Evidence for a Sharp CO Snow Line Transition in a Protoplanetary Disk and Implications for Millimeter-wave Observations of CO Isotopologues

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Observations of CO isotopologue emission from protoplanetary disks at millimeter wavelengths are a powerful tool for probing the CO snow line, an important marker for disk chemistry, and also for estimating total disk gas mass, a key quantity for planet formation. We use simple models to demonstrate that the vertical thickness of an isothermal layer around the disk midplane has important effects on the CO column density radial profile, with a thick layer producing a sharp CO snow line transition. We simulate next-generation Very Large Array (ngVLA) and Atacama Large Millimeter/submillimeter Array (ALMA) images to show that this sharp change in the CO column density can be detected in the derivative of the radial profile of emission from optically thin CO isotopologue lines. We apply this method to archival ALMA observations of the disk around the Herbig Ae star HD 163296 in the C17O and C18O J = 1–0 and J = 2–1 lines to identify a sharp CO snow line transition near ∼80 au (0.″8 at 101 pc), and show the CO column density decreases by more than a factor of 20. This finding is consistent with previous inferences from the steep rise of N2H+ emission, which marks the location where CO depletes. We also demonstrate that the disk’s thermal structure introduces significant systematic uncertainty to estimates of total disk gas mass derived from these lines. The substantial improvement in sensitivity envisioned for the ngVLA over ALMA for observations of ground-state lines of CO isotopologues has the potential to extend this approach to a much larger population of disks.