Imaging the 44 au Kuiper Belt Analog Debris Ring around HD 141569A with GPI Polarimetry

Item Type Article; text

Authors Bruzzone, J.S.; Metchev, S.; Duchene, G.; Millar-Blanchaer, M.A.; Dong, R.; Esposito, T.M.; Wang, J.J.; Graham, J.R.; Mazoyer, J.; Wolff, S.; Ammons, S.M.; Schneider, A.C.; Greenbaum, A.Z.; Matthews, B.C.; Arriaga, P.; Bailey, V.P.; Barman, T.; Bulger, J.; Chilcote, J.; Cotten, T.; De Rosa, R.J.; Doyon, R.; Fitzgerald, M.P.; Follette, K.B.; Gerard, B.L.; Goodsell, S.J.; Hibon, P.; Hom, J.; Hung, L.-W.; Ingraham, P.; Kalas, P.; Konopacky, Q.; Larkin, J.E.; MacIntosh, B.; Maire, J.; Marchis, F.; Marois, C.; Morzinski, K.M.; Nielsen, E.L.; Oppenheimer, R.; Palmer, D.; Patel, R.; Patience, J.; Perrin, M.; Poyneer, L.; Pueyo, L.; Rajan, A.; Rameau, J.; Rantakyrö, F.T.; Savransky, D.; Sivaramakrishnan, A.; Song, I.; Soummer, R.; Thomas, S.; Wallace, J.K.; Ward-Duong, K.; Wiktorowicz, S.

Citation Bruzzone, J. S., Metchev, S., Duchene, G., Millar-Blanchaer, M. A., Dong, R., Esposito, T. M., Wang, J. J., Graham, J. R., Mazoyer, J., Wolff, S., Ammons, S. M., Schneider, A. C., Greenbaum, A. Z., Matthews, B. C., Arriaga, P., Bailey, V. P., Barman, T., Bulger, J., Chilcote, J., … Wiktorowicz, S. (2020). Imaging the 44 au Kuiper Belt Analog Debris Ring around HD 141569A with GPI Polarimetry. Astronomical Journal, 159(2).

DOI 10.3847/1538-3881/ab5d2e

Publisher IOP Publishing Ltd

Journal Astronomical Journal

Rights © 2020. The American Astronomical Society. All rights reserved.

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Link to Item http://hdl.handle.net/10150/660718 The Astronomical Journal, 159:53 (13pp), 2020 February https://doi.org/10.3847/1538-3881/ab5d2e © 2020. The American Astronomical Society. All rights reserved.

Imaging the 44 au Kuiper Belt Analog Debris Ring around HD 141569A with GPI Polarimetry

Juan Sebastián Bruzzone1 , Stanimir Metchev1,2,3 , Gaspard Duchêne4,5 , Maxwell A. Millar-Blanchaer6, Ruobing Dong7 , Thomas M. Esposito4 , Jason J. Wang4 , James R. Graham4, Johan Mazoyer6 , Schuyler Wolff8 , S. Mark Ammons9 , Adam C. Schneider10 , Alexandra Z. Greenbaum11 , Brenda C. Matthews7,12 , Pauline Arriaga13 , Vanessa P. Bailey6 , Travis Barman14 , Joanna Bulger15 , Jeffrey Chilcote16,17 , Tara Cotten18 , Robert J. De Rosa4 , Rene Doyon19, Michael P. Fitzgerald13 , Katherine B. Follette20 , Benjamin L. Gerard7,12 , Stephen J. Goodsell21 , Pascale Hibon22 , Justin Hom10 , Li-Wei Hung13 , Patrick Ingraham23 , Paul Kalas4,24, Quinn Konopacky25 , James E. Larkin13 , Bruce Macintosh16 , Jérôme Maire25, Franck Marchis24 , Christian Marois7,12 , Katie M. Morzinski26 , Eric L. Nielsen16,24 , Rebecca Oppenheimer27 , David Palmer9 , Rahul Patel28 , Jennifer Patience10, Marshall Perrin8 , Lisa Poyneer9, Laurent Pueyo8, Abhijith Rajan10 , Julien Rameau19 , Fredrik T. Rantakyrö29 , Dmitry Savransky30 , Anand Sivaramakrishnan8 , Inseok Song18 , Remi Soummer8 , Sandrine Thomas23 , J. Kent Wallace6, Kimberly Ward-Duong20 , and Sloane Wiktorowicz31 1 Department of Physics and Astronomy, The University of Western Ontario, London, ON N6A 3K7, Canada 2 Institute of Earth and Space Exploration, London, ON N6A 3K7, Canada; [email protected] 3 Department of Physics & Astronomy, Stony Brook University, Stony Brook, NY 11794–3800, USA 4 Astronomy Department, University of California, Berkeley, CA 94720, USA 5 Université Grenoble Alpes, CNRS, IPAG, Grenoble, F-38000, France 6 Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA 7 University of Victoria, 3800 Finnerty Road, Victoria, BC V8P 5C2, Canada 8 Space Telescope Science Institute, Baltimore, MD 21218, USA 9 Lawrence Livermore National Laboratory, Livermore, CA 94551, USA 10 School of Earth and Space Exploration, Arizona State University, P.O. Box 871404, Tempe, AZ 85287, USA 11 Department of Astronomy, University of Michigan, Ann Arbor, MI 48109, USA 12 National Research Council of Canada Herzberg, 5071 West Saanich Road, Victoria, BC V9E 2E7, Canada 13 Department of Physics & Astronomy, University of California, Los Angeles, CA 90095, USA 14 Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721, USA 15 Subaru Telescope, NAOJ, 650 North A’ohoku Place, Hilo, HI 96720, USA 16 Kavli Institute for Particle Astrophysics and Cosmology, Stanford University, Stanford, CA 94305, USA 17 Department of Physics, University of Notre Dame, 225 Nieuwland Science Hall, Notre Dame, IN 46556, USA 18 Department of Physics and Astronomy, University of Georgia, Athens, GA 30602, USA 19 Institut de Recherche sur les Exoplanètes, Département de Physique, Université de Montréal, Montréal, QC H3C 3J7, Canada 20 Physics and Astronomy Department, Amherst College, 21 Merrill Science Drive, Amherst, MA 01002, USA 21 Gemini Observatory, 670 North A’ohoku Place, Hilo, HI 96720, USA 22 European Southern Observatory, 3107 Alonso de Córdova, Vitacura, Santiago, Chile 23 Large Synoptic Survey Telescope, 950 North Cherry Avenue, Tucson, AZ 85719, USA 24 SETI Institute, Carl Sagan Center, 189 Bernardo Avenue, Mountain View, CA 94043, USA 25 Center for Astrophysics and Space Science, University of California San Diego, La Jolla, CA 92093, USA 26 Steward Observatory, University of Arizona, Tucson, AZ 85721, USA 27 Department of Astrophysics, American Museum of Natural History, New York, NY 10024, USA 28 Infrared Processing and Analysis Center, California Institute of Technology, Pasadena, CA 91125, USA 29 Gemini Observatory, Casilla 603, La Serena, Chile 30 Sibley School of Mechanical and Aerospace Engineering, Cornell University, Ithaca, NY 14853, USA 31 Department of Astronomy, UC Santa Cruz, 1156 High Street, Santa Cruz, CA 95064, USA Received 2018 May 11; revised 2019 November 25; accepted 2019 November 26; published 2020 January 16

