Empirical Constraints on the CO Snowline Transition in HD 163296: The Local CO Column and ¹³ C ¹⁸ O Optical Depth
- 1Boston University
- 2National Astronomical Observatory of Japan
- 3Center for Astrophysics ∣ Harvard & Smithsonian
Abstract
CO isotopolog 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, line width, and isotope ratios. Inferring CO/H ₂ additionally requires an independent constraint on the local hydrogen column. We use high-resolution ¹³ C ¹⁸ O 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 line widths of 0.30 and 0.50 km s ⁻¹ , we find N CO beam = ( 1.6 –2.4) × 10 ²⁰ cm ⁻² and ¹³ C ¹⁸ O line-center optical depths τ = 0.39–0.97. Thus, even this rare isotopolog 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 ¹⁷ O-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.
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BibTeX
@article{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},
journal = {The Astrophysical Journal},
year = {2026},
month = oct,
doi = {10.3847/1538-4357/aea097},
url = {https://iopscience.iop.org/article/10.3847/1538-4357/aea097},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - Empirical Constraints on the CO Snowline Transition in HD 163296: The Local CO Column and ¹³ C ¹⁸ O Optical Depth
AU - Chunhua Qi
AU - Takahiro Ueda
AU - David J. Wilner
AU - Catherine C. Espaillat
PY - 2026
DA - 2026/10/01
JO - The Astrophysical Journal
DO - 10.3847/1538-4357/aea097
UR - https://iopscience.iop.org/article/10.3847/1538-4357/aea097
AB - CO isotopolog 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, line width, and isotope ratios. Inferring CO/H ₂ additionally requires an independent constraint on the local hydrogen column. We use high-resolution ¹³ C ¹⁸ O 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 line widths of 0.30 and 0.50 km s ⁻¹ , we find N CO beam = ( 1.6 –2.4) × 10 ²⁰ cm ⁻² and ¹³ C ¹⁸ O line-center optical depths τ = 0.39–0.97. Thus, even this rare isotopolog 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 ¹⁷ O-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.
SN - 0004-637X
SN - 1538-4357
ER -