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Gas Absorption Detected from the Edge-on Debris Disk Surrounding HD 32297
Near-infrared and optical images of HD 32297 indicate that it has anedge-on debris disk, similar to β Pic. I present high-resolutionoptical spectra of the Na I doublet toward HD 32297 and stars in closeangular proximity. A circumstellar absorption component is clearlyobserved toward HD 32297 at the stellar radial velocity, which is notobserved toward any of its neighbors, including the nearest only 0.9'away. An interstellar component is detected in all stars >90 pc,including HD 32297, likely due to the interstellar material at theboundary of the Local Bubble. Radial velocity measurements of thenearest neighbors, BD +07 777s and BD +07 778, indicate that they areunlikely to be physically associated with HD 32297. The measuredcircumstellar column density around HD 32997, logNNaI~11.4,is the strongest Na I absorption measured toward any nearbymain-sequence debris disk, even the prototypical edge-on debris diskβ Pic. Assuming that the morphology and abundances of the gascomponent around HD 32297 are similar to β Pic, I estimate an upperlimit to the gas mass in the circumstellar disk surrounding HD 32297 of~0.3 M⊕.
| The Signature of Primordial Grain Growth in the Polarized Light of the AU Microscopii Debris Disk
We have used the Hubble Space Telescope Advanced Camera for Surveyscoronagraph to make the first polarization maps of the AU Microscopiidebris disk. The polarization rises from 5% at 20 AU to 40% at 80 AU.The polarization is perpendicular to the disk, indicating that thescattered light originates from micron-sized grains in an optically thindisk. Disk models show that interior to the ``birth ring'' (40-50 AU)there is a hole in the dust distribution where micron-sized dust isdepleted by a factor of more than 300. The disk is collision dominated,and grains that fall inward due to drag forces undergo a destructivecollision. The presence of this hole implies that the localizedenhancements in surface brightness that occur at projected radiiinterior to the birth ring are caused by nonaxisymmetric structures inthe outer disk. The grains exhibit strong forward scattering and highpolarization. Spherical grains composed of conventional materials cannotreproduce these optical properties. A Mie/Maxwell-Garnett analysisdemands highly porous (91%-94%) particles. In the inner solar system,porous particles form in cometary dust, where the sublimation of icesleaves a ``bird's nest'' of refractory material. In AU Mic, the grainporosity may be primordial, because the dust birth ring lies beyond theice sublimation point. The observed porosities span the range of valuesimplied by laboratory studies of particle coagulation by ballisticcluster-cluster aggregation. To avoid compactification, the upper sizelimit for the parent bodies is in the decimeter range, in agreement withtheoretical predictions based on collisional lifetime arguments.Consequently, AU Mic may exhibit the signature of the primordialagglomeration process whereby interstellar grains first assembled toform macroscopic objects.
| Discovery of an 86 AU Radius Debris Ring around HD 181327
HST NICMOS PSF-subtracted coronagraphic observations of HD 181327 haverevealed the presence of a ringlike disk of circumstellar debris seen in1.1 μm light scattered by the disk grains, surrounded by a diffuseouter region of lower surface brightness. The annular disk appears to beinclined by 31.7d+/-1.6d from face-on, with the disk major-axis P.A. at107deg+/-2deg. The total 1.1 μm flux density ofthe light scattered by the disk (at 1.2"
| Nearby Debris Disk Systems with High Fractional Luminosity Reconsidered
By searching the IRAS and ISO databases, we compiled a list of 60 debrisdisks that exhibit the highest fractional luminosity values(fd>10-4) in the vicinity of the Sun (d<120pc). Eleven out of these 60 systems are new discoveries. Special carewas taken to exclude bogus disks from the sample. We computed thefractional luminosity values using available IRAS, ISO, and Spitzer dataand analyzed the Galactic space velocities of the objects. The resultsrevealed that stars with disks of high fractional luminosity oftenbelong to young stellar kinematic groups, providing an opportunity toobtain improved age estimates for these systems. We found thatpractically all disks with fd>5×10-4 areyounger than 100 Myr. The distribution of the disks in the fractionalluminosity versus age diagram indicates that (1) the number of oldsystems with high fd is lower than was claimed before, (2)there exist many relatively young disks of moderate fractionalluminosity, and (3) comparing the observations with a currenttheoretical model of debris disk evolution, a general good agreementcould be found.
