Circumstellar Dust Research Articles

V532 Oph Is a New R Coronae Borealis Star

ABSTRACTV532 Oph has been found to be a member of the rare, hydrogen-deficient R Coronae Borealis (RCB) stars from new photometric and spectroscopic data reported in this article. The light curve of V532 Oph shows the sudden, deep, irregularly spaced declines characteristic of RCB stars. Its optical spectrum is typical of a warm (Teff ∼ 7000 K) RCB star, showing weak or absent hydrogen lines, C2 Swan bands, and no evidence for 13C. In addition, the star shows small pulsations typical of an RCB star and an infrared excess due to circumstellar dust. It also appears to be significantly reddened by foreground dust. The distance to V532 Oph is estimated to be 5.5–8.7 kpc. These new data show that this star was misclassified as an eclipsing binary in the General Catalog of Variable Stars. The new data presented here for V532 Oph reveal the power of high-quality, high-cadence, all-sky photometric surveys, such as ASAS-3, to identify new RCB candidates on the basis of light-curve data alone, now that they have been collecting data for durations sufficiently long to reveal multiple declines. Despite their small numbers, RCB stars may be of great importance in understanding the late stages of stellar evolution. In particular, their measured isotopic abundances imply that many, if not most, RCB stars are produced by WD mergers, which may be the low-mass counterparts of the more massive mergers thought to produce type Ia supernovae. Therefore, establishing the population of RCB stars in the Galaxy will help constrain the frequency of these WD mergers.

THE MASS LOSS RETURN FROM EVOLVED STARS TO THE LARGE MAGELLANIC CLOUD: EMPIRICAL RELATIONS FOR EXCESS EMISSION AT 8 AND 24 μm

We present empirical relations describing excess emission from evolved stars in the Large Magellanic Cloud (LMC) using data from the Spitzer Space Telescope Surveying the Agents of a Galaxy's Evolution (SAGE) survey which includes the Infrared Array Camera (IRAC) 3.6, 4.5, 5.8, and 8.0 {mu}m and Multiband Imaging Photometer (MIPS) 24, 70, and 160 {mu}m bands. We combine the SAGE data with the Two Micron All Sky Survey (2MASS; J, H, and K {sub s}) and the optical Magellanic Cloud Photometric Survey (MCPS; U, B, V, and I) point source catalogs in order to create complete spectral energy distributions (SEDs) of the asymptotic giant branch (AGB) star candidates in the LMC. AGB star outflows are among the main producers of dust in a galaxy, and this mass loss results in an excess in the fluxes observed in the 8 and 24 {mu}m bands. The aim of this work is to investigate the mass loss return by AGB stars to the interstellar medium of the LMC by studying the dependence of the infrared excess flux on the total luminosity. We identify oxygen-rich, carbon-rich, and extreme AGB star populations in our sample based on their 2MASS and IRAC colors. The SEDs ofmore » oxygen- and carbon-rich AGB stars are compared with appropriate stellar photosphere models to obtain the excess flux in all the IRAC bands and the MIPS 24 {mu}m band. Extreme AGB stars are dominated by circumstellar emission at 8 and 24 {mu}m; thus we approximate their excesses with the flux observed in these bands. We find about 16,000 O-rich, 6300 C-rich, and 1000 extreme sources with reliable 8 {mu}m excesses, and about 4500 O-rich, 5300 C-rich, and 960 extreme sources with reliable 24 {mu}m excesses. The excesses are in the range 0.1 mJy to 5 Jy. The 8 and 24 {mu}m excesses for all three types of AGB candidates show a general increasing trend with luminosity. The color temperature of the circumstellar dust derived from the ratio of the 8 and 24 {mu}m excesses decreases with an increase in excess, while the 24 {mu}m optical depth increases with excess. The extreme AGB candidates are the major contributors to the mass loss, and we estimate the total AGB mass loss return to the LMC to be (5.9-13) x 10{sup -3} M {sub sun} yr{sup -1}.« less

The spin-orbit alignment of the Fomalhaut planetary system probed by optical long baseline interferometry

We discuss the spin-orbit orientation of the Fomalhaut planetary system composed of a central A4V star, a debris disk, and a recently discovered planetary companion. We use spectrally resolved, near-IR long baseline interferometry to obtain precise spectro-astrometric measurements across the brG absorption line. The achieved astrometric accuracy of 3 nu-as and the spectral resolution R=1500 from the AMBER/VLTI instrument allow us to spatially and spectrally resolve the rotating photosphere. We find a position angle PAstar=65deg pm 3deg for the stellar rotation axis, perfectly perpendicular with the literature measurement for the disk position angle (PAdisk=156deg pm 0.3deg). This is the first time such test can be performed for a debris disk, and in a non-eclipsing system. Additionally, our measurements suggest unexpected backward-scattering properties for the circumstellar dust grains. Our observations validate the standard scenario for star and planet formation, in which the angular momentum of the planetary systems are expected to be collinear with the stellar spins.

