* Astronomy

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AU Microscopii Debris Disk

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RE: AU Microscopii

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Title: A Resolved Millimetre Emission Belt in the AU Mic Debris Disk
Authors: David J. Wilner, Sean M. Andrews, Meredith A. MacGregor, A. Meredith Hughes

We present imaging observations at 1.3 millimetres of the debris disk surrounding the nearby M-type flare star AU Mic with beam size 3 arcsec (30 AU) from the Submillimeter Array. These data reveal a belt of thermal dust emission surrounding the star with the same edge-on geometry as the more extended scattered light disk detected at optical wavelengths. Simple modelling indicates a central radius of ~35 AU for the emission belt. This location is consistent with the reservoir of planetesimals previously invoked to explain the shape of the scattered light surface brightness profile through size-dependent dust dynamics. The identification of this belt further strengthens the kinship between the debris disks around AU Mic and its more massive sister star beta Pic, members of the same ~10 Myr-old moving group.

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Title: X-raying the AU Microscopii debris disk
Authors: P. C. Schneider, J. H. M. M. Schmitt

AU Mic is a young, nearby X-ray active M-dwarf with an edge-on debris disk. Debris disk are the successors of the gaseous disks usually surrounding pre-main sequence stars which form after the first few Myrs of their host stars' lifetime, when - presumably - also the planet formation takes place. Since X-ray transmission spectroscopy is sensitive to the chemical composition of the absorber, features in the stellar spectrum of AU Mic caused by its debris disk can in principle be detected. The upper limits we derive from our high resolution Chandra LETGS X-ray spectroscopy are on the same order as those from UV absorption measurements, consistent with the idea that AU Mic's debris disk possesses an inner hole with only a very low density of sub-micron sized grains or gas.

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Title: A Low-Mass H2 Component to the AU Microscopii Circumstellar Disk
Authors: Kevin France (1), Aki Roberge (2), Roxana E. Lupu (3), Seth Redfield (4), Paul D. Feldman (3) (1-CITA/U Toronto, 2-NASA GSFC, 3-JHU, 4-U Texas-Austin)

We present a determination of the molecular gas mass in the AU Microscopii circumstellar disk. Direct detection of a gas component to the AU Mic disk has proven elusive, with upper limits derived from ultraviolet absorption line and submillimeter CO emission studies. Fluorescent emission lines of H2, pumped by the OVI 1032 resonance line through the C-X (1 -- 1) Q(3) 1031.87 \AA\ transition, are detected by the Far Ultraviolet Spectroscopic Explorer. These lines are used to derive the H2 column density associated with the AU Mic system. The derived column density is in the range N(H2) = 1.9 x 10^{17} - 2.8 x 10^{15} cm^{-2}, roughly two orders of magnitude lower than the upper limit inferred from absorption line studies. This range of column densities reflects the range of H2 excitation temperature consistent with the observations, T(H2) = 800 -- 2000 K, derived from the presence of emission lines excited by OVI in the absence of those excited by LyA. Within the observational uncertainties, the data are consistent with the H2 gas residing in the disk. The inferred N(H2) range corresponds to H2-to-dust ratios of < 1/30:1 and a total M(H2) = 4.0 x 10^{-4} - 5.8 x 10^{-6} Earth masses. We use these results to predict the intensity of the associated rovibrational emission lines of H2 at infrared wavelengths covered by ground-based instruments, HST-NICMOS, and the Spitzer-IRS.

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A flurry of lint-like particles discovered swirling around a small, distant star could help explain how miniscule interstellar dust grains clump together to form planets, astronomers say.

"We have seen many seeds of planets and we have seen many planets, but how they go from one to the other is a mystery. These observations help us to fill in that gap" said study team member James Graham of the University of California, Berkeley.

The newfound fluffy particles are about ten times larger than interstellar dust grains and about as porous as newly fallen snow, which is composed of about 97% air and only 3% ice.

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New observations from NASA's Hubble Space Telescope have begun to fill gaps in the early stages of planet birth.
Hubble observed a "blizzard" of particles in a disk around a young star revealing the process by which planets grow from tiny dust grains. The particles are as fluffy as snowflakes and are roughly ten times larger than typical interstellar dust grains. They were detected in a disk encircling the 12-million-year-old star AU Microscopii. The star is 32 light-years away in the southern constellation of Microscopium, the Microscope.
The particles' fluffiness suggests that they were shed by much larger, but unseen snowball-sized objects that had gently collided with each other. These unseen objects are believed to reside in a region dubbed the "birth ring," first hypothesized in 2005 by Berkeley astronomers Linda Strubbe and Eugene Chiang. The ring is between