Empirical Constraints on the CO Snowline Transition in HD 163296: The Local CO Column and 13C18O Optical Depth
Abstract
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/H2 additionally requires an independent constraint on the local hydrogen column. We use high-resolution 13C18O $2−1$ 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 NCObeam=(1.6−2.4)×1020 cm−2 and 13C18O line-center optical depths τ=0.39−0.97. Thus, even this rare isotopologue is not safely optically thin at the snowline edge. Adopting NH2=2.5×1024 cm−2 from a published parametric gas surface-density profile gives the conditional abundance CO/H2=(6.4−9.5)×10−5. 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 C17O-based MAPS estimate. For the adopted hydrogen column, the inferred CO/H2 ratio is consistent with a near-canonical abundance on the warm side of the snowline.
Related
- Empirical Constraints on the CO Snowline Transition in HD 163296: The Local CO Column and ¹³ C ¹⁸ O Optical Depth
- The IACOB project XIX. Revisiting massive-star evolution with empirical TAMS constraints: updated models, overshoot calibration, and the population of blue supergiants
- HII-CHI-mistry for active galactic nuclei: optical sulphur abundances without empirical ionisation correction factors
- HII-CHI-mistry for active galactic nuclei: optical sulphur abundances without empirical ionisation correction factors
- Around the water snowline: I. Effects of dynamical and chemical interplay on mid-infrared water spectra of protoplanetary discs
BibTeX
@misc{qi2026empirical,
title = {{Empirical Constraints on the CO Snowline Transition in HD 163296: The Local CO Column and $^13$C$^18$O Optical Depth}},
author = {Chunhua Qi and Takahiro Ueda and David J. Wilner and Catherine C. Espaillat},
howpublished = {arXiv},
year = {2026},
month = aug,
doi = {10.48550/arxiv.2608.30140},
eprint = {2608.30140},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2608.30140},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Empirical Constraints on the CO Snowline Transition in HD 163296: The Local CO Column and ^13C^18O Optical Depth
AU - Chunhua Qi
AU - Takahiro Ueda
AU - David J. Wilner
AU - Catherine C. Espaillat
PY - 2026
DA - 2026/08/31
PB - arXiv
DO - 10.48550/arxiv.2608.30140
UR - https://arxiv.org/abs/2608.30140
AB - 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 ^13C^18O $2−1$ 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)×10^20 cm^−2 and ^13C^18O 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=2.5×10^24 cm^−2 from a published parametric gas surface-density profile gives the conditional abundance CO/H_2=(6.4−9.5)×10^−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^17O-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.
ER -