THE PHYSICAL PROPERTIES OF THE RED SUPERGIANT WOH G64: THE LARGEST STAR KNOWN?

WOH G64 is an unusual red supergiant (RSG) in the Large Magellanic Cloud (LMC), with a number of properties that set it apart from the rest of the LMC RSG population, including a thick circumstellar dust torus, an unusually late spectral type, maser activity, and nebular emission lines. Its reported physical properties are also extreme, including the largest radius for any star known and an effective temperature that is much cooler than other RSGs in the LMC, both of which are at variance with stellar evolutionary theory. We fit moderate-resolution optical spectrophotometry of WOH G64 with the MARCS stellar atmosphere models, determining an effective temperature of 3400 +/- 25 K. We obtain a similar result from the star's broadband V - K colors. With this effective temperature, and taking into account the flux contribution from the aysmmetric circumstellar dust envelope, we calculate log(L/L_sun) = 5.45 +/- 0.05 for WOH G64, quite similar to the luminosity reported by Ohnaka and collaborators based on their radiative transfer modeling of the star's dust torus. We determine a radius of R/R_sun = 1540, bringing the size of WOH G64 and its position on the H-R diagram into agreement with the largest known Galactic RSGs, although it is still extreme for the LMC. In addition, we use the Ca II triplet absorption feature to determine a radial velocity of 294 +/- 2 km/s for the star; this is the same radial velocity as the rotating gas in the LMC's disk, which confirms its membership in the LMC and precludes it from being an unusual Galactic halo giant. Finally, we describe the star's unusual nebula emission spectrum; the gas is nitrogen-rich and shock-heated, and displays a radial velocity that is significantly more positive than the star itself by 50 km/s.

Circumstellar dust shells around long-period variables

Context. Miras and long-period variables (LPVs) are radially pulsating, highly evolved stars on the Asymptotic Giant Branch. Because of peculiar conditions of this objects, their cool, extended atmospheres are ideal sites for the formation of dust particles. Carbon-rich circumstellar dust shells (CDSs) surrounding stars with high stellar luminosity, tend to become dynamically unstable. They develop a self-maintaining oscillatory pattern caused entirely by dust formation even without the additional input of mechanical momentum from the star (exterior κ-mechanism). Since this system obviously has an eigenmode, it is interesting to consider the interaction with an exterior mechanical force, i.e. the radial pulsation at the inner boundary. Aims. We investigate in great detail the complex dynamical behaviour of carbon-rich CDSs in a more systematic way. This is done by established methods of nonlinear dynamics. Methods. We consider CDSs as multioscillatory systems that can be analysed with tools of nonlinear dynamics. We also use a discrete Fourier transform to examine the eigenmodes and their behaviour and apply this to a typical model CDS as a representative example. Results. In the absence of external excitation, the dynamics of the shell are dominated by its eigenmode, which is determined by the characteristic timescale of the coupled system of dust formation, hydrodynamics, and thermodynamics. The input mechanical energy and momentum of an underlying stellar pulsation introduces a new timescale to the system. Depending on the ratio of these two timescales, the dynamical behaviour of the system is either dominated by the eigenmode of the shell, by the excitation force, or can be irregular. In the latter case, the strength of the excitation becomes especially important. However, even for a small excitation period when the system is dominated by the shell’s eigenmode, the oscillation of the shell tends to synchronise with the excitation force.

