MIRI-JWST mid-infrared direct imaging of the debris disk of HD 106906: Structure and mass of the disk

Context. We report MIRI-JWST coronagraphic observations at 11.3 and 15.5 μm of the debris disk around the young star HD 106906. The wavelength range is sensitive to the thermal emission of the dust heated by the central star. Aims. The observations were made to characterize the structure of the disk through the thermal emission, to search for clues to the presence of a central void of dust particles, and to derive the mass of the dust and the temperature distribution. Another goal was also to constrain the size distribution of the grains. Methods. The data were reduced and calibrated using the JWST pipeline. The analysis was based on a forward-modeling of the images using a multiparameter radiative transfer model coupled to an optical code for coronagraphy processing. Results. The disk is clearly detected at both wavelengths. The slight asymmetry is geometrically consistent with the asymmetry observed in the near-IR, but it is inconsistent the brightness distribution. The observed structure is well reproduced with a model of a disk (or belt) with a critical radius 70 au, a mildly inward-increasing density (index 2) and a steeper decrease outward (index ‑6). This indication of a filled disk inside the critical radius is inconsistent with sculpting from an inner massive planet. The size distribution of the grains that cause the mid-IR emission is well constrained by the flux ratio at the two wavelengths : 0.45–10 and 0.65–10 μm for silicate and graphite grains, respectively. The minimum size is consistent with predictions of blowout through radiative pressure. Conclusions. We derive a mass of the dust that causes the mid-IR emission of 3.3–5.0 10<SUP>‑3</SUP> M<SUB>⊕</SUB>. When the larger grains (up to 1 cm) that cause the millimeter emission are included, we extrapolate this mass to 0.10–0.16 M<SUB>⊕</SUB>. We point out to that this is fully consistent with ALMA observations of the disk in terms of dust mass and of its millimeter flux. We estimate the average dust temperature in the planetesimal belt to be 74 K, but the temperature range within the whole disk is rather wide: from 40 to 130 K.