Abstract We present the first polarimetric detection of the inner disk component around the pre-main-sequence B9.5 star HD 141569A. Gemini Planet Imager H-band (1.65 μm) polarimetric differential imaging reveals the highest signal-to-noise ratio detection of this ring yet attained and traces structure inward to 0 25 (28 au at a distance of 111 pc).Theradial polarized intensity image shows the east side of the disk, peaking in intensity at 0 40 (44 au) and extending out to 0 9 (100 au). There is a spiral arm–like enhancement to the south, reminiscent of the known spiral structures on the outer rings of the disk. The location of the spiral arm is coincident with 12CO J=3–2 emission detected by ALMA and hints at a dynamically active inner circumstellar region. Our observations also show a portion of the middle dusty ring at ∼220 au known from previous observations of this system. We fit the polarized H-band emission with a continuum radiative transfer Mie model. Our best-fit model favors an optically thin disk with a minimum dust grain size close to the blowout size for this system, evidence of ongoing dust production in the inner reaches of the disk. The thermal emission from this model accounts for virtually all of the far-infrared and millimeter flux from the entire HD141569A disk, in agreement with the lack of ALMA continuum and CO emission beyond ∼100 au. A remaining 8–30 μm thermal excess afactorof∼2 above our model argues for an as-yet-unresolved warm innermost 5–15 au component of the disk. Unified Astronomy Thesaurus concepts: Debris disks (363); Circumstellar disks (235); Polarimetry (1278); Astronomical techniques (1684); Coronographic imaging (313); Direct imaging (387)

1 The Astronomical Journal, 159:53 (13pp), 2020 February Bruzzone et al.

1. Introduction confirmed the detection of the inner disk component around HD 141569A in L -band scattered light with a vortex coronagraph in Debris disks are established laboratories to study planet p Keck/NIRC2. The combination of optical/near-IR scattered light formation and evolution. Planetesimals and infant planets and 870 μm/2.9 mm Atacama Large Millimeter/submillimeter interact with the dusty disk and create gaps, asymmetries, Array (ALMA) observations (White et al. 2016, 2018) limit the offsets, and local enhancements through various dynamical outer radius of the inner disk to ∼55 au. mechanisms that help infer their presence (e.g., the β Pictoris Also known to be a gas-rich disk on large scales, HD disk and planet; Lagage & Pantin 1994; Kalas & Jewitt 1995; – ) 141559A has a total estimated mass in the 13 200 M⊕ Lagrange et al. 2010 . Of special interest are those young disks ( – × −4 ) ( ( ) 0.39 6.0 10 Me range Zuckerman et al. 1995; Jonkheid up to 40 Myr transitioning between the protoplanetary disk et al. 2006; Thi et al. 2014; Flaherty et al. 2016). The CO and debris disk stage. These “hybrid disks” contain gas that can ( ) rovibrational emission lines trace the existence of the gas be either primordial or secondary e.g., Moór et al. 2017 and between 17 and 500 au (Brittain et al. 2003; Goto et al. 2006; often show a significant deficit in near- or mid-IR emission or ) ( Flaherty et al. 2016; White et al. 2016 . However, spatially an inner clearing in resolved images e.g., HD 163296, Doucet resolved ALMA 12CO (J=3–2) observations reveal that et al. 2006; 49 Ceti, Hughes et al. 2008; HD 21997, Kóspál the <210 au region of the disk may contain only a fraction, et al. 2013). The study of hybrid disks is attractive because of ∼ ( × −6 ) ( – 1.5 M⊕ 4.5 10 Me , of this gas mass White et al. the implications for gas giant planet formation and gas dust 2016), and that the inner <50 au hold only tenuous amounts of interaction models in nascent planetary systems. ( = CO gas. The star HD 141569A V 7.12 mag, Høg et al. 2000; We present the first H-band polarimetric observations of the H=6.86 mag, Cutri et al. 2003) is a well-known hybrid disk ( ) ± ( ) inner disk of HD 141569A Section 2 . We use polarimetric host. A young 5 3 Myr Weinberger et al. 2000 B9.5V star differential imaging (PDI) with the Gemini Planet Imager (GPI; (Jaschek & Jaschek 1992), HD 141569A is at a distance of ) ± ( ) Macintosh et al. 2014 . The PDI excels in high-contrast 110.5 0.5 pc Gaia Collaboration et al. 2018 with sensitivity to polarized circumstellar emission, as it eliminates / ∼ × −3 ( ) LIR Lstar 8 10 Sylvester et al. 1996 and two low-mass the need of further point-spread function (PSF) subtraction that ″ ( stellar companions, B and C, at roughly 9 Weinberger et al. can hamper the detection of extended emission. Our PDI ) – μ 2000 . Early 12 100 m photometry with IRAS indicated a observations resolve the inner disk into a ring with polarized ∼ population of 100 K circumstellar dust at an estimated intensity peaking at 44 au and extending inward to 0 25 (28 au; – ( distance of 47 60 au Jaschek et al. 1986; Walker & Section 3). We model the linear polarized intensity image to ) Wolstencroft 1988 . derive the physical parameters of the disk (Section 4). We also fi The rst high-contrast coronagraphic images in scattered compare the predicted thermal emission from our best-fit model ( )/ light with the Hubble Space Telescope HST NICMOS at against the spectral energy distribution (SED) assembled from ( ) ~1.6m m F160W revealed a bright dusty disk inclined at previous studies (Section 5). We present our conclusions in °± ° ( ) °± ° ( 53 5 and a position angle PA of 355 1 Augereau Section 6. et al. 1999a; Weinberger et al. 1999). Weinberger et al. (1999) reported a disk extending out to 4″ with two conspicuous nested rings peaking at 2 0 (220 au) and 3 3 (360 au; along the 2. Observations and Data Reduction semimajor axis of the disk), separated by a 1″ wide gap devoid We observed HD 141569A on 2014 March 22 UT in of scattering material. In this work, we refer to the 220 and polarimetry mode at the H band during GPI commissioning at 360 au rings as the middle and outer rings, respectively. The the Gemini South Telescope. We acquired 50 frames of 60 s ◦ brighter eastern side of the system is likely nearer to us under each over 45 of parallactic field rotation at an average airmass the assumption of preferentially forward scattering by dust of 1.12. Between each observation, the half wave plate (HWP) (Weinberger et al. 1999). Subsequently, HST optical corona- modulator was rotated in 22°.5 steps. The HWP introduces graphic observations with the Space Telescope Imaging modulation in the signal and thereby allows for reconstruction Spectrograph (STIS; 365–806 μm, 50 CCD; Mouillet et al. of the Stokes vector later in the reduction steps. During these 2001) and Advanced Camera for Surveys (ACS) at 430 nm observations, the average Gemini differential image motion (F435W), 590 nm (F606W), and 830 nm (F814W; Clampin monitor (DIMM) seeing at Cerro Pachón was 0 70. et al. 2003) revealed asymmetries and two prominent tightly We reduced the data with the GPI data reduction pipeline wound spiral substructures: an inner arm between 1 8 and 2 2 (DRP; Maire et al. 2014; Perrin et al. 2014) following the (200 and 240 au) and an outer broad ring at 3 0–4 0 procedure described in Millar-Blanchaer et al. (2015) and (330–440 au). In a new analysis of archival Gemini/NICI Perrin et al. (2015). This starts with dark subtraction, correction and HST/NICMOS data, Mazoyer et al. (2016) reported a split for instrument flexure, microphonics noise, and bad pixels. in two fine rings in the eastern part of the disk and showed that Each frame is then assembled into a polarization data cube, the 2″ ring shows a small 0 03 offset relative to the central star. where the third dimension comprises two image slices, each Marsh et al. (2002) were the first to suggest a peak in the dust containing one of the two orthogonal polarization states yielded optical depth inward of ∼70 au from mid-IR imaging with Keck/ by the Wollaston prism analyzer. Each data cube is divided by MIRLIN (Ressler et al. 1994). High-contrast coronagraphic a Gemini facility calibration unit flat field for throughput observations in the near-IR with the Very Large Telescope correction across the field. We correct for instrumental (VLT)/SPHERE further revealed the presence of a third inner ring polarization as outlined in Millar-Blanchaer et al. (2015). at ∼44 au (Perrot et al. 2016). This resolved inner disk component The position of the central star is determined using GPI’s wasalsoseenasanarc-likerimbyKonishietal.(2016) in optical four fiducial satellite spots (Wang et al. 2014). In broadband broadband light with HST/STISandmarginallydetectedbyCurrie images, the satellite spots are smeared radially outward from et al. (2016) with Keck/NIRC2 at Lp. Mawet et al. (2017) the central star and form bright elongated streaks that can be

2 The Astronomical Journal, 159:53 (13pp), 2020 February Bruzzone et al.

Figure 1. Observations of the HD 141569A dusty disk in H-band linear polarization with GPI in a total of 50 minutes of integration. Left: linear polarization intensity Qr image. An arrow points to the location of the surface brightness enhancement—an arc feature (see Figure 8)—to the south. Right: Stokes Ur image shown on the same color scale. A star symbol marks the position of the star, and a circular aperture of radius 0 25 centered on the star indicates the region affected by uncorrected instrumental polarization. Beyond this region, the Ur image scatters uniformly around zero flux, which indicates that the dust seen in the Stokes Qr is optically thin. The previously known middle ring is visible at low surface brightness at 1″ to the east in the Stokes Qr image.