The young active star SAO 51891 (V383 Lacertae)

Our aim is investigating surface inhomogeneities of the young late-type star SAO51891, from photosphere to upper chromosphere, analyzing contemporaneous high-resolution spectra and broad-band photometry. The FOCES@CAHA spectral range is used to determine spectral classification and derive vsini and Vrad. The Li abundance is measured to estimate the age. The BVRIJHKs bands are used to construct the SED. The variations of our BV fluxes and Teff are used to infer the presence of photospheric spots and observe their behavior over time. The chromospheric activity is studied applying the spectral subtraction technique to Halpha, CaII H&K, Heps, and CaII IRT lines. We find SAO51891 to be a young K0-1V star with Li abundance close to the Pleiades upper envelope, confirming its youth (~100 Myr), also inferred from its kinematical membership to the Local Association. We detect no IR excess from SED analysis, and rotational modulation of luminosity, Teff, CaII, and Heps total fluxes. A spot model with two active regions, ~240 K cooler than the surrounding photosphere, fits our light/Teff curves, and reproduces the small-amplitude Vrad variations. The anti-correlation of light curves and chromospheric diagnostics indicates plages spatially associated with spots. The large amplitude observed in the Heps-flux curve suggests that this line is very sensitive to the plage presence. Finally, SAO51891 is a young active star, lacking significant amounts of circumstellar dust or any evidence for low mass companions. The spots turn out to be larger and warmer than those in less active MS stars. The Vrad variation produced by spots has an amplitude comparable with those induced by Jupiter-mass planets orbiting close to the star. SAO51891 is a good example of star where the detection of planets may be hampered by the high activity level.

THE MID-INFRARED VIEW OF RED SEQUENCE GALAXIES IN ABELL 2218 WITH AKARI

We present the {\it AKARI} InfraRed Camera (IRC) imaging observation of early-type galaxies in A2218 at z $\simeq$ 0.175. Mid-infrared (MIR) emission from early-type galaxies traces circumstellar dust emission from AGB stars or/and residual star formation. Including the unique imaging capability at 11 and 15 $\mu$m, our {\it AKARI} data provide an effective way to investigate MIR properties of early-type galaxies in the cluster environment. Among our flux-limited sample of 22 red sequence early-type galaxies with precise dynamical and line strength measurements ($<$ 18 mag at 3 $\mu m$), we find that at least 41% have MIR-excess emission. The $N3-S11$ versus $N3$ (3 and 11 $\mu$m) color-magnitude relation shows the expected blue sequence, but the MIR-excess galaxies add a red wing to the relation especially at the fainter end. A SED analysis reveals that the dust emission from AGB stars is the most likely cause for the MIR-excess, with low level of star formation being the next possible explanation. The MIR-excess galaxies show a wide spread of $N3-S11$ colors, implying a significant spread (2--11 Gyr) in the estimated mean ages of stellar populations. We study the environmental dependence of MIR-excess early-type galaxies over an area out to a half virial radius ($\sim$1 Mpc). We find that the MIR-excess early-type galaxies are preferentially located in the outer region. From these evidences, we suggest that the fainter, MIR-excess early-type galaxies have just joined the red sequence, possibly due to the infall and subsequent morphological/spectral transformation induced by the cluster environment.

HOT DEBRIS DUST AROUND HD 106797

Photometry of the A0 V main-sequence star HD 106797 with AKARI and Gemini/T-ReCS is used to detect excess emission over the expected stellar photospheric emission between 10 and 20 μm, which is best attributed to hot circumstellar debris dust surrounding the star. The temperature of the debris dust is derived as T d ~ 190 K by assuming that the excess emission is approximated by a single temperature blackbody. The derived temperature suggests that the inner radius of the debris disk is ~14 AU. The fractional luminosity of the debris disk is 1000 times brighter than that of our own zodiacal cloud. The existence of such a large amount of hot dust around HD 106797 cannot be accounted for by a simple model of the steady state evolution of a debris disk due to collisions, and it is likely that transient events play a significant role. Our data also show a narrow spectral feature between 11 and 12 μm attributable to crystalline silicates, suggesting that dust heating has occurred during the formation and evolution of the debris disk of HD 106797.

