Christine Chen, Space Telescope Science Institute
Johan Mazoyer, Jet Propulsion Laboratory
Charles A. Poteet, Space Telescope Science Institute
Bin Ren, Johns Hopkins University
Gaspard Duch ne, University of California, Berkeley
Justin Hom, School of Earth and Space Exploration
Pauline Arriaga, University of California, Los Angeles
Maxwell A. Millar-Blanchaer, California Institute of Technology
Jessica Arnold, Carnegie Institution of Washington
Vanessa P. Bailey, Jet Propulsion Laboratory
Juan Sebastián Bruzzone, Western University
Jeffrey Chilcote, University of Notre Dame
lodie Choquet, Laboratoire d'Astrophysique de Marseille
Robert J. De Rosa, European Southern Observatory Santiago
Zachary H. Draper, University of Victoria
Thomas M. Esposito, University of California, Berkeley
Michael P. Fitzgerald, University of California, Los Angeles
Katherine B. Follette, Amherst College
Pascale Hibon, European Southern Observatory Santiago
Dean C. Hines, Space Telescope Science Institute
Paul Kalas, University of California, Berkeley
Franck Marchis, SETI Institute
Brenda Matthews, SETI Institute
Julien Milli, European Southern Observatory Santiago
Jennifer Patience, School of Earth and Space Exploration
Marshall D. Perrin, Space Telescope Science Institute
Laurent Pueyo, Space Telescope Science Institute
Abhijith Rajan, Space Telescope Science Institute
Fredrik T. Rantakyrö, Gemini ObservatorySouthern Operations Center
Timothy J. Rodigas, Carnegie Institution of Washington
Gael M. Roudier, Jet Propulsion Laboratory
Glenn Schneider, The University of Arizona
Kimberly Ward-Duong, Amherst CollegeFollow
et al, Various Institutions

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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.










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