Isotopic Ratios in the Disk of HD 163296

Abstract 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 13 C 18 O(2–1) at ∼0 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mover accent="true"> <mml:mrow> <mml:mi>.</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>″</mml:mi> </mml:mrow> </mml:mover> </mml:math> 3 resolution from the disk around the Herbig Ae star HD 163296, combined with archival observations of C 17 O(2–1), C 18 O(1–0), and C 17 O(1–0), to empirically constrain carbon and oxygen isotopic ratios without detailed disk modeling. Both the C 17 O/ 13 C 18 O(2–1) and C 18 O/C 17 O(1–0) flux ratios rise sharply across the CO snowline and flatten beyond 1 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mover accent="true"> <mml:mrow> <mml:mi>.</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>″</mml:mi> </mml:mrow> </mml:mover> </mml:math> 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 12 C/ 13 C = <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>75</mml:mn> <mml:mo>.</mml:mo> <mml:msubsup> <mml:mrow> <mml:mn>3</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>11.4</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>14.7</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> and 18 O/ 17 O = <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>3.2</mml:mn> <mml:msubsup> <mml:mrow> <mml:mn>8</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.26</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.31</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> , both consistent with local interstellar medium values. The 16 O/ 18 O ratio remains weakly constrained due to moderate optical depth in the C 18 O(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.