JWST high-contrast spectroscopy with speckle modelling: Atmospheric retrievals of the T-dwarf companion HD 19467 B

High-contrast medium-resolution spectroscopy with JWST can resolve molecular and isotopic features in cool substellar atmospheres, but for close-in companions, the extracted spectra can be biased by wavelength-dependent residual stellar contamination. We assess the impact of residual speckles on atmospheric inference for the T-dwarf companion HD 19467 B and compare the results to those for the field T dwarf 2MASS J0415-0935. We analysed JWST/NIRSpec G395H spectra (2.87--5.2 (R coupled with nested sampling using . We used a flexible parametrised pressure--temperature profile with free constant-with-altitude molecular abundances. For HD 19467 B, we fitted the PSF-subtracted spectrum with a linear model that included the atmospheric model and a set of speckle spectra from the integral field unit. and performed Bayesian atmospheric retrievals with ultranest We detected _ CO, and NH_3 in both atmospheres and measured carbon isotopic ratios from CO isotopologues, finding +19 -17 $ for HD 19467 B and for 2MASS J0415-0935. Speckle contamination primarily affects the low-frequency spectral shape at 3.0--3.7 and can affect the retrieved abundances when it is not accounted for. We obtained apparently constrained posteriors for some additional species (e.g. SiO and H_2S) in some cases, but treated them as tentative because a cross-correlation yielded no significant detections; PH_3 is not detected in either target. A joint fitting of the atmospheric spectrum and the speckle contamination enabled us to perform native-resolution retrievals of the high-contrast companion HD 19467 B with JWST/NIRSpec without continuum subtraction. Over 2.87--5.2 medium-resolution spectroscopy constrains the elemental and isotopic composition. Both objects exhibit near-solar metallicity and subsolar C/O ratios.