Reproduce the observed CO line amplitudes in HD 13724 B atmospheric models

Reproduce the observed amplitudes of the CO absorption lines between 4.4 and 5.0 microns in forward models and atmospheric retrievals of HD 13724 B.

Background

The JWST NIRSpec/G395H spectrum of HD 13724 B contains the CO fundamental band near 4.5 microns, but the data have low signal-to-noise and exhibit correlated residuals in this region. The retrievals respond to these residuals by inferring implausibly high CO abundances and, in some cases, unphysical atmospheric compositions.

The paper reports that neither the self-consistent forward-model grids nor the retrieval models reproduce the observed CO-line amplitudes. Improving the treatment of CO opacity, chemistry, vertical mixing, and/or the data-model comparison is therefore an unresolved modeling problem relevant to reliable atmospheric characterization.

References

Notably, none of the retrievals or forward model grids were able reproduce the amplitude of the CO lines between 4.4 and 5.0 micron.

— The Dyn-Atmo Survey: High-contrast Imaging Spectroscopy of the Substellar Companion HD 13724 B with the JWST NIRSpec IFU  (2609.11827 - Hoch et al., 10 Sep 2026) in Section 5.4, “Dependence on dynamical mass”