Draft version May 14, 2021 Typeset using LATEX twocolumn style in AASTeX63 A Deep Polarimetric Study of the Asymmetrical Debris Disk HD 106906 Katie A. Crotts ,1 Brenda C. Matthews ,1, 2 Thomas M. Esposito ,3, 4 Gaspard Duchene^ ,3, 5 Paul Kalas ,3, 4, 6 Christine H. Chen ,7 Pauline Arriaga ,8 Maxwell A. Millar-Blanchaer ,9, ∗ John H. Debes ,7 Zachary H. Draper ,1, 2 Michael P. Fitzgerald ,8 Justin Hom ,10 Meredith A. MacGregor,11, 12, y Johan Mazoyer ,13, z Jennifer Patience,10 Malena Rice ,14 Alycia J. Weinberger ,11 David J. Wilner ,15 and Schuyler Wolff 16 1University of Victoria, 3800 Finnerty Rd, Victoria, BC, V8P 5C2, Canada 2National Research Council of Canada Herzberg, 5071 West Saanich Road, Victoria, BC V9E 2E7, Canada 3Astronomy Department, University of California, Berkeley, CA 94720, USA 4SETI Institute, Carl Sagan Center, 189 Bernardo Ave., Mountain View CA 94043, USA 5Universit´eGrenoble Alpes/CNRS, Institut de Plan´etologie et d'Astrophysique de Grenoble, 38000 Grenoble, France 6Institute of Astrophysics, FORTH, GR-71110 Heraklion, Greece 7Space Telescope Science Institute (STScI), 3700 San Martin Drive, Baltimore, MD 21218, USA 8Department of Physics and Astronomy, 430 Portola Plaza, University of California, Los Angeles, CA 90095, USA 9NASA Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA 10School of Earth and Space Exploration, Arizona State University, PO Box 871404, Tempe, AZ 85287, USA 11Department of Terrestrial Magnetism, Carnegie Institution for Science, 5421 Broad Branch Road, Washington, DC 20015, USA 12Department of Astrophysical and Planetary Sciences, University of Colorado, 2000 Colorado Avenue, Boulder, CO 80309, USA 13LESIA, Observatoire de Paris, Universit´ePSL, CNRS, Sorbonne Universit´e,Universit´ede Paris, 5 place Jules Janssen, 92195 Meudon, France 14Department of Astronomy, Yale University, New Haven, CT 06511, USA 15Center for Astrophysics, Harvard & Smithsonian, 60 Garden St., Cambridge, MA 02138, USA 16Department of Astronomy, The University of Arizona, 933 N Cherry Ave, Tucson, AZ, 85750, USA Submitted to ApJ ABSTRACT HD 106906 is a young, binary stellar system, located in the Lower Centaurus Crux (LCC) group. This system is unique among discovered systems in that it contains an asymmetrical debris disk, as well as an 11 MJup planet companion, at a separation of ∼735 AU. Only a handful of other systems are known to contain both a disk and directly imaged planet, where HD 106906 is the only one in which the planet has apparently been scattered. The debris disk is nearly edge on, and extends roughly to >500 AU, where previous studies with HST have shown the outer regions to have high asymmetry. To better understand the structure and composition of the disk, we have performed a deep polarimetric study of HD 106906's asymmetrical debris disk using newly obtained H-, J-, and K1-band polarimetric data from the Gemini Planet Imager (GPI). An empirical analysis of our data supports a disk that is asymmetrical in surface brightness and structure, where fitting an inclined ring model to the disk arXiv:2105.05995v1 [astro-ph.EP] 12 May 2021 spine suggests that the disk may be highly eccentric (e & 0:16). A comparison of the disk flux with the stellar flux in each band suggests a blue color that also does not significantly vary across the disk. We discuss these results in terms of possible sources of asymmetry, where we find that dynamical interaction with the planet companion, HD 106906b, is a likely candidate. Keywords: circumstellar matter | stars: individual (HD 106906) | polarization | scattering | infrared: planetary systems Corresponding author: Katie Crotts [email protected] 2 Crotts et al. 1. INTRODUCTION observable through scattered light. Through this cam- Debris disks can be characterized as gas-poor, dusty paign, 26 debris disk targets were resolved via total in- disks around main sequence stars, and which are impor- tensity and/or polarized scattered light (Esposito et al. tant objects to study in relation to better understand- 2020). ing the evolution of exoplanetary systems. Debris disks One of these disks is HD 106906 (HIP 59960), which are formed through collisional cascade, in which collid- was first discovered by Spitzer through an infrared sur- ing planetismals, such as comets and asteroids, break vey of 25 stars in the Lower Centaurus Crux group into milli-meter to sub-micron sized dust grains (Hughes (Chen et al. 2005) and is located ∼103.3 pc away (Gaia et al. 2018; Matthews et al. 2014; Wyatt 2008). For Collaboration et al. 2018). HD 106906 is a unique and this reason, debris disks are indicators of the success- interesting system for several reasons. First, it contains ful formation of at least >1 meter-sized objects during an eccentric, tight binary (e=0.669±0.002), with two the protoplanetary disk phase. However, processes from