Early High-contrast Imaging Results with Keck/NIRC2-PWFS: The SR 21 Disk

TL;DR

This paper reports on high-contrast imaging of the SR 21 disk using Keck/NIRC2 with a near-infrared pyramid wavefront sensor, achieving better image quality and revealing spiral features in total intensity at L' band, demonstrating improved adaptive optics performance for red targets.

Contribution

First demonstration of NIR pyramid wavefront sensor with Keck/NIRC2, improving AO performance and enabling detailed imaging of red targets like SR 21.

Findings

Achieved ~20% better Strehl ratio compared to optical wavefront sensing.

Detected spiral features in SR 21 in total intensity at L' band.

Improved contrast limits at small angular separations.

Abstract

High-contrast imaging of exoplanets and protoplanetary disks depends on wavefront sensing and correction made by adaptive optics instruments. Classically, wavefront sensing has been conducted at optical wavelengths, which made high-contrast imaging of red targets such as M-type stars or extincted T Tauri stars challenging. Keck/NIRC2 has combined near-infrared (NIR) detector technology with the pyramid wavefront sensor (PWFS). With this new module we observed SR~21, a young star that is brighter at NIR wavelengths than at optical wavelengths. Compared with the archival data of SR~21 taken with the optical wavefront sensing we achieved $\sim$20\% better Strehl ratio in similar natural seeing conditions. Further post-processing utilizing angular differential imaging and reference-star differential imaging confirmed the spiral feature reported by the VLT/SPHERE polarimetric observation, which is the first detection of the SR~21 spiral in total intensity at $L^\prime$ band. We also compared the contrast limit of our result ($10^{-4}$ at $0\farcs4$ and $2\times10^{-5}$ at $1\farcs0$) with the archival data that were taken with optical wavefront sensing and confirmed the improvement, particularly at $\leq0\farcs5$. Our observation demonstrates that the NIR PWFS improves AO performance and will provide more opportunities for red targets in the future.