A Cavity in the Compact Disk of the Forming Companion GQ Lup B Revealed by JWST/MIRI

GQ Lup B is a forming brown dwarf companion ($M\sim10-30~\MJ$) showing evidence for an infrared excess associated with a disk surronding the companion itself. Here we present mid-infrared (MIR) observations of GQ~Lup~B with the Medium Resolution Spectrograph (MRS) on JWST, spanning 4.8--11.7 $\mu$m. We remove the stellar contamination using reference differential imaging based on principal component analysis, demonstrating that the MRS can perform high-contrast science. Our observations provide a sensitive probe of substructures in the disk surrounding GQ~Lup~B. We find that the dust disk around the companion is compact and exhibits an inner cavity. While the size of the cavity depends on the model assumptions, we find that it ranges between $\sim8.2$ and $\sim40.3~\RJ$. Several mechanisms like dust sublimation, magnetospheric accretion or satellite formation could be at responsible for the cavity, depending on its actual size. Furthermore, we find a disk outer radius between 38.6 and $112.4~\RJ$, consistent with archival ALMA upper limits. This approach paves the path to the future study of circumplanetary disks and their physical properties. Our results demonstrate that MIR spectroscopic observations can reveal the physical characteristics of disks around forming companions, providing unique insights into the formation of giant planets, brown dwarfs and their satellites.