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

TL;DR

This paper presents a novel reference star differential imaging method using HST/WFC3 archival PSF library to detect accreting protoplanets, demonstrating improved efficiency and potential for studying planet formation.

Contribution

The study develops a new RDI approach with a PSF library and morphology criteria, enabling effective detection of accreting protoplanets in space-based imaging datasets.

Findings

Detected PDS 70 b with RDI at S/N of 5.3

Calibrated astrometry and photometry of PDS 70 b

Highlighted the potential for short-timescale accretion variability studies

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 ($\mathrm{H}\alpha$) 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 morphology-significance 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 $186 \pm 13$ mas, a position angle of $142 \pm 5^\circ$, and an H$\alpha$ flux of $(1.7 \pm 0.3)\times10^{-15}$ erg s$^{-1}$ cm$^{-2}$. The lower detection significance with RDI can be attributed to the $\sim$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 $\mathrm{H}\alpha$ around targets inaccessible to ground-based adaptive optics systems, such as faint transition disk hosts.