| First Scattered Light Images of Debris Disks around HD 53143 and HD 139664
We present the first scattered light images of debris disks around a Kstar (HD 53143) and an F star (HD 139664) using the coronagraphic modeof the Advanced Camera for Surveys (ACS) on board the Hubble SpaceTelescope (HST). With ages of 0.3-1 Gyr, these are among the oldestoptically detected debris disks. HD 53143, viewed ~45° from edge-on,does not show radial variation in disk structure and has a width >55AU. HD 139664 is seen close to edge-on and has a beltlike morphologywith a dust peak 83 AU from the star and a distinct outer boundary at109 AU. We discuss evidence for significant diversity in the radialarchitecture of debris disks that appears unconnected to stellarspectral type or age. HD 139664 and possibly the solar system belong ina category of narrow belts 20-30 AU wide. HD 53143 represents a class ofwide-disk architecture with a characteristic width >50 AU.
| Dust in the solar system and in extra-solar planetary systems
Among the observed circumstellar dust envelopes a certain population,planetary debris disks, is ascribed to systems with optically thin dustdisks and low gas content. These systems contain planetesimals andpossibly planets and are believed to be systems that are most similar toour solar system in an early evolutionary stage. Planetary debris diskshave been identified in large numbers by a brightness excess in thenear-infrared, mid-infrared and/or submillimetre range of their stellarspectral energy distributions. In some cases, spatially resolvedobservations are possible and reveal complex spatial structures. Actingforces and physical processes are similar to those in the solar systemdust cloud, but the observational approach is obviously quite different:overall spatial distributions for systems of different ages for theplanetary debris disks, as opposed to detailed local information in thecase of the solar system. Comparison with the processes of dustformation and evolution observed in the solar system therefore helpsunderstand the planetary debris disks. In this paper, we review ourpresent knowledge of observations, acting forces, and major physicalinteractions of the dust in the solar system and in similar extra-solarplanetary systems.
| First Optical Images of Circumstellar Dust Surrounding the Debris Disk Candidate HD 32297
Near-infrared imaging with the Hubble Space Telescope recently revealeda circumstellar dust disk around the A star HD 32297. Dust-scatteredlight is detected as far as 400 AU radius, and the linear morphology isconsistent with a disk ~10° away from an edge-on orientation. Herewe present the first optical images that show the dust-scattered lightmorphology from 560 to 1680 AU radius. The position angle of theputative disk midplane diverges by ~31°, and the color of dustscattering is most likely blue. We associate HD 32297 with a wall ofinterstellar gas and the enigmatic region south of the Taurus molecularcloud. We propose that the extreme asymmetries and blue disk colororiginate from a collision with a clump of interstellar material as HD32297 moves southward, and discuss evidence consistent with an age of 30Myr or younger.
| Discovery of a Nearly Edge-on Disk around HD 32297
We report the discovery of a nearly edge-on disk about the A0 star HD32297 seen in light scattered by the disk grains revealed in NICMOSPSF-subtracted coronagraphic images. The disk extends to a distance ofat least 400 AU (3.3") along its major axis with a 1.1 μm fluxdensity of 4.81 +/- 0.57 mJy beyond a radius of 0.3" from thecoronagraphically occulted star. The fraction of 1.1 μm starlightscattered by the disk, 0.0033 +/- 0.0004, is comparable to itsfractional excess emission at 25 + 60 μm of ~0.0027 as measured fromIRAS data. The disk appears to be inclined 10.5d +/- 2.5d from anedge-on viewing geometry, with its major axis oriented 236.5d +/- 1°eastward of north. The disk exhibits unequal brightness on opposingsides and a break in the surface brightness profile along the NE-sidedisk major axis. Such asymmetries might implicate the existence of oneor more (unseen) planetary mass companions.
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