Molecules in the shadows of the protoplanetary disk HD 143006: Evidence for C/O ≳ 1 from simultaneously modeling fourteen ALMA lines
Abstract
The inner disk of the T Tauri star HD 143006 is inclined and casts a broad shadow on a large portion of the outer disk. We use this extraordinary geometry to study the impact of stellar ultraviolet (UV) irradiation on disk chemistry, temperature, and line formation. We present new ALMA observations of this disk in CO, 13CO, C18O, CN, HCN, HCO+, CS and C3H2 and combine these with previously-published data to get a rich dataset, which is modeled with ProDiMo models for the illuminated and shadowed side by including an optional inclined inner disk to partially block stellar illumination of the outer disk. The observed line flux contrasts between the illuminated and the shadowed disk sides are small. We explain this by introducing ``equilibrated models'' which consider a slow heating/cooling relaxation of gas parcels orbiting in and out of the shadow. We fit the radial line intensity profiles of all 14 ALMA lines for both halves of the disk, the SED beyond 20μm, and two ALMA continuum images, with an overall reduced χ of about 2.3, in one combined model. Line intensities only increase slightly on the illuminated side because the gas warms up slowly, although fast UV photochemistry significantly reduces the observable molecular concentrations. For the lines of N2H+ and CS, which probe deeper layers where the gas and dust temperatures are coupled, slow UV-desorption of ices such as NH3 and H2S plays a crucial role. This can result in a brightening of these lines on the shadowed side. The carbon-to-oxygen ratio in the outer disk of HD 143006 is slightly greater than one (∼ 1.05). The disk mass is fitted to be as low as 0.0012 M⊙, but almost equally well-fitting models with a fixed disk mass of 0.021 M⊙ can be obtained after re-adjusting the disk shape, dust, and gas heating/cooling parameters. (abridged)
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BibTeX
@misc{terwisga2026molecules,
title = {{Molecules in the shadows of the protoplanetary disk HD 143006: Evidence for C/O $\gtrsim 1$ from simultaneously modeling fourteen ALMA lines}},
author = {S. E. van Terwisga and P. Woitke and A. M. Weiss and A. D. Bosman and L. Trapman},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.26433},
eprint = {2609.26433},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.26433},
}
RIS
TY - UNPB
TI - Molecules in the shadows of the protoplanetary disk HD 143006: Evidence for C/O ≳ 1 from simultaneously modeling fourteen ALMA lines
AU - S. E. van Terwisga
AU - P. Woitke
AU - A. M. Weiss
AU - A. D. Bosman
AU - L. Trapman
PY - 2026
DA - 2026/09/22
PB - arXiv
DO - 10.48550/arxiv.2609.26433
UR - https://arxiv.org/abs/2609.26433
AB - The inner disk of the T Tauri star HD 143006 is inclined and casts a broad shadow on a large portion of the outer disk. We use this extraordinary geometry to study the impact of stellar ultraviolet (UV) irradiation on disk chemistry, temperature, and line formation. We present new ALMA observations of this disk in CO, ^13CO, C^18O, CN, HCN, HCO^+, CS and C_3H_2 and combine these with previously-published data to get a rich dataset, which is modeled with ProDiMo models for the illuminated and shadowed side by including an optional inclined inner disk to partially block stellar illumination of the outer disk. The observed line flux contrasts between the illuminated and the shadowed disk sides are small. We explain this by introducing ``equilibrated models'' which consider a slow heating/cooling relaxation of gas parcels orbiting in and out of the shadow. We fit the radial line intensity profiles of all 14 ALMA lines for both halves of the disk, the SED beyond 20μm, and two ALMA continuum images, with an overall reduced χ of about 2.3, in one combined model. Line intensities only increase slightly on the illuminated side because the gas warms up slowly, although fast UV photochemistry significantly reduces the observable molecular concentrations. For the lines of N_2H^+ and CS, which probe deeper layers where the gas and dust temperatures are coupled, slow UV-desorption of ices such as NH_3 and H_2S plays a crucial role. This can result in a brightening of these lines on the shadowed side. The carbon-to-oxygen ratio in the outer disk of HD 143006 is slightly greater than one (∼ 1.05). The disk mass is fitted to be as low as 0.0012 M_⊙, but almost equally well-fitting models with a fixed disk mass of 0.021 M_⊙ can be obtained after re-adjusting the disk shape, dust, and gas heating/cooling parameters. (abridged)
ER -