F5V type stars (De Rosa & Kalas 2019). Secondly, it the central star(s), such as radiation pressure and stel- has a possibly scattered 11±2 MJup planet companion lar winds, cause dust grains under a certain size to be outside of the disk, at a separation of 735±5 AU (Bailey blown out from the system (Augereau & Beust 2006; et al. 2014; Daemgen et al. 2017). And finally, the HD Strubbe & Chiang 2006). Therefore, in order for a colli- 106906 system hosts a debris disk that features a bright- sional cascade to occur and continually produce smaller ness asymmetry, with a south-east (SE) extension that dust grains to replenish the disk, planetesimals within appears brighter than the north-west (NW) extension the system need to have a sufficient relative velocity to (Kalas et al. 2015; Lagrange et al. 2016). cause fragmentation upon impact, meaning that some The Hubble Space Telescope (HST), which has a much sort of stirring mechanism is required to perturb their larger field of view than GPI, has also taken images of orbits. this system at optical wavelengths. The results from There are several stirring mechanisms that could oc- HST have shown that the NW side of the disk halo cur, such as late planetismal formation at large dis- seems to extend to >500 AU, creating a \needle" like tances from the star (Kennedy & Wyatt 2010; Kenyon & structure (Kalas et al. 2015). On the other hand, this Bromley 2008), interaction with the interstellar medium same feature is not observed for the SE-extension. The (ISM, Debes et al. 2009), close encounters with nearby HST data also show that the planet has a position an- stars (Kennedy et al. 2014), and planetary scattering gle (PA) ∼21 degrees from the NW-extension. Because (Kalas et al. 2015). The source of stirring may be de- of the wide separation of HD 106906b, its orbit is chal- termined by direct observation, as each of these mech- lenging to constrain; however, recent analysis has pre- anisms can affect the morphology and properties of the sented the first detection of the planet's orbit. If the disk in slightly different ways, such as creating warps, planet is bound, this analysis shows a highly eccentric, eccentric disks, gaps, and rings. In any of these cases, non-coplanar orbit with a periastron distance that comes the chances of creating an asymmetrical debris disk in reasonably close to the disk (Nguyen et al. 2021). terms of brightness and structure are high. For this rea- The fact that HD 106906 hosts an asymmetric debris son, asymmetrical debris disks are especially interesting disk, as well as an planet companion external to the and make great candidates for study as they allow us to disk, raises questions about whether or not HD 106906b gain more insight into the architecture of exoplanetary is responsible for this asymmetry. Previous work has systems. shown that if HD 106906b indeed formed within the The Gemini Planet Imager (GPI, Macintosh et al. disk, it could have migrated inwards, and consequently 2014) has not only searched for exoplanets, but has im- been ejected through dynamical interactions with the aged many debris disks as well. From November 2014 stellar binary (Rodet et al. 2017). While an in situ for- to October 2018, GPI conducted its Exoplanet Survey mation scenario for the planet is possible, this is less (GPIES, PI B. Macintosh, Macintosh et al. 2018), which favored compared to the scattering scenario due to the was an 860-hour campaign given to search roughly 600 lack of circumstellar gas at that distance, as well as the stars for Jupiter-type planets, as well as debris disks planet formation timescale being greater than the lifes- pan of gas in a fiducial protoplanetary disk. In both formation scenarios, however, secular effects from the ∗ NASA Hubble Fellow orbiting planet could create major asymmetries in the y NSF Astronomy and Astrophysics Postdoctoral Fellow disk (Rodet et al. 2017; Nesvold et al. 2017). If the z NFHP Sagan Fellow scattering scenario were true, the planet may have also HD 106906 3 had dynamical interactions with the disk on its scattered polarization is measured from the apparent stellar polar- path, leading to a perturbed disk. ization in each datacube by measuring the average frac- The goal of this paper is to better understand the tional polarization inside the occulting mask, which is source of the disk's asymmetry. To do this, we analyzed then subtracted from each cube (Millar-Blanchaer et al. the latest GPI data, revealing evidence of a geometri- 2015). Additionally, each cube is smoothed with a Gaus- cally asymmetric and eccentric disk. We further studied sian kernel (FWHM of 1 pixel) and the satellite-spot-to- the disk through 3D radiative-transfer modelling, gain- star flux is measured in order to determine a photomet- ing a deeper look at the disk's density structure and ric calibration factor.