* Astronomy

Blobrana · L

HD 181327

Blobrana · L

Title: An icy Kuiper-Belt around the young solar-type star HD 181327
Authors: J. Lebreton, J.-C. Augereau, W.-F. Thi, A. Roberge, J. Donaldson, G. Schneider, S. T. Maddison, F. Ménard, P. Riviere-Marichalar, G. S. Mathews, I. Kamp, C. Pinte, W. R. F. Dent, D. Barrado, G. Duchêne, J.-F. Gonzalez, C. A. Grady, G. Meeus, E. Pantin, J. P. Williams, P. Woitke

HD 181327 is a young F5/F6V star belonging to the Beta Pictoris moving group (12 Myr). It harbours an optically thin belt of circumstellar material at 90 AU. We aim to study the dust properties in the belt in details, and to constrain the gas-to-dust ratio. We obtained far-IR observations with the Herschel/PACS instrument, and 3.2 mm observations with the ATCA array. The geometry of the belt is constrained with newly reduced HST/NICMOS images that break the degeneracy between the disk geometry and the dust properties. We use the radiative transfer code GRaTer to compute a large grid of models, and we identify the grain models that best reproduce the Spectral Energy Distribution through a Bayesian analysis. We attempt to detect the [OI] and [CII] lines with PACS spectroscopy, providing observables to our photochemical code ProDiMo. The HST observations confirm that the dust is confined in a narrow belt. The continuum is detected in the far-IR with PACS and the disk is resolved with both PACS and ATCA. A medium integration of the gas spectral lines only provides upper limits on the line fluxes. We show that the HD 181327 dust disk consists of micron-sized grains of porous amorphous silicates and carbonaceous material surrounded by an important layer of ice, for a total dust mass of 0.05 Solar masses plus (up to 1 mm). We discuss evidences that the grains are fluffy aggregates. The upper limits on the gas lines do not provide unambiguous constraints: only if the PAH abundance is high, the gas mass must be lower than 17 Solar masses plus. Despite the weak constraints on the gas disk, the age of HD 181327 and the properties of the dust disk suggest that it has passed the stage of gaseous planets formation. The dust reveals a population of icy planetesimals, similar to the primitive Kuiper Belt, that may be a source for the future delivery of water and volatiles onto forming terrestrial planets.

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Blobrana · L

RE: HD181327

Blobrana · L

Astronomers have spotted a disk of dust and ice ringing a young Sun-like star 165 light years away. The icy signature of the disk and the collisions between bodies inferred to be taking place there suggest it is similar to the Sun's Kuiper belt, a disk of small icy bodies that extends beyond Neptune.

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Blobrana · L

Title: A Possible Icy Kuiper Belt around HD 181327
Authors: Christine H. Chen, Michael P. Fitzgerald, Paul S. Smith

We have obtained a Gemini South T-ReCS Qa-band (18.3 micron) image and a Spitzer MIPS SED-mode observation of HD181327, an F5/F6V member of the ~12 Myr old beta Pictoris moving group. We resolve the disk in thermal-emission for the first time and find that the northern arm of the disk is 1.4x brighter than the southern arm. In addition, we detect a broad peak in the combined Spitzer IRS and MIPS spectra at 60 - 75 micron that may be produced by emission from crystalline water ice. We model the IRS and MIPS data using a size distribution of amorphous olivine and water ice grains (dn/da proportional to a^{-2.25} with a_{min} consistent with the minimum blow out size and a_{max} = 20 micron) located at a distance of 86.3 AU from the central star, as observed in previously published scattered-light images. Since the photo-desorption lifetime for the icy particles is ~1400 yr, significantly less than the estimated ~12 Myr age of the system, we hypothesise that we have detected debris that may be steadily replenished by collisions among icy Kuiper belt object-like parent bodies in a newly forming planetary system.

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Blobrana · L

Title: Discovery of an 86 AU Radius Debris Ring Around HD 181327
Authors: Glenn Schneider, Murray D. Silverstone, Dean C. Hines, Jean-Charles Augereau, Christophe Pinte, Francois Menard, John Krist, Mark Clampin, Carol Grady, David Golimowski, David Ardila, Thomas Henning, Sebastian Wolf, Jens Rodmann

HST/NICMOS PSF-subtracted coronagraphic observations of HD 181327 have revealed the presence of a ring-like disk of circumstellar debris seen in 1.1 micron light scattered by the disk grains, surrounded by a diffuse outer region of lower surface brightness. The annular disk appears to be inclined by 31.7 ±1.6 deg from face on with the disk major axis PA at 107 ±2 deg . The total 1.1 micron flux density of the light scattered by the disk (at 1.2" < r < 5.0") of 9.6 mJy ±0.8 mJy is 0.17% ±0.015% of the starlight. Seventy percent of the light from the scattering grains appears to be confined in a 36 AU wide annulus centred on the peak of the radial surface brightness (SB) profile 86.3 ±3.9 AU from the star, well beyond the characteristic radius of thermal emission estimated from IRAS and Spitzer flux densities assuming blackbody grains (~ 22 AU). The light scattered by the ring appears bilaterally symmetric, exhibits directionally preferential scattering well represented by a Henyey-Greenstein scattering phase function with g = 0.30 ±0.03, and has an azimuthally medianed SB at the 86.3 AU radius of peak SB of 1.00 ±0.07 mJy arcsec^-2. No photocentric offset is seen in the ring relative to the position of the central star. A low surface brightness diffuse halo is seen in the NICMOS image to a distance of ~ 4" Deeper 0.6 micron HST/ACS PSF-subtracted coronagraphic observations reveal a faint outer nebulosity, asymmetrically brighter to the North of the star. We discuss models of the disk and properties of its grains, from which we infer a maximum vertical scale height of 4 - 8 AU at the 87.6 AU radius of maximum surface density, and a total maximum dust mass of collisionally replenished grains with minimum grain sizes of ~ 1 micron of ~ 4 M(moon).

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