Imaging Detection of the Inner Dust Belt and the Four Exoplanets in the HR8799 System with JWST's MIRI Coronagraph

The multi planet system HR8799 is the first target observed with MIRI'scoronagraphs as part of the MIRI-EC Guaranteed Time Observations exoplanetsprogramme in Nov. 2022. We obtained deep observations in three coronagraphicfilters from 10 to 15mic (F1065C, F1140C, F1550C), and one standard imagingfilter at 20 mic (F2100W), with the goal to extract the photometry of the fourplanets, as well as to detect and investigate the distribution of circumstellardust. Using dedicated observations of a reference star, we tested severalalgorithms to subtract the stellar diffraction pattern while preserving thefluxes of planets, which can be significantly affected by over-subtraction.Measuring correctly the planet's flux values requires accounting for theattenuation by the coronagraphs as a function of their position, and toestimate the normalisation with respect to the central star. We tested severalprocedures to derive averaged photometric values and error bars. Theseobservations have enabled us to obtain two main results. First of all, the fourplanets in the system are well recovered, and their mid-IR fluxes, combinedwith near-IR flux values from the literature, are compared to two exoplanetatmosphere models, ATMO and Exo-REM. As a main outcome, the MIRI photometricdata points imply larger radii (0.86 or 1.07 RJ for planet b) and coolertemperatures (950 or 1100 K for planet b), especially for planet b, in betteragreement with evolutionary models. Second of all, these JWST/MIRIcoronagraphic data also deliver the first spatially resolved detection of theinner warm debris disk, the radius of which is constrained to about 15 au, withflux densities comparable, but lower than former unresolved spectroscopicmeasurements with Spitzer. abridged...