2026ApJ...997L..19Q

Isotopic abundance ratios in protoplanetary disks are critical for understanding volatile inheritance and chemical evolution in planet-forming environments. We present Atacama Large Millimeter/submillimeter Array observations of the rare isotopologue ¹³C¹⁸O(2-1) at ∼0$\mathop{.}\limits^{^{\prime\prime} }$3 resolution from the disk around the Herbig Ae star HD 163296, combined with archival observations of C¹⁷O(2-1), C¹⁸O(1-0), and C¹⁷O(1-0), to empirically constrain carbon and oxygen isotopic ratios without detailed disk modeling. Both the C¹⁷O/¹³C¹⁸O(2-1) and C¹⁸O/C¹⁷O(1-0) flux ratios rise sharply across the CO snowline and flatten beyond 1$\mathop{.}\limits^{^{\prime\prime} }$5 (r ≳ 150 au), where the emission becomes optically thin. This transition, reflecting a steep drop in CO column density set by the disk's thermal structure, makes HD 163296 an optimal case for isotopic analysis. Using beam-averaged intensities of the four transitions measured in this optically thin region, we derive isotopic ratios of ¹²C/¹³C = $75.{3}_{-11.4}^{+14.7}$ and ¹⁸O/¹⁷O = $3.2{8}_{-0.26}^{+0.31}$, both consistent with local interstellar medium values. The ¹⁶O/¹⁸O ratio remains weakly constrained due to moderate optical depth in the C¹⁸O(1-0) line and degeneracy with CO column density. These results demonstrate that rare CO isotopologues can provide robust, empirical constraints on isotopic ratios in disks when sharp structural transitions allow for the identification of optically thin regions, and establish HD 163296 as a benchmark for extending such studies to other systems with resolved snowline structures.