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 the field T dwarf 2MASS J0415-0935. We analyse JWST/NIRSpec G395H spectra (2.87 -- 5.2 mu m; R about2700) and perform Bayesian atmospheric retrievals with petitRADTRANS coupled to nested sampling using ultranest. We use a flexible, parameterised pressure-temperature profile with free, constant-with-altitude molecular abundances. For HD 19467 B we fit the PSF-subtracted spectrum with a linear model that includes the atmospheric model and a set of speckle spectra from the integral field unit. We detect H 2 O, CH 4, CO, CO 2, and NH 3 in both atmospheres and measure carbon isotopic ratios from CO isotopologues, finding 12 C/ 13 C =154+19-17 for HD 19467 B and 12 C/ 13 C =85 plus or minus5 for 2MASS J0415-0935. Speckle contamination primarily affects the low-frequency spectral shape at 3.0 -- 3.7 mu m and can affect retrieved abundances if not accounted for. We obtain seemingly constrained posteriors for some additional species (e.g. SiO and H 2 S) in some cases, but treat these as tentative because cross-correlation does not yield significant detections; PH 3 is not detected in either target. Joint fitting of the atmospheric spectrum and the speckle contamination enables native-resolution retrievals of the high-contrast companion HD 19467 B with JWST/NIRSpec without continuum subtraction. Over 2.87 -- 5.2 mu m, medium-resolution spectroscopy constrains elemental and isotopic composition; both objects exhibit near-solar metallicity and subsolar C/O ratios.