− used to estimate the location of the obscured star (Pueyo et al. obtain a conversion factor of (1.05´ 0.06) 10-8 Jy ADU 1 −1 2015). Following Wang et al. (2014), we use a technique that coadd . The final Qr and Ur images are shown in Figure 1, implements a Radon transform of the flux distribution in each and a signal-to-noise ratio (S/N) map of the Qr image is shown polarized image to compute the line integral over all lines in Figure 2. passing through an initial guess for the position of the star. We In addition to the Stokes Qr image, we also reduced the total sum the squares of all line integrals and repeat the procedure intensity image (Stokes I slice) for the entire sequence with for the next guess for the stellar position within a small search pyKLIP (Wang et al. 2015), which implements the Karhunen– box. The point within the search box that contains the most Loève Image Projection (KLIP) algorithm (Soummer et al. light pointing toward it corresponds to the stellar position. This 2012) for optimal PSF subtraction. That reduced image is way, we attain the position of the obscured central star to shown in Figure 3. ∼1 mas precision (Wang et al. 2014). We then perform a Immediately after the polarimetry sequence was completed, double differencing between the two polarization states to we also acquired integral field spectroscopic (IFS; nonpolari- correct for non-common path errors (Perrin et al. 2015). metric) observations of HD 141569A with GPI in the H band. We use the series of differenced data cubes obtained at The IFS data contain an independent measurement of the total different HWP angles to construct a single Stokes [I, Q, U, V] intensity flux from the disk. The sequence comprised 32 frames data cube, the 2D slices of which hold the total intensity and of 60 s each. The sequence started at an airmass of 1.13, with ◦ linear and circular polarization information for the entire an average Gemini DIMM seeing of 1 01 and 27 of observation sequence. The details of this procedure are cumulative field rotation. We reduced the data and assembled presented in Appendix B.2 of Perrin et al. (2015). Because the spectral data cubes with the standard recipes provided in the GPI’s HWP is not exactly one half wave at all wavelengths, GPI DRP. The entire reduced data set was then processed with GPI is only weakly sensitive to circular polarization, Stokes V. pyKLIP. However, unlike our polarimetric observations, this Thus, we disregard the Stokes V cube slice. Afterward, shorter IFS sequence did not reveal the disk and is not following Schmid et al. (2006), the Stokes cube was shown here. transformed into the radial Stokes convention [I, Qr, Ur, V]. In this formalism, positive values of Stokes Qr represent linear 3. Results polarization perpendicular to the radial direction from the star, We present the imaging results in polarized and total while negative values represent polarization parallel to the intensity in Sections 3.1 and 3.2, respectively. We revisit these radial direction. For Rayleigh-like scatterers in an optically thin further in Sections 5.1 and 5.2 in the context of previous fl U debris disk, no ux is expected in the Stokes r, image as only observations and a scattered-light model of the disk (Section 4). the perpendicular macroscopic polarization state (azimuthal polarization) prevails (Millar-Blanchaer et al. 2015). Thus, 3.1. Polarized Intensity H-band Image light from single scattering events by dust grains will lead to positive values in Stokes Qr. However, we note that multiple The H-band Stokes Qr image in Figure 1 shows the first scattering in optically thick disks could have a Stokes Ur signal polarized-light detection of the HD 141569A inner disk. We of a few percent of the Stokes Qr signal (Canovas et al. 2015), clearly resolve the eastern portion of the disk, likely the result which is below the sensitivity of our observations. of predominantly forward scattering by micron-sized dust Our final reduction step it to flux-calibrate the data. grains. This is dictated by the combined effects of phase Following the procedure outlined in Hung et al. (2016) and function and polarization dependence with scattering angle. adopting 1.85±0.07 Jy as the H-band flux of HD 141569A Most known dust compositions preferentially scatter light in from the Two Micron All Sky Survey (Cutri et al. 2003),we the forward direction for dust particles a few times larger than

3 The Astronomical Journal, 159:53 (13pp), 2020 February Bruzzone et al. However, we do not anticipate this to generally be true, as evidenced by the unusual case of the HR4796A debris disk (Perrin et al. 2015; Milli et al. 2017) and by theoretical projections for polarization phase functions in Canovas et al. (2015). In Section 5.2 we further discuss a residual arc-like structure to the south, which is also detectable as a brightness enhancement in the Stokes Qr image (Figure 1, left panel). Uncorrected instrumental polarization affects the signal within 0 25 of the star. We have delimited this region with a dashed circle in Figures 1–3 and excluded it from our analysis. No coherent structures are observed in the Stokes Ur image to indicate significant optical depth. Hence, the Stokes Ur image can be used as a noise map for our measurements, and it reassures the astrophysical nature of the Stokes Qr emission, specifically, the polarized morphology exterior to 0 25 and the middle ring near the edge to the east. Figure 2 shows an S/N map created by dividing the Qr image at every position by local noise estimated as the standard deviation of pixels within a 3

Figure 2. The H-band Stokes Qr S/N map of HD 141569A. Each point on the pixel wide annulus in the Stokes Ur image at the same angular map is constructed by dividing the Qr image by the standard deviation of all separation from the star. pixels within a 3 pixel wide annulus in the Ur image at the same angular separation from the star. 3.2. Total Intensity H-band Image We attempted to detect the total intensity emission from the inner ring in two different ways: from the combined polarization states in our PDI observations and from the unpolarized IFS observations. Figure 3 shows the result of the PSF subtraction of the Stokes I image after applying pyKLIP (Wang et al. 2015) and angular differential imaging (ADI; Marois et al. 2006). Our shorter IFS sequence with GPI did not reveal either the inner disk detected in Stokes Qr or the previously known middle ring. Our inability to detect the inner ring in total intensity is not entirely surprising. In extracting the Stokes I signal from high- contrast observations, we lose the differential imaging advantage of PDI. Moreover, the PSF subtraction process in total intensity also removes the smooth structure of the disk. Hence, we expect greater sensitivity to scattered light in our polarized-light Stokes Qr image. In view of the low-S/N detection in total intensity, we use only the polarimetric detection in Stokes Qr in the remainder of this study. Figure 3. The H-band total intensity (Stokes I) pyKLIP+ADI reduction of HD 141569A (Section 3.2). Self-subtraction degrades the residuals, and there is no 4. Disk Modeling significant evidence of the presence of the inner ring. The image hints at the presence of the middle 220 au ring 1″ to the east. We model the resolved polarized Stokes Qr image of the inner disk with a radiative transfer model to determine the disk geometry (Section 4.1) and dust properties (Section 4.2). The ( ) the wavelength of the scattered light van de Hulst 1958 . For same modeling tool can predict the SED for the disk models, such particles, optically thin Mie models also suggest that but since the SED is dominated by emission from dust outside „ ° polarized intensity curves peak at scattering angles 90 the GPI field of view, we only compute a model SED as a test ( ) Perrin et al. 2015 . to check against gross inconsistency (Section 4.3). The emission peaks to the east at a semimajor axis of 0 40 (44 au) and extends out to ∼0 9 (100 au), where it blends into 4.1. Parameterization of the Dust Model the background. At 1″ to the east, the Stokes Qr surface brightness increases again, revealing the previously detected We use the Monte Carlo continuum radiative transfer and middle dusty ring at 220 au. The clearing between the inner ray-tracing code MCFOST (Pinte et al. 2006, 2009) to compute disk and the middle ring indicates a region deficient in synthetic observations of the SED and the Stokes Qr images of scattering material, assuming that the disk is optically thin. the disk around HD 141569A. The MCFOST code computes The western part of the Stokes Qr image shows no significant the scattering, absorption, and reemission events by dust grains emission. We set a 1σ lower limit of 4.0 on the ratio of forward by propagating photon packets throughout a cylindrical spatial to backward scattering in polarized intensity. If the polarization grid in accordance with Mie theory. phase function is symmetric around 90°, then this also sets a Dust grains are assumed to be in radiative equilibrium lower limit on the forward to backward scattering intensity. embedded in the radiation field from the host star. The sampling