Composite grains: Effects of porosity and inclusions on the [formula omitted] silicate feature

We calculate the absorption efficiency of the composite grains, made up of host silicate spheroids and inclusions of ices/graphites/or voids, in the spectral region 7.0 – 14.0 μ m . The absorption efficiencies of the composite spheroidal grains for three axial ratios are computed using the discrete dipole approximation (DDA) as well as using the effective medium approximation and T-matrix (EMT–T-matrix) approach. We study the absorption as a function of the volume fraction of the inclusions and porosity. In particular, we study the variation in the 10.0 μ m feature with the volume fraction of the inclusions and porosity. We then calculate the infrared fluxes for these composite grains and compare the model curves with the average observed IRAS–LRS curve, obtained for several circumstellar dust shells around stars. These results on the composite grains show that the wavelength of the peak absorption shifts and the width of the 10.0 μ m feature varies with the variation in the volume fraction of the inclusions. The model curves for composite grains with axial ratios not very large ( AR ∼ 1.3 ) and volume fractions of inclusions with f = 0.20 , and dust temperature of about 250–300 K, fit the observed emission curves reasonably well.

INFRARED SIGNATURES OF DISRUPTED MINOR PLANETS AT WHITE DWARFS

Spitzer Space Observatory IRAC and MIPS photometric observations are presented for 20 white dwarfs with T < 20,000 K and metal-contaminated photospheres. A warm circumstellar disk is detected at GD 16 and likely at PG 1457-086, while the remaining targets fail to reveal mid-infrared excess typical of dust disks, including a number of heavily polluted stars. Extending previous studies, over 50% of all single white dwarfs with implied metal accretion rates dM/dt > 3e8 g/s display a warm infrared excess from orbiting dust; the likely result of a tidally-destroyed minor planet. This benchmark accretion rate lies between the dust production rates of 1e6 g/s in the solar system zodiacal cloud and 1e10 g/s often inferred for debris disks at main sequence A-type stars. It is estimated that between 1% and 3% of all single white dwarfs with cooling ages less than around 0.5 Gyr possess circumstellar dust, signifying an underlying population of minor planets.

Accretion and diffusion in white dwarfs

<i>Context. <i/>A number of cool white dwarfs with metal traces, of spectral types DAZ, DBZ, and DZ have been found to exhibit infrared excess radiation due to circumstellar dust. The origin of this dust is possibly a tidally disrupted asteroid that formed a debris disk now supplying the matter accreting onto the white dwarf. To reach any clear conclusions from the observed composition of the white dwarf atmosphere to that of the circumstellar matter, we need a detailed understanding of the accretion and diffusion process, in particular the diffusion timescales.<i>Aims. <i/>We aim to provide data for a wide range of white dwarf parameters and all possible observed chemical elements.<i>Methods. <i/>Starting from atmosphere models, we calculate the structure of the outer envelopes, obtaining the depth of the convection zone and the physical parameters at the lower boundary. These parameters are used to calculate the diffusion velocities using calculations of diffusion coefficients available in the literature.<i>Results. <i/>With a simple example, we demonstrate that the observed element abundances are not identical to the accreted abundances. Reliable conclusions are possible only if we know or can assume that the star has reached a steady state between accretion and diffusion. In this case, most element abundances differ only by factors in the range 2-4 between atmospheric values and the circumstellar matter. Knowing the diffusion timescales, we can also accurately relate the accreted abundances to the observed ones. If accretion has stopped, or if the rates vary by large amounts, we cannot determine the composition of the accreted matter with any certainty.

FORMATION AND EVOLUTION OF PLANETARY SYSTEMS: PROPERTIES OF DEBRIS DUST AROUND SOLAR-TYPE STARS

We present Spitzer photometric (IRAC and MIPS) and spectroscopic (IRS low resolution) observations for 314 stars in the Formation and Evolution of Planetary Systems Legacy program. These data are used to investigate the properties and evolution of circumstellar dust around solar-type stars spanning ages from approximately 3 Myr-3 Gyr. We identify 46 sources that exhibit excess infrared emission above the stellar photosphere at 24 μm, and 21 sources with excesses at 70 μm. Five sources with an infrared excess have characteristics of optically thick primordial disks, while the remaining sources have properties akin to debris systems. The fraction of systems exhibiting a 24 μm excess greater than 10.2% above the photosphere is 15% for ages < 300 Myr and declines to 2.7% for older ages. The upper envelope to the 70 μm fractional luminosity appears to decline over a similar age range. The characteristic temperature of the debris inferred from the IRS spectra range between 60 and 180 K, with evidence for the presence of cooler dust to account for the strength of the 70 μm excess emission. No strong correlation is found between dust temperature and stellar age. Comparison of the observational data with disk models containing a power-law distribution of silicate grains suggests that the typical inner-disk radius is ≳10 AU. Although the interpretation is not unique, the lack of excess emission shortward of 16 μm and the relatively flat distribution of the 24 μm excess for ages ≾300 Myr is consistent with steady-state collisional models.

