Radial profiles of the same-transition isotopologue intensity ratios. Left: C¹⁷O/¹³C¹⁸O(2–1) ratio, flat at small radii, rising steeply across the CO snowline, and converging to ∼20 beyond 1﹩\mathop{.}\limits^{^{\prime\prime} }﹩5. Uncertainties include both statistical errors and a 7.1% systematic calibration term, as the two lines were observed in different bands and epochs. Right: C¹⁸O/C¹⁷O(1–0) ratio, showing similar radial behavior and approaching ∼2.7 beyond 1﹩\mathop{.}\limits^{^{\prime\prime} }﹩5. Only statistical uncertainties are shown, since both lines were observed in the same band and share a common amplitude calibration. In both panels, the rapid increase followed by a flat plateau indicates the sharp decline in CO column density across the snowline and identifies the outer disk as an optically thin region suitable for isotopic ratio analysis.