Empirical Constraints on the CO Snowline Transition in HD 163296: The Local CO Column and 13 C 18 O Optical Depth

CO isotopologue emission is widely used to infer gas masses and volatile carbon abundances in protoplanetary disks, but converting line emission into a CO column depends on optical depth, temperature structure, linewidth, and isotope ratios. Inferring CO/H 2 additionally requires an independent constraint on the local hydrogen column. We use high-resolution 13 C 18 O 2 - 1 observations of HD 163296 to derive a spatially localized empirical constraint on the CO column at the resolved CO snowline edge. We focus on the 70-75 au annulus, on the inner, high-column side of the observed profile steepening near 75 au. Using RADEX slab calculations conditioned on a two-dimensional temperature structure, we infer an effective beam-averaged CO column from the absolute integrated intensity. Across representative temperatures, isotope-ratio pairs, and effective local linewidths of 0.30 and 0.50 km s-1, we find N CO beam=(1.6-2.4) times10 20 cm-2 and 13 C 18 O line-center optical depths τ=0.39-0.97. Thus, even this rare isotopologue is not safely optically thin at the snowline edge. Adopting N H2=2.5 times10 24 cm-2 from a published parametric gas surface-density profile gives the conditional abundance CO/H 2 =(6.4-9.5) times10-5. A beam-forward radial-profile analysis gives consistent columns, and a physical disk model with a near-canonical warm-layer CO abundance supplies a comparable CO column. The measurement is consistent with the higher C 17 O-based MAPS estimate. For the adopted hydrogen column, the inferred CO/H 2 ratio is consistent with a near-canonical abundance on the warm side of the snowline.