SIX WHITE DWARFS WITH CIRCUMSTELLAR SILICATES

Spitzer Space Telescope spectra reveal 10 micron silicate emission from circumstellar dust orbiting six externally-polluted white dwarfs. Micron-size glasses with an olivine stoichiometry can account for the distinctively broad wings that extend to 12 microns; these particles likely are produced by tidal-disruption of asteroids. The absence of infrared PAH features is consistent with a scenario where extrasolar rocky planets are assembled from carbon-poor solids.

Laboratory studies of circumstellar and interstellar materials

When the most primitive types of meteorite (chondrites) and interplanetary dust particles (IDPs) formed in the solar protoplanetary disk (solar nebula), all of them incorporated at least two types of presolar material: interstellar organic matter and circumstellar grains. The organic matter probably formed in the protosolar molecular cloud. To date, 10 types of circumstellar grain have been found and their isotopic compositions show that they formed around RGB/AGB stars, supernovae and, possibly, novae and WR stars. It has been proposed that interstellar silicates are also present in IDPs, but the evidence for this is less compelling. The range of isotopic compositions exhibited by the circumstellar grains require a minimum of 35–40 stellar sources, suggesting that they are a fairly representative sample of circumstellar dust production in the Galaxy. In general, the relative abundances of circumstellar grains from different sources are similar to what is expected from astronomical estimates provided that the contribution from supernovae is relatively small. The one exception is graphite grains from AGB stars that are highly depleted. Collisional erosion, radiation damage and sputtering are all expected to affect grains in the ISM on relatively short timescales. Yet, for reasons that are unclear, examination of the circumstellar grains has found little evidence for any of these processes. The circumstellar grains also provide information about grain nucleation and growth in stellar winds. For instance, only graphite grains from AGB stars seem to have formed by heterogeneous nucleation. Graphite and SiC grains from AGB stars as large as 10–20 µm have been found in meteorites, probably requiring regions of high gas density rather than the usual assumption of uniformly expanding outflows. However, few graphite and SiC grains have similar C isotopic compositions, suggesting that they tend to form at different stages of AGB evolution.

Infrared Spectroscopy of Interstellar Dust

IR spectroscopy is the premier tool to study the composition of interstellar dust. Broad absorption and emission bands provide direct identification of the solid compounds present in space and allow measurement of accurate abundances. Systematic studies of large samples of sources allow then inferences on the origin and evolution of dust in space. Overall, the observed infrared spectra of interstellar and circumstellar dust reveals an incredibly rich and varied composition. This chapter briefly reviews the principles of infrared spectroscopy. This is then applied to the composition of circumstellar oxides and minerals and interstellar Polycyclic Aromatic Hydrocarbon molecules.

AKARI IRC Survey of the Large Magellanic Cloud: Outline of the Survey and Initial Results

Abstract We observed an area of 10deg$^2$ of the Large Magellanic Cloud using the Infrared Camera (IRC) on board AKARI. The observations were carried out using five imaging filters (3, 7, 11, 15, and 24$\mu$m) and a dispersion prism (2-5$\mu$m, $\lambda / \Delta\lambda \sim$ 20) equipped in the IRC. This paper describes an outline of our survey project, and presents some initial results using imaging data that detected over 5.9$\times$10$^5$ near-infrared and 6.4$\times$10$^4$ mid-infrared point sources. The 10$\sigma$ detection limits of our survey are about 16.5, 14.0, 12.3, 10.8, and 9.2 in Vega-magnitude at 3, 7, 11, 15, and 24$\mu$m, respectively. The 11 and 15$\mu$m data, which are unique to AKARI IRC, allow us to construct color-magnitude diagrams that are useful to identify stars with circumstellar dust. We found a new sequence in the color-magnitude diagram, which is attributed to red giants with luminosity fainter than that of the tip of the first red-giant branch. We suggest that this sequence is likely to be related to the broad emission feature of aluminum oxide at 11.5$\mu$m. The 11 and 15$\mu$m data also indicate that the ([11]$-$[15]) color of both oxygen-rich and carbon-rich red giants once becomes blue, and then turns red again in the course of their evolution, probably due to a change in the flux ratio of the silicate or silicon carbide emission feature at 10 or 11.3$\mu$m to the 15$\mu$m flux.

