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Reference star differential imaging (RDI) is a powerful strategy for highcontrast imaging. Using example observations taken with the vortex coronagraphmode of Keck/NIRC2 in $L^\prime$ band, we demonstrate that RDI providesimproved sensitivity to point sources at small angular separations compared toangular differential imaging (ADI). Applying RDI to images of the low-massstellar companions HIP 79124 C (192 mas separation, $\Delta L^\prime$=4.01) andHIP 78233 B (141 mas separation, $\Delta L^\prime$=4.78), the latter a firstimaging detection, increases the significance of their detections by up to afactor of 5 with respect to ADI. We compare methods for reference framesselection and find that pre-selection of frames improves detection significanceof point sources by up to a factor of 3. In addition, we use observations ofthe circumstellar disks around MWC 758 and 2MASS J16042165-2130284 to show thatRDI allows for accurate mapping of scattered light distributions withoutself-subtraction artifacts.

Reference star differential imaging of close-in companions and circumstellar disks with the NIRC2 vortex coronagraph at W.M. Keck Observatory