4 The Astronomical Journal, 159:53 (13pp), 2020 February Bruzzone et al. of our synthetic images is defined to cover the entire field of stellocentric disk populated by a single dust grain composition view of observations using GPI’s pixel scale of 14.166± of amorphous magnesium-rich olivine with a dust grain density −1 −3 0.007 mas lenslet (De Rosa et al. 2015). The star is located at ρdust=3.5 g cm . We adopted this dust grain composition to the center of the computational grid, and the disk is centered on match the composition used by Thi et al. (2014) and Mawet the star. To obtain the stellar luminosity, we fit the optical–to– et al. (2017). near-IR SED of HD 141569A (Høg et al. 2000; Cutri et al. Our disk model parameterization thus comprises Rin, Rout, i, 2003; Mendigutía et al. 2012), keeping the stellar radius and PA, H0, Md, αin, αout, Rc, amin, and p. To reduce the burden of foreground extinction as variables. We assumed a fixed distance an 11 dimensional parameter exploration, we fixed Rin and Rout of 111 pc with an effective temperature of 10,000 K for the star at 20 and 110 au, respectively. These disk boundaries are (Merín et al. 2004) and used NextGen photospheric models from motivated by our inability to detect the inner disk within Hauschildt et al. (1999).WeobtainL=25.48 Le for the stellar ∼0 25 and beyond 1″ from the star (Figure 2). We also set luminosity. It agrees within the uncertainties with the previous αin=14 as motivated by preliminary modeling that pointed to -2.2 ( ) α fi estimate of 25.77+1.63 Le by Merín et al. 2004 . high in values. Such a steep pro le implies a sharp drop in The disk geometry in cylindrical coordinates is described by density inside of rmax. The probability density distribution for α α   the radial extent of the disk, with inner and outer radii Rin and in in those models plateaued at in 14. – Rout, inclination i, PA, and dust density distribution ρ(r, z)= By exploring each of the variable parameters at 5 11 discrete Σ(r)Z(r, z). Following Augereau et al. (1999b), the dust values, we construct a model grid with over 3×107 grid distribution in the vertical direction is parameterized within points for the remaining eight free parameters. Table 1 details the MCFOST framework as an exponential with a shape the full set of parameters involved in the modeling, including parameter γ, additional fixed parameters for the star and disk. ⎛ ∣∣z ⎞g Zrz(),exp,=-⎜ ⎟ () 1 4.2. Polarized Scattered-light Modeling and Best-fit Estimates ⎝ Hr()⎠ In preparation for the fitting procedure, we first binned the r b images in 3×3 pixels to reduce the effects of correlated where the scale height is defined as H ()rH= 0 at a fixed ()R0 errors. The GPI resolution element is about 3 pixels in the H = β fl reference radius R0 45 au, and is a disk aring parameter. band; therefore, the binning should not lead to a loss of spatial Radially, the dust distribution follows a smooth combination of information. Following Millar-Blanchaer et al. (2015), we used two power laws, the 3×3 binned Stokes Ur to derive the uncertainties used to fi -12 t the binned Stokes Qr image. For each position in the binned ⎧⎛ ⎞--22aain⎛ ⎞ out⎫ ⎪ r r ⎪ Stokes Q image, the uncertainty is calculated as the standard Sµ()r ⎨⎜ ⎟ +⎜ ⎟ ⎬ ,2 () r ⎪⎝ ⎠ ⎝ ⎠ ⎪ ⎩ RccR ⎭ deviation of a 3 pixel wide annulus on the binned Ur image. These steps return the uncertainty map σ to use in the χ2 where ain > 0, αout<0, and Rc is the radial distance of the minimization procedure: peak density of the grain distribution. The surface density of dust grains is represented as ⎛ Obs- Mod ⎞2 c2 =SNpix⎜ ii⎟ .5() i ⎝ s ⎠ +¥ ⎛ r ⎞b i srs()rrzdzr==´S (,,3 ) ()⎜ ⎟ () ò 0 ⎝ ⎠ -¥ R0 The fitting occurs within a 0 85×0 61 (radius) elliptical region centered on the star and excludes the central 0 25 radius where σ0=2H0ρ(R0,0) for γ=1. The maximum of the circular region to avoid PSF subtraction residuals (Figure 4, surface density is not at Rc but rather at left). The fitting region in the binned image has 767 resolution -1 ⎛ G ⎞()22G-Gin out elements. We opt for an elliptical fitting region rather than a rR= ⎜ in ⎟ ,4() max()s ⎝ ⎠ c circular one because Poisson noise from the disk rather than the -Gout stellar halo is the main limiting factor for our sensitivity. where Γ1=αin+β and Γ2=αout+β, which in our case is The outcome of our modeling strategy is presented in nearly identical to Rc. Figure 4, which shows the Stokes Qr image (left), the best-fit We assume a disk with a constant opening angle, thus no model (middle), and the residuals (right), all displayed on the flaring (β=1), and adopt γ=1. Without better initial same color and intensity scale as the Stokes Ur image in 2 constrains on these parameters, our assumption is reasonable Figure 1. Our best-fit model returns a reduced χr=0.93 and for an optically thin debris disk. so provides a good match to the Stokes Qr image. Our best-fit The surface brightness and thermal flux of the disk are model Stokes Ur image contains very little flux, on the order of controlled by the total mass in grains Md in the disk. To 0.1% of the model Stokes Qr. This indicates that the disk is characterize the dust content in the disk, we adopt a power-law optically thin with an inferred midplane optical depth of − n -2 grain size distribution dN(a) ∝ a da with n=3.5, as is t1.65m m = 10 from MCFOST. commonly assumed for debris disks in a steady-state collisional As a consistency check on our best-fit solution, we plot the cascade (Dohnanyi 1969). The size distribution has limits amin radial profiles of the polarized emission along the semimajor axis and amax and grain porosity p.Wefix amax=1 mm and leave of the disk and compare them to the prediction from the model amin as a free parameter in the model. Within the MCFOST (Figure 5). We see that the model follows the radial profile well framework, the refractive index of porous grains is approxi- and stays within the 1σ uncertainty band at most separations, mated from a mixture of solid grains with void particles except inward of 31 au to the north, where it overestimates the following the Bruggeman mixing rule. Modeling is limited to a observed emission. There is also residual emission to the south

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Table 1 Parameters Probed in Our Exploration Grid of Models with MCFOST and Best-fit Values for the Stokes Qr Image Fixed Stellar Parameters Distance d(pc) 111 Stellar radius R* (Re) 1.66 Effective temperature Teff (K) 10,000 Extinction EB−V (mag) 0.144 RV (mag) 3.1 Disk Model Fixed Parameters

Inner radius Rin (au) 20 Outer radius Rout (au) 110 Inner exponent αin 14 Reference radius R0 (au) 45 −3 Solid material dust density ρdust (gcm ) 3.5 Flaring exponent β 1 Vertical exponent γ 1