Late stages of stellar evolution

Mass loss is an important ingredient in the evolution of stars, in particular during late stages of stellar evolution, when mass loss often is the dominant factor. The far-IR window enables to probe the outflows of evolved stars at different depths, providing direct access to the driving mechanisms. The spatial and spectral resolution of Herschel offer important clues to study the geometry and the composition of the circumstellar gas and dust, and to describe and understand the spatial and temporal variability of the mass loss from initially spherical stars.

MOST photometry of the enigmatic PMS pulsator HD 142666

We present precise photometry of the pulsating Herbig Ae star HD 142666 obtained in two consecutive years with the MOST (Microvariability & Oscilations of STars) satellite. Previously, only a single pulsation period was known for HD 142666. The MOST photometry reveals that HD 142666 is multi-periodic. However, the unique identification of pulsation frequencies is complicated by the presence of irregular variability caused by the star's circumstellar dust disk. The two light curves obtained with MOST in 2006 and 2007 provided data of unprecedented quality to study the pulsations in HD 142666 and also to monitor the circumstellar variability. We attribute 12 frequencies to pulsation. Model fits to the three frequencies with the highest amplitudes lie well outside the uncertainty box for the star's position in the HR diagram based on published values. The models suggest that either (1) the published estimate of the luminosity of HD 142666, based on a relation between circumstellar disk radius and stellar luminosity, is too high and/or (2) additional physics such as mass accretion may be needed in our models to accurately fit both the observed frequencies and HD 142666's position in the HR diagram.

A dense disk of dust around the born-again Sakurai's object

In 1996, Sakurai's object (V4334 Sgr) suddenly brightened in the centre of a faint Planetary Nebula (PN). This very rare event was interpreted as the reignition of a hot white dwarf that caused a rapid evolution back to the cool giant phase. From 1998 on, a copious amount of dust has formed continuously, screening out the star which has remained embedded in this expanding high optical depth envelope. The new observations, reported here, are used to study the morphology of the circumstellar dust in order to investigate the hypothesis that Sakurai's Object is surrounded by a thick spherical envelope of dust. We have obtained unprecedented, high-angular resolution spectro-interferometric observations, taken with the mid-IR interferometer MIDI/VLTI, which resolve the dust envelope of Sakurai's object. We report the discovery of a unexpectedly compact (30 x 40 milliarcsec, 105 x 140 AU assuming a distance of 3.5 kpc), highly inclined, dust disk. We used Monte Carlo radiative-transfer simulations of a stratified disk to constrain its geometric and physical parameters, although such a model is only a rough approximation of the rapidly evolving dust structure. Even though the fits are not fully satisfactory, some useful and robust constraints can be inferred. The disk inclination is estimated to be 75+/-3 degree with a large scale height of 47+/-7 AU. The dust mass of the disk is estimated to be 6 10^{-5} solar mass. The major axis of the disk (132+/-3 degree) is aligned with an asymmetry seen in the old PN that was re-investigated as part of this study. This implies that the mechanism responsible for shaping the dust envelope surrounding Sakurai's object was already at work when the old PN formed.

STIS IMAGING OF THE HR 4796A CIRCUMSTELLAR DEBRIS RING

We have obtained high spatial resolution imaging observations of the HR 4796A circumstellar debris dust ring using the broad optical response of the Hubble Space Telescope Imaging Spectrograph in coronagraphic mode. We use our visual wavelength observations to improve upon the earlier measured geometrical parameters of the ring-like disk. Two significant flux density asymmetries are noted: (1) preferential forward scattering by the disk grains and (2) an azimuthal surface brightness anisotropy about the morphological minor axis of the disk with corresponding differential ansal brightness. We find the debris ring offset from the location of the star by ~1.4 AU, a shift insufficient to explain the differing brightnesses of the NE and SW ansae simply by the 1/$r^2$ dimmunition of starlight. The STIS data also better quantify the radial confinement of the starlight-scattering circumstellar debris, to a characteristic region <14 AU in photometric half-width, with a significantly steeper inner truncation than outward falloff in radial surface brightness. The inferred spatial distribution of the disk grains is consistent with the possibility of one or more unseen co-orbital planetary-mass perturbers, and the colors of the disk grains are consistent with a collisionally evolved population of debris, possibly including ices reddened by radiation exposure to the central star.