mm wavelength range (ALMA Bands 6 and 7) with antenna configurations involving maximum baselines of several hundred meters, yielding subarcsecond-resolution images in more than a dozen molecular species and isotopologues. Isotopologues of CO and HCN display centrally peaked morphologies of integrated emission-line intensity, whereas the line emission from complex nitrile group molecules (HC₃N, CH₃CN), deuterated molecules (DCN, DCO⁺), hydrocarbons (as traced by C₂H), and potential CO ice line tracers (N₂H⁺, and H₂CO) appears as a sequence of sharp and diffuse rings. The dimensions and morphologies of HC₃N and CH₃CN emission are suggestive of photodesorption of organic ices from the surfaces of dust grains, while the sequence of increasing radius of peak intensity represented by DCN (smallest), DCO⁺, N₂H⁺, and H₂CO (largest) is qualitatively consistent with the expected decline of midplane gas temperature with increasing disk radius. Empirical modeling indicates that the sharp-edged C₂H emission ring lies at relatively deep disk layers, leaving open the question of the origin of C₂H abundance enhancements in evolved disks. This study of the "molecular anatomy" of V4046 Sgr should serve as motivation for additional subarcsecond ALMA molecular line imaging surveys of nearby, evolved protoplanetary disks aimed at addressing major uncertainties in protoplanetary disk physical and chemical structure and molecular production pathways.