Isotopic Ratios in the Disk of HD 163296

TL;DR

This study measures isotopic ratios of carbon and oxygen in the disk of HD 163296 using ALMA observations, revealing ratios consistent with interstellar values and demonstrating a method for empirical isotopic analysis in protoplanetary disks.

Contribution

It provides the first empirical constraints on isotopic ratios in HD 163296's disk using rare CO isotopologues without detailed modeling, highlighting the importance of structural transitions.

Findings

Isotopic ratios of 12C/13C and 18O/17O are consistent with interstellar medium values.

Sharp transitions across the CO snowline enable optically thin regions for analysis.

HD 163296 serves as a benchmark for isotopic studies in protoplanetary disks.

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 13C18O(2-1) at approximately 0.3 arcsec 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 1.5 arcsec (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 (+14.7/-11.4) and 18O/17O = 3.28 (+0.31/-0.26), 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.