  High-contrast imaging presents us with the opportunity to study circumstellar
disks and the planets still embedded within them -- providing key insights into
the formation and evolution of planetary systems. However, the post-processing
techniques that are often needed to suppress stellar halo light typically
result in significant and variable loss of circumstellar light, even when using
relatively conservative approaches like reference star differential imaging
(RDI). We introduce ``constrained reference star differential imaging"
(constrained RDI), a new class of RDI point spread function (PSF)-subtraction
techniques for systems with circumstellar disks. Constrained RDI utilizes
either high-resolution polarized intensity (PI) images or disk models to
severely limit or even eliminate the signal loss due to oversubtraction that is
common to RDI.
  We demonstrate the ability of constrained RDI utilizing polarimetric data to
yield an oversubtraction-free detection of the AB Aurigae protoplanetary disk
in total intensity. PI-constrained RDI allows us to decisively recover the
spectral signature of the confirmed, recently-discovered protoplanet, AB
Aurigae b (Currie et al. 2022). We further demonstrate that constrained RDI can
be a powerful analysis tool for soon-to-be-acquired James Webb Space Telescope
coronagraphic imaging of disks. In both cases, constrained RDI provides
analysis-ready products that enable more detailed studies of disks and more
robust verification of embedded exoplanets.
