* Astronomy

Title: An insight in the surroundings of HR4796
Authors: A.-M. Lagrange, J. Milli, A. Boccaletti, S. Lacour, P. Thebault, G. Chauvin, D. Mouillet, J.C. Augereau, M. Bonnefoy, D. Ehrenreich, Q. Kral

HR4796 is a young, early A-type star harbouring a well structured debris disk, shaped as a ring with sharp inner edges. It forms with the M-type star HR4796B a binary system, with a proj. sep. ~560 AU. Our aim is to explore the surroundings of HR4796A and B, both in terms of extended or point-like structures. Adaptive optics images at L'-band were obtained with NaCo in Angular Differential Mode and with Sparse Aperture Masking (SAM). We analyse the data as well as the artefacts that can be produced by ADI reduction on an extended structure with a shape similar to that of HR4796A dust ring. We determine constraints on the presence of companions using SAM and ADI on HR4796A, and ADI on HR4796B. We also performed dynamical simulations of a disk of planetesimals and dust produced by collisions, perturbed by a planet located close to the disk outer edge. The disk ring around HR4796A is well resolved. We highlight the potential effects of ADI reduction of the observed disk shape and surface brightness distribution, and side-to-side asymmetries. No planet is detected around the star, with masses as low as 3.5 Jupiter masses at 0.5" (58 AU) and less than 3 Jupiter masses in the 0.8-1" range along the semi-major axis. We exclude massive brown dwarfs at separations as close as 60 mas (4.5 AU) from the star thanks to SAM data. The detection limits obtained allow us to exclude a possible close companion to HR4796A as the origin of the offset of the ring center with respect to the star; they also allow to put interesting constraints on the (mass, separation) of any planet possibly responsible for the inner disk steep edge. Using detailed dynamical simulations, we show that a giant planet orbiting outside the ring could sharpen the disk outer edge and reproduce the STIS images published by Schneider et al. (2009).

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Subaru's Sharp Eye Confirms Signs of Unseen Planets in the Dust Ring of HR 4796 A

The SEEDS (Strategic Exploration of Exoplanets and Disks with Subaru Telescope/HiCIAO) project, a five-year international collaboration launched in 2009 and led by Motohide Tamura of NAOJ (National Astronomical Observatory of Japan) has yielded another impressive image that contributes to our understanding of the link between disks and planet formation. Researchers used Subaru's planet-finder camera, HiCIAO (High Contrast Instrument for the Subaru Next Generation Adaptive Optics), to take a crisp high-contrast image of the dust ring around HR 4796 A, a young (8-10 million years old) nearby star, only 240 light years away from Earth. The ring consists of dust grains in a wide orbit, roughly twice the size of Pluto's orbit, around the central star.
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Herschel Finds Oceans of Water in Disk of Nearby Star

Using data from the Herschel Space Observatory, astronomers have detected for the first time cold water vapour enveloping a dusty disk around a young star. The findings suggest that this disk, which is poised to develop into a solar system, contains great quantities of water, suggesting that water-covered planets like Earth may be common in the universe. Herschel is a European Space Agency mission with important NASA contributions.
Scientists previously found warm water vapour in planet-forming disks close to a central star. Evidence for vast quantities of water extending out into the cooler, far reaches of disks where comets take shape had not been seen until now. The more water available in disks for icy comets to form, the greater the chances that large amounts eventually will reach new planets through impacts.
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Title: Emission Lines from the Gas Disk around TW Hydra and the Origin of the Inner Hole
Authors: Uma Gorti, David Hollenbach, Joan Najita, Ilaria Pascucci

We compare line emission calculated from theoretical disk models with optical to sub-millimeter wavelength observational data of the gas disk surrounding TW Hya and infer the spatial distribution of mass in the gas disk. The model disk that best matches observations has a gas mass ranging from ~10^{-4}-10^{-5}\ms\ for 0.06 AU <r<3.5AU and ~ 0.06\ms\ for  3.5 AU <r<200AU. We find that the inner dust hole (r<3.5AU) in the disk must be depleted of gas by ~ 1-2 orders of magnitude compared to the extrapolated surface density distribution of the outer disk. Grain growth alone is therefore not a viable explanation for the dust hole. CO vibrational emission arises within r~ 0.5AU from thermal excitation of gas. [OI] 6300\AA\ and 5577\AA\ forbidden lines and OH mid-infrared emission are mainly due to prompt emission following UV photodissociation of OH and water at r\lesssim0.1AU and at r~ 4AU. [NeII] emission is consistent with an origin in X-ray heated neutral gas at r\lesssim 10AU, and may not require the presence of a significant EUV (h\nu>13.6eV) flux from TW Hya. H_2 pure rotational line emission comes primarily from r~ 1-30AU. [OI]63 \mu m, HCO^+ and CO pure rotational lines all arise from the outer disk at r~30-120AU. We discuss planet formation and photoevaporation as causes for the decrease in surface density of gas and dust inside 4 AU. If a planet is present, our results suggest a planet mass ~ 4-7M_J situated at ~ 3AU. Using our photoevaporation models and the best surface density profile match to observations, we estimate a current photoevaporative mass loss rate of  4 x 10^{-9}\ms\ yr^{-1} and a remaining disk lifetime of ~ 5 million years.

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