Efficiently Imaging Accreting Protoplanets from Space: Reference Star Differential Imaging of the PDS 70 Planetary System with the HST/WFC3 PSF Library

Accreting protoplanets provide key insights into how planets form within their natal protoplanetary disks. The direct detections of Hα emission from newly formed planets have constrained planetary-mass accretion rates and enabled quantitative studies of accretion physics, planet-disk interactions, and planetary luminosity evolution. Recently, Zhou et al. 2021 used angular differential imaging (ADI) with Hubble Space Telescope’s Wide Field Camera 3 (HST/WFC3) to recover the young accreting planet PDS 70 b in F656N (Hα) at a S/N of 7.9. Here we demonstrate the applicability of reference star differential (RDI) with the same dataset. We compile a reference library from the database of WFC3 point-spread functions (PSFs) provided by Space Telescope Science Institute and develop a set of morphology-significance criteria for pre-selection of reference frames to improve RDI subtraction. We explore different implementations of RDI by varying the library size, reference star subsets, and subtraction regions to find the optimal setups for planet detections with HST/WFC3. RDI with this PSF library results in a detection of PDS 70 b at a S/N of 5.3, opening up the possibility of imaging accreting planets more efficiently than with ADI. The lower detection significance with RDI can be attributed to the ~100 times lower S/N of the reference PSFs compared to the ADI PSFs. Building a high-quality reference library with WFC3 will enable unique opportunities to monitor planetary accretion variability and efficiently search for actively accreting protoplanets in Hα around targets inaccessible to current ground-based adaptive optics systems, such as faint transition disk hosts.