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

Abstract

Accreting protoplanets provide key insights into how planets assemble from their natal protoplanetary disks. 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. In this paper, we demonstrate a promising approach to efficiently imaging accreting planets by applying reference star differential imaging (RDI) to 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 morphologysignificance criteria for pre-selection of reference frames to improve RDI subtraction. RDI with this PSF library results in a detection of PDS 70 b at a S/N of 5.3. Astrometry and photometry of PDS 70 b are calibrated using a forward-modeling method and injection-recovery tests, resulting in a separation of 186 ± 13 mas, a position angle of 142 ± 5 • , and an Hα flux of (1.7 ± 0.3) × 10 −15 erg s −1 cm −2 . The lower detection significance with RDI can be attributed to the ∼100 times lower peak-to-background ratios of the reference PSFs compared to the ADI PSFs. Building a high-quality reference library with WFC3 will provide unique opportunities to study accretion variability on short timescales not limited by roll angle scheduling constraints and efficiently search for actively accreting protoplanets in Hα around targets inaccessible to ground-based adaptive optics systems, such as faint transition disk hosts.

Companion mass limits for 17 binary systems obtained with binary differential imaging and MagAO/Clio

“…An improvement on RDI utilizes a library of PSF reference images (e.g. Sanghi et al 2021) and a Locally Optimized Combination of Images (LOCI; Lafrenière et al 2007) to optimally reconstruct the PSF, but is still susceptible to time variation between reference and science images. Spectral Differential Imaging (SDI; Racine et al 1999;Marois et al 2000), in which the science target is imaged simultaneously in multiple filter bands, does not obtain photon-noise limited PSF subtraction due to the chromatic variation in speckles that doesn't scale with wavelength (Rameau et al 2015), suf-fers from a difference in Strehl ratios between images in different bands, and depends on spectral features like Methane in the companion's atmosphere.…”