Disk Model Free Parameters Sampling Range Number of Values Best-fit Value Inclination i(°) Lin. in cosine [40, 80] 10 60°±10° Position angle PA (°) Lin. [−25, 15] 95°±10° ( ) [ ] +9 Reference scale height H0 au Lin. 2.2, 36 9 14-5 −6 −6 Dust mass Md (Me) Lin. [0.2, 2.0]×10 10 1.0±0.4×10 α [− ] +0.5 Outer exponent out Lin. 3.5, 0.0 8 -1.0-1.0 ( ) [ ] +8 Rc Rc au Lin. 24, 64 11 44-12 (μ ) [ ] +4 Minimum grain size amin m Log. 0.5, 16 6 4-2 Porosity p (%) Lin. [0, 20, 40, 60, 80] 50

Minimum reduced χ2 0.93

around PA=150° that has no counterpart to the north (Figure 4, (which, at a reference radius of R0=45 au, gives an opening ) +0.5 right and that we discuss in Sections 5.1 and 5.2. angle of 17°), and a shallow outer exponent αout=−1.0−1.0.The −6 The overall consistency of the model and image radial total dust mass of 1.0±0.4×10 Me is within the range −5 profiles give us confidence that we have an adequate under- (0.03–1)×10 Me estimated from SED modeling (Zuckerman standing of the dust disk parameters. Assuming that our et al. 1995; Sylvester et al. 1996; Merín et al. 2004;Thietal. ) fi +4 μ ( observational errors are approximately Gaussian, and adopting 2014 .Thebest- t minimum grain size is amin = 4-2 m 90% a flat prior for each parameter, the Bayesian probability of our confidence interval). The minimum grain size agrees with the model given the data is blowout grain radius (Burns et al. 1979) in a gas-poor disk, with ⎛ ⎞ r blowout = 4.2 mm for spherical silicate grains of density c2 − P µ-exp ⎜ ⎟.6() ρ=3.5 g cm 3, assuming a radiation pressure efficiency factor of ⎝ ⎠ 2 Qp=1. We discuss the blowout radius with respect to the gas fi To estimate the probability density distribution for any content of the inner disk further in Section 5.4.Thebest- tmodel favors a population of solid dust grains with porosity p=0%, parameter, we marginalize P over the remaining parameters although it is consistent with porosity up to p=40% within the as shown in Figure 6. 90% confidence limit. We obtain good constraints for αout, M, and amin, whose As seen in Figure 6, the total dust mass Md is strongly probability density distributions are approximately Gaussian. degenerate with several disk parameters, notably the minimum Unfortunately, our modeling returns poor constraints on the grain size a , the outer exponent α , and, surprisingly, the viewing geometry of the disk and some of the parameters that min out disk scale height H0. There is no evident mechanism by which describe the spatial distribution of the dust. The posterior the scale height would be degenerate with the disk mass for an distributions for i and PA in Figure 6 are broader than we optically thin disk. A third parameter could set this correlation, anticipated. Hence, we cannot establish a proper uncertainty. but the relationship remains unclear. All of these degeneracies This suggests that polarized intensity alone is not adequate for preclude us from placing fully independent constraints on M , determining the geometry, since only the eastern half of the d amin, H0,orαout. In Section 5.4 we discuss an independent and disk is detected. In addition, for H0 and Rc, our modeling more stringent constraint on the minimum grain size a returns nonsymmetric skewed distributions. We use the 68% min fi arising from the lack of measurable signal in the Stokes Ur con dence intervals as estimates for the uncertainties on the image. best-fit parameter values for i, PA, H0, αout, Rc, and Md. For a and p, we use the 90% confidence interval instead, as min 4.3. Comparison of the Scattered-light Model to the SED motivated by our coarser sampling in these parameters. Our best-fit disk model has i∼60°,PA∼5°, scale radius We compare the predicted thermal emission from our best-fit +8 +9  Rc = 44-12 au, a rather large reference scale height H0 = 14-5 au model to the Stokes Qr image against the SED of HD 141569A

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Figure 4. Modeling of the inner HD141569A dust ring with MCFOST. The central 0 25 circular region is the same as in Figures 1–3 and is ignored in the modeling. Left: observed Stokes Qr image with the fitting region delimited between the circular and elliptical dashed lines. Middle: Stokes Qr image from the best-fit model. Right: residuals of the Stokes Qr image minus the best-fit model. A residual arc-like polarized emission is seen to the south. We have used the same intensity scale and color stretch as for the Stokes Ur image in Figure 1. in Figure 7. Fitting the SED is not a part of our search for the that we did not include any SED information in our modeling. model that best matches the scattered-light emission. It is The published far-IR and millimeter flux measurements are merely a consistency check on our scattered-light modeling, obtained from much wider beams that incorporate the outer two assuming that the same dust population is responsible for both debris rings, so we expect that the fluxes should be higher than the disk tracers. predicted brightness of the 44 au ring. While an SED analysis of We use the compendium of photometric data from Thi et al. HD141569A is relegated to a future study, there are two (2014) with updated photometry in the optical (Høg et al. 2000; independent lines of evidence that suggest that the 44 au ring Mendigutía et al. 2012). We expand this data set by including dominates the >100 μm thermal emission. First, the emission recent submillimeter and millimeter photometry with ALMA resolved in the ALMA 870 μm continuum and CO maps in White (White et al. 2018) and the Submillimeter Array (SMA; et al. (2016) is consistent with an origin in an ∼50 au ring, likely Flaherty et al. 2016). We note that the photometric measure- the same dust ring as resolved by GPI. ALMA shows no evidence ments in Thi et al. (2014), Flaherty et al. (2016), and White of significant millimeter emission associated with the middle et al. (2018) come from instruments with different resolutions (220 au) dust ring. Second, with the middle dust ring being about and beam sizes. These range from FWHW ≈1″ seeing-limited five times wider than the inner 44 au ring, its characteristic dust optical/near-IR measurements, to a beam of 5 1×4 2 for the temperature will be ∼ 5 cooler, so ∼30K. A strong 30Kdust 2.8 mm SMA photometry, up to 2′×5′ for 60 μm photometry component will produce a notable bump in the SED around from the Infrared Astronomical Satellite (IRAS). 100 μm. However, the observed 60 μm fluxes closely follow a We also include a polycyclic aromatic hydrocarbon (PAH) Rayleigh–Jeans distribution. Hence, we believe that the 44 au dust component, motivated by resolved (FWHM=0 26) observa- disk resolved by GPI accounts for most of the long-wavelength tions with the VLT Imager and Spectrometer for the mid-IR flux from the HD141569A circumstellar disk. (VISIR; Lagage et al. 2004) in the PAH1 filter (λc=8.6 μm, Finally, we note that even while it reproduces the observed Δλ=0.42). These reveal a disk out to 1″ along the semimajor 60 μm fluxes adequately, the combined SED of our scattered axis (Thi et al. 2014). The PAHs in the circumstellar light and PAH emission model is underluminous between 8 and environment of HD 141569A are responsible for the emission 30 μm. A fourth component, interior to the one seen by GPI, is features in the mid-IR at 7.7, 8.7, 11.3, and 12.7 μm (Sylvester likely present around the star. To correct for this flux deficit in et al. 1996). We include the PAH component (magenta dotted the mid-IR, we model an innermost 5–15 au dust disk (light line) with the sole goal of approximating the emission features blue dashed line in Figure 7) in MCFOST. We discuss this in the mid-IR modeled in previous studies (Li & Lunine 2003; innermost disk further in Section 5.5. Thi et al. 2014). The PAHs were added with the adopted single −10 molecule size of 6.84 Å and fixed mass of 1.6×10 Me from Thi et al. (2014). 4.4. The Effect of Mie Scattering Assumptions on Disk The thermal emission from our scattered-light model of the Modeling Results 44 au disk reproduces the observed 50 μm fluxes adequately. The synthetic models in our modeling procedure rely on Mie The emission peaks at a wavelength of 45 μm (and so has scattering theory to compute the scattering, opacity, and a characteristic dust temperature of 67 K) and a flux of absorption cross sections of dust grains in the circumstellar − − 1.6×10 13 Wm 2. This is within factors of 1.0–1.8 of the environment of HD 141569A. Although Mie theory has been observed 60–100 μm far-IR fluxes measured with IRAS, extensively used in numerous debris disk studies, it has Spitzer, and Herschel (Thi et al. 2014, and references therein). limitations. The range is entirely due to the scatter in the flux measurements In Mie scattering, individual dust grains are idealized as from different instruments. uniform solid spheres, an assumption likely not applicable to At >140 μm wavelengths, the predicted thermal emission from dust grains growing through agglomeration in debris disks. the 44 au disk matches the SED unexpectedly well, considering Hence, Mie theory could struggle to accurately predict the light

7 The Astronomical Journal, 159:53 (13pp), 2020 February Bruzzone et al.

Figure 5. Radial profiles along the semimajor axis (PA=5°) of the Stokes Qr image (dashed line) and the best-fit models with MCFOST (solid line) inward to 30 au (0 27), north (top) and south (bottom). Shaded areas indicate the dispersion calculated using the uncertainty map derived from the Stokes Ur image (Figure 1, right panel). The best-fit model falls within the uncertainties and in general agrees with the observed profiles. scattering and thermal processes in debris disks. More elaborate observations with VLT/SPHERE (FWHM≈0 040, IWA= models with irregularly shaped dust grain aggregates indicate 0 093),asdidKonishietal.(2016) with HST/STIS (FWHM= that Mie theory does not reproduce well the scattering phase 0 04, IWA=0 40).ComparedtoourPDIwithGPI,allofthese function at angles θ>90° (Min et al. 2016). Accordingly, previous nonpolarimetric observations have lower S/N because of Milli et al. (2017) showed that Mie models fail to reproduce the the inability to employ simultaneous differential imaging through scattering phase function for the HR 4796A dusty debris ring. polarimetry but relying solely on ADI instead. Diffuse structures Such discrepancies likely bias the determination of certain with significant axial symmetry, such as a debris disk seen at model parameters, in particular the viewing geometry, mini- low-to-moderate inclinations, are challenging to extract with mum grain size, dust composition, dust mass, and porosity. / / – ADI KLIP. A high-S N detection of the disk with much lower While we have adopted Mie theory based models for our reduction-dependent systematics has been produced by PDI analysis for the sake of comparison with previous radiative with GPI. transfer modeling on the dust-scattered light from the Our GPI polarized-light image confirms the clearing within HD141569A debris disk (Jonkheid et al. 2006; Thi et al. the 44 au ring first reported by Perrot et al. (2016). Thi et al. 2014; Mawet et al. 2017), we note the above limitations ahead (2014), Konishi et al. (2016), and Mawet et al. (2017) reported of the ensuing discussion. PAH or scattered-light emission from dust at similar separa- tions but described its radial dependence with a single- 5. Discussion exponent power law that decreases with separation. As seen in 5.1. Morphology of the HD 141569A Inner Disk in Polarized Figure 5, we clearly resolve the peak at 44 au that is well Scattered Light: Comparison to Previous Observations modeled with exponential drop-offs on either side. The width ( ∼ ) The clearest view of the inner disk around HD 141569A of the ring in polarized light FWHM 30 au is greater ( ∼ ) / inward to an inner working angle (IWA) of 0 25 (28 au) was than reported FWHM 10 au from the unpolarized VLT ( ) revealed by PDI with GPI. The IWA achieved with GPI shows SPHERE observation of Perrot et al. 2016 , which we attribute the 44 au ring morphology that supersedes any lower-S/N to self-subtraction in the various KLIP reductions of the detections from images with larger IWAs. SPHERE images. The highest S/N previously obtained on the 44 au ring in Consistent with Perrot et al. (2016) and Mawet et al. (2017), fi – scattered light is through Lp-band AO and vortex coronagraphy we nd a north south asymmetry in the brightness of the 44 au with Keck/NIRC2 (FWHM≈0 08, effective IWA ;0 16) ring, which we now reveal as a high-S/N arc-like structure to by Mawet et al. (2017) and also reported with Keck/NIRC2 in the south. We do not see evidence of the other clumps reported the Lp band by Currie et al. (2016) down to 0 25. Perrot et al. in these studies and suspect that they may be related to image (2016) reported even higher angular resolution H-band AO artifacts.

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Figure 6. Normalized probability density distributions for each parameter in the grid of models explored to fit the Stokes Qr image with MCFOST. Brighter yellow regions in the color maps correspond to higher probability densities. A blue bar in each of the histogram panels along the diagonal shows the best-fit value for each parameter.

The viewing geometry of our best-fit model, i∼60° and It is at first surprising that our polarimetric image does not PA∼5°, is similar to previous findings from scattered-light produce better constraints on the geometry. The S/N of all of total intensity observations at lower S/N. Perrot et al. our own and the previously published total intensity observa- (2016) reported that the inner ring has an inclination of tions is much lower. The reduced total intensity images also i=57°.9±1°.3 and a PA of 353°.7±1°.1 from observations suffer from the typical PSF subtraction systematics for with VLT/SPHERE, while Mawet et al. (2017) reported extended emission around bright stars, the result of ADI mode i=53°±6° and PA=349°±8° from Lp observations with observations and image reduction with the KLIP algorithm. Keck/NIRC2. However, unlike all previous direct imaging Thus, it is possible that the uncertainties in Perrot et al. (2016) (nonpolarimetric) observations, which point to a relatively have been underestimated, and our values are closer to well-constrained slightly west of due north orientation of the agreement with theirs because of larger uncertainties (as in semimajor axis of the inner ring, our polarimetric observation Mawet et al. 2017). Nonetheless, the discrepancy between the produces a less well constrained, slightly east of due north PA. polarized and the total intensity geometry is still unusual, as is Our values for i and PA follow broad distributions in Figure 6 the inability to get better geometric constraints from our that suggest uncertainties of about 10°. Our own lower-S/N polarized intensity images. H-band total intensity image (Figure 3) suggests a slightly We suspect that the failure of our modeling to produce better clockwise (PA∼350°) orientation of the scattered-light viewing geometry constraints may be a consequence of emission, in agreement with Perrot et al. (2016) and Mawet detecting only half of a radially extended disk in polarized et al. (2017) and contrary to our polarized image. Effectively, intensity. The detection of this nonaxisymmetric half disk, the orientation of the inner disk from previous total intensity combined with the broader arc-like feature to the south observations is in good alignment with the outer disk, whereas (Section 4.2; Figure 4, right), may favor models with PAs the PDI observations are less conclusive. flipped around the north–south axis. In addition, as discussed in

9 The Astronomical Journal, 159:53 (13pp), 2020 February Bruzzone et al. emission prefers that geometry. In this case, the arc contributes up to 40% of the surface brightness of the debris disk in the range 120°<PA<170° and has a net integrated surface brightness of ≈5mJy within a 10 au wide region centered on the peak emission at 44 au. In the semimajor axis PA=353°.7 case, the residuals are more uniform, and also more positive. The arc contributes 50% of the disk surface brightness at PA=130° and wraps counterclockwise to at least PA=190°, where it is still at ∼40% of the disk’s total brightness before the signal diffuses into the residual noise. The net integrated surface brightness of the arc in the same 120°<PA<170° region is ∼15 and 25mJy if extended up to PA=190°. Without a higher-S/N total intensity image of the disk, we cannot decide in favor of one disk PA versus the other. However, both point to the existence of an arc on the inner ring of the HD141569A disk that contributes between 40% and Figure 7. Measured data points and predicted MCFOST SEDs. Observations 50% of the total disk flux to the southeast. Such arc-like comprise the photometric compendium by Thi et al. (2014) and references structures are known on the outer two rings of HD141569A therein (filled circles), SMA photometry by Flaherty et al. (2016; filled ( squares), and ALMA photometry by White et al. (2018; filled triangles). The Mouillet et al. 2001; Clampin et al. 2003; Perrot et al. 2016; ) fi model SEDs represent the best-fit model to the Stokes Qr image of the HD Mawet et al. 2017 . We believe this to be the rst convincing 141569A inner disk (dark blue dashed line), a fourth innermost disk (light blue detection of such an arc on the inner ring. The location and dashed line), and PAH emission (magenta dotted line). The total emission from these components and the star is shown with a solid line. extent of the feature match the observed brightness enhance- ment in the southeast section of the inner ring in Perrot et al. (2016) and the enhancement in the CO zeroth-moment map in Section 4.4, it is possible that the scattering phase function ( ) fi derived in our Mie-based radiative transfer MCFOST model White et al. 2016 . Our best- t model indicates an average ∼ may not agree well with the behavior of debris disk dust grains. temperature of 90 K at the location of the arc, well above the The detected side of the debris ring is seen at a >90° scattering sublimation point of CO. The CO production mechanism may phase angle, and, as in the case of the HR4796A debris ring thus be linked to the dust overdensity, pointing to ongoing (Milli et al. 2017), the polarization and intensity of the scattered destruction of CO ice-rich planetesimals. The destruction light may not be well represented by Mie theory. cascade itself may be triggered by an unseen body that is also responsible for producing the arc as a spiral arm. Spiral arm structures have been discovered in near-IR 5.2. The Arc-like Structure: A Spiral Arm? scattered-light imaging observations of a few circumstellar ( The arc-like asymmetry spans between 120° and 170° in PA. disks e.g., AB Aur, Hashimoto et al. 2011; HD 142527, This feature is not an outcome of the data reduction process Avenhaus et al. 2014; SAO 206462, Muto et al. 2012; MWC ) because the PDI image requires no further PSF subtraction; 758, Grady et al. 2013; HD 100453, Wagner et al. 2015 . Two rather, it is a true brightness enhancement in the ring. A disk mechanisms are capable of driving such arms in gas-rich with a stellocentric offset could offer a possible explanation for protoplanetary (and transition) disks: gravitational instability the enhanced emission from the arc-like structure to the south. (e.g., Dong et al. 2015a) and interaction between the disk and a The portion of the disk closer to the star would appear brighter planetary or stellar companion (e.g., Dong et al. 2015b, 2016b). than the other side, leading to pericenter glow (Wyatt et al. In optically thin debris disks with much lower gas-to-dust 1999). Our modeling procedure did not include stellocentric ratios, photoelectric instability (Klahr & Lin 2005; Besla & offsets, so we cannot check for pericenter glow. Perrot et al. Wu 2007) may also lead to clumping of dust into structures. (2016) found that the inner ring has a stellocentric offset of Spiral density waves are one of the hypotheses invoked for 15.4±3.4 mas (1.7±0.4 au) to the west but only a negligible explaining the radially moving dust enhancements in the edge- ± ( ± ) one, 1.2 9.4 mas 0.1 1.1 au , to the north. Therefore, a on AU Mic debris disk (Boccaletti et al. 2015). However, – north south stellocentric offset is not the likely cause of the typically, multiple rings and arcs (i.e., broken rings), instead of brightness enhancement to the south. one or two spiral arms, are seen in simulations (Lyra & For a clearer view of the morphology of the asymmetry, we Kuchner 2013; Richert et al. 2018). While the HD 141569A mirror and subtract the northern portion of the disk from the ’ disk is almost certainly too low in mass to be gravitationally southern half. Given the ambiguity of the disk s orientation, we – fi unstable, the possibility that the detected spiral arm like feature perform two different subtractions. In the rst case, we mirror ( ) around the semiminor axis of the best-fit model of the polarized is driven by an unseen planet is exciting. Dong et al. 2016a emission seen with GPI; i.e., the PA of the semimajor axis is 5° showed that spiral arms driven by giant planets in disks at (Figure 8, middle panel). In the second case, we use the modest-to-high inclinations may appear very close to, or be geometry inferred from the total intensity image from part of, the disk ring sculpted by the planet. In particular, the SPHERE, with a semimajor axis PA of 353°.7 (Figure 8, right 50° and 60° inclination panels in Figure 8 of Dong et al. panel). Both subtractions show the excess emission to the south (2016a) show intriguing similarities with the HD 141569A as a remnant arc at S/N levels of 4–5 per GPI pixel along the inner disk spiral arm in Figure 7. The weak contrast of the arm peak of the emission. The residuals in the PA=5° case are in HD 141569A indicates that if it is planet-driven, the planet is closer to zero, which is why our modeling of the polarized-light most likely Jovian or smaller (Dong & Fung 2017).

10 The Astronomical Journal, 159:53 (13pp), 2020 February Bruzzone et al.

Figure 8. Revealing the southern arc on the 44au dust ring around HD141569A by mirroring the northern part of the disk and subtracting it from the southern part. Left: polarized intensity image from GPI. Middle: mirroring and subtraction, assuming the best-fit geometry of the model of the GPI polarized-light H-band emission. The arc extends over 120°–170° and contributes ∼40% of the total disk brightness level. Right: mirroring and subtraction, assuming the inferred geometry of the SPHERE total intensity H-band emission. The arc can be traced counterclockwise to at least 190° at 40%–50% of the overall disk brightness.

−4 5.3. Disk Opening Angle Based on a total gas mass estimate of 200 M⊕ (6×10 Me), Thi et al. (2014) found an average gas-to-dust ratio of 50–100 While it is a much less powerful constraint than it is in ∼ (optically thick) protoplanetary disks, our modeling allows us over the full extent of the 500 au disk. This would preclude a meaningful radiation pressure estimate of the blowout radius. to place approximate constraints on the opening angle or scale ( ) ( ) height of the resolved inner disk. The best-fit model indicates a However, White et al. 2016 found a much lower H2 gas mass of 1.5 M⊕ within 210 au and observed that the inner ∼50 au rather large reference scale height of H0=14 au at the ◦ region shows only tenuous CO emission. Hence, the inner disk R0=45 au reference radius, so an opening angle of 17 . This is above expectations even for a transitional disk, although that we resolved with GPI is substantially more deprived of gas. ( ) fi values as small as 10% are within the 84% confidence limit. Adopting the White et al. 2016 gas mass and our best- tdust ∼ −6 = ( ) With such a large disk opening angle, the best-fit model mass of 10 Me 0.3 M⊕ for the 44 au ring Table 1 yields a ∼ incorporates significant scattering at angles smaller than the gas-to-dust ratio of 5. The actual gas-to-dust ratio in the 44 au ( ) ∼30° expected from a perfectly flat disk (given an inclination ring is likely lower, since the White et al. 2016 gas mass of i∼60°). However, if our Mie theory–derived scattering estimate encompasses both the inner 44 au and parts of the middle phase function is wrong, this constraint is not to be trusted. 220 au ring. Therefore, we expect that the smallest grains in the Previous determinations of the opening angle range from 5%– inner ring are more strongly affected by radiation pressure than 10% (Thietal.2014) to 23% (Merín et al. 2004). The latter is gas drag. fi +4 from SED fitting alone, and, while consistent with our finding, it Our best- t value of amin = 4-2 mm for the minimum grain is not very well constrained. The Thi et al. determination pertains size is marginally consistent with previous findings from to the gas disk geometry and is constrained from Herschel scattered-light observations (Marsh et al. 2002; Mawet et al. measurements of the [C II]/CO J=3–2lineflux ratio, which 2017) but differ from values inferred from SED modeling (Thi traces the efficiency of CO photodissociation as a function of gas et al. 2014; Mawet et al. 2017). The resolved Keck II mid-IR 2 scale height. However, without having spatially resolved the inner observations of Marsh et al. (2002) yield a best-fit (χν=1.23) disk, Thi et al. adopted a model with a gas surface density peak at minimum grain size of 1–3 μm—in agreement with our ≈28 au (Figure 6 in that paper), whereas we resolve the brightness findings—although fits with either interstellar medium–sized ( ) μ ( 2 ) ( μ ) peak of the inner dust disk at 44 au Figure 5 . If the gas and dust 0.1 m cn = 1.40 or large 30 m blackbody grains are well mixed, as assumed by Thi et al., then a cooler gas disk (c2 = 1.50) are also satisfactory. Using Mie scattering [ ]/ n would require a greater scale height to produce the same C II CO assumptions and MCFOST for modeling, Mawet et al. (2017) = – fl J 3 2line ux ratio. It is therefore likely that under the joint found that a population of dust particles of pure olivine with constraints from the Herschel gas abundances and the GPI- a =10 μm provides the best fit to the resolved L -band > min p resolved dust disk morphology, the gas disk has a 10% opening scattered-light emission between 20 and 90 au. At the same angle, consistent with the wide dust disk opening angle found here. time, Mawet et al. also found that a minimum grain size of 0.5 μm best fits the SED, echoing the findings from SED model fitting by Thi et al. (2014). The combined best fit to the Lp 5.4. Independent Constraint on the Minimum Grain Size from image and SED in Mawet et al. (2017) yields an even smaller Polarimetry minimum grain size: amin=0.1 μm. Our model of the polarized scattered light (Section 4.2) The preference for very small (0.1–0.5 μm) grains in SED +4 produced a minimum grain size of amin = 4-2 mm, consistent modeling points to the presence of a warm dust component that with the 4.2 μm blowout grain radius around HD141569A. maynotbewellrepresentedbyanextrapolationofanindex The result for the blowout radius is meaningful as long as the n=3.5 (collisionally dominated) grain size distribution below inner disk remains gas-poor. Conversely, in gas-rich disks with 1 μm. The collisional cascade may not be equally efficient at all interstellar medium–like gas-to-dust ratios of ∼100, gas drag grain sizes or radial separations in the disk. Such is the case at least dominates the dynamics of small grains. for large grains around HD141569A, as multiband 0.9–9mm

11 The Astronomical Journal, 159:53 (13pp), 2020 February Bruzzone et al. ALMA and Very Large Array observations show that at can be described radially with a combination of two power laws millimeter sizes, the index is n=2.95±0.1 (White et al. 2018). that peaks at 44 au and extends out to 100 au. The disk also Our polarization observations are uniquely diagnostic of the features an arc-like overdensity along the southern part that is presence of submicron grains because of their polarization reminiscent of the spiral arm structures previously known at properties. Specifically, scattered-light models with a significant larger scales in this system. The existence of this inner spiral arm population of amin<0.8 μm are rejected because they produce structure and its colocation with CO emission detected by ALMA negative polarization in Stokes Qr that is not observed by GPI. We indicates that this may be a site of ongoing icy grain destruction, similarly rule out highly porous (p>60%) materials. Thus, with perhaps driven by an unseen planetary perturber. The best-fit the added power of near-IR polarimetry, we conclude that the model to our polarimetry data indicates an optically thin disk with „ μ +8 population of 1.0 m grains in the 44 au dust ring is not a maximum surface density of rmax  Rc = 44-12 au, a steep significant enough to be detectable in polarized light. A trace inner gradient (αin=14), and a shallower outer exponent ( +0.5) population may nonetheless exist and could be responsible for the aout =-1.0-1.0 . The polarimetric observations are best observed PAH emission. We use this result to argue for a fourth described by a dust population model with a minimum size of  +4 ( ± )× −6 innermost and unseen component of the HD 141569A debris disk 4m-2m and a mass of 1.0 0.4 10 Me for nonporous in Section 5.5. grains up to 1mm in size. A significant population of submicron We again caution that this analysis is rooted in Mie theory, grains is independently excluded by the lack of negative signal in ( which may not yield the correct ratio of scattered to absorbed and the H-band Stokes Qr image. We use the thermal emission from emitted) flux. A non-Mie treatment could yield different constraints our best-fit Mie model to estimate the amount of unseen dust on the minimum grain size from the observed polarization inward of 28 au. We find that a fourth innermost dust population, signatures. However, an additional argument against the presence potentially a 5–15 au belt, is required to fully reproduce the of a large reservoir of submicron grains is the lack of a 10 μm 8–30 μm SED under Mie theory assumptions. silicate feature in the Spitzer IRS spectrum of HD141569A (Sloan With our new high-S/N polarimetric detection of the 44 au et al. 2005),asalsoarguedbyThietal.(2014). Hence, we find that ring, the richness of the circumstellar environment around HD a warm ring of dust grains several microns in size and lying 141569A can be appreciated in a new light. Considering interior to the one resolved with GPI offers the simplest resolved imaging data from other high-contrast facilities, the explanation for the extra 8–30 μmemissionfromHD141569A. HD 1415169A debris disk shapes up to be made of at least three, and potentially four, nested rings, with spiral structures 5.5. An Unseen Innermost (Fourth) Ring on the three spatially resolved rings. As such, it is an excellent laboratory for studying dynamically perturbed disks. Our best-fit model to the light-polarizing dust offers a good matchtotheλ50 μmSED(Figure 7). However, there is We thank our referee for very insightful feedback on the dust remnant excess emission between 8 and 30 μmthatisnot considerations in our modeling. This research was supported in reproduced by our dust model. The presence of warm circumstellar part by a Discovery Grant by the Canadian Natural Sciences material well within 100 au has been inferred not only from the and Engineering Council (NSERC) to S.M. and NSF grant abovementioned SED fitting by Thi et al. (2014) and Mawet et al. AST-1413718 (GD). P.K. and J.R.G. are thankful for support (2017) but also from CO observations (Fisher et al. 2000;Merín from NSF AST-1518332 and NASA NNX15AC89G and et al. 2004;Gotoetal.2006; Thi et al. 2014;Flahertyetal.2016; NNX15AD95G/NEXSS. J.M. acknowledges support from the White et al. 2016). Our detection of an inner clearing within the NASA through the NASA Hubble Fellowship grant HST-HF2- 44 au ring and the lack of polarization signal from submicron-sized 51414.001 awarded by the Space Telescope Science Institute, grains (Section 5.4) imposes new constraints on the spatial extent which is operated by the Association of Universities for of the warm dust responsible for the excess thermal emission at Research in Astronomy, Inc., for NASA, under contract NAS5- shorter wavelengths. 26555. This work benefited from NASA’s Nexus for Exoplanet To account for this missing flux, we employ a simple model System Science (NExSS) research coordination network, in MCFOST assuming the same grain size distribution, sponsored by NASA’s Science Mission Directorate. Portions = μ = amin 4 m, amax 1 mm, porosity of 0%, and magnesium- of this work were performed under the auspices of the U.S. rich olivines from our best-fit model. Motivated by Thi et al. Department of Energy by Lawrence Livermore National (2014), the dust is characterized by a radial density distribution Laboratory under contract DE-AC52-07NA27344. p R∝r with p=1, no flaring (β=1), and an inner disk radius Facility: Gemini:South(GPI). of Rin¢ = 5au. We keep the outer radius Rou¢ t and dust mass M¢ of the innermost disk as free parameters. dust ORCID iDs The best-fit thermal model for the innermost disk (light blue dashed line in Figure 7) indicates 300 K dust with a mass of Juan Sebastián Bruzzone https://orcid.org/0000-0002- −8 ( × −3 ) 10 Me 3 10 M⊕ ranging from Rin¢ = 5.0 au up to 2731-0397 // / Rout¢ = 15 au. This is well within the coronagraph IWA of GPI Stanimir Metchev https: orcid.org 0000-0003-3050-8203 and thus presently undetectable in scattered light. Gaspard Duchêne https://orcid.org/0000-0002-5092-6464 Ruobing Dong https://orcid.org/0000-0001-9290-7846 // / 6. Conclusion Thomas M. Esposito https: orcid.org 0000-0002- 0792-3719 We have presented the first polarimetric detection of the inner Jason J. Wang https://orcid.org/0000-0003-0774-6502 44 au disk component of the pre-main-sequence star HD Johan Mazoyer https://orcid.org/0000-0002-9133-3091 141569A. Our H-band PDI with GPI has revealed that there is Schuyler Wolff https://orcid.org/0000-0002-9977-8255 a nonuniform ring-shaped optically thin dusty disk inward to S. Mark Ammons https://orcid.org/0000-0001-5172-7902 0 25 at the highest S/N attained to date. We find that the disk Adam C. Schneider https://orcid.org/0000-0002-6294-5937

12 The Astronomical Journal, 159:53 (13pp), 2020 February Bruzzone et al.

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