Multiband GPI Imaging of the HR 4796A Debris Disk

TL;DR

This study used multi-band GPI observations and advanced PSF subtraction techniques to analyze the HR 4796A debris disk, revealing its geometric parameters and scattering properties, and comparing them across different wavelengths.

Contribution

It introduces a comprehensive analysis combining multiple PSF subtraction methods and disk modeling to measure the debris disk's properties and assess wavelength dependence of scattering.

Findings

Geometric parameters of the disk are robustly measured.

Scattering phase function is challenging to reconstruct directly from PSF-subtracted images.

The disk shows a blue color at small angles and red at large angles, consistent with large grain predictions.

Abstract

We have obtained Gemini Planet Imager (GPI) J-, H-, K1-, and K2-Spec observations of the iconic debris ring around the young, main-sequence star HR 4796A. We applied several point-spread function (PSF) subtraction techniques to the observations (Mask-and-Interpolate, RDI-NMF, RDI-KLIP, and ADI-KLIP) to measure the geometric parameters and the scattering phase function for the disk. To understand the systematic errors associated with PSF subtraction, we also forward-modeled the observations using a Markov Chain Monte Carlo framework and a simple model for the disk. We found that measurements of the disk geometric parameters were robust, with all of our analyses yielding consistent results; however, measurements of the scattering phase function were challenging to reconstruct from PSF-subtracted images, despite extensive testing. As a result, we estimated the scattering phase function using disk modeling. We searched for a dependence of the scattering phase function with respect to the GPI filters but found none. We compared the H-band scattering phase function with that measured by Hubble Space Telescope STIS at visual wavelengths and discovered a blue color at small scattering angles and a red color at large scattering angles, consistent with predictions and laboratory measurements of large grains. Finally, we successfully modeled the SPHERE H2 HR 4796A scattered phase function using a distribution of hollow spheres composed of silicates, carbon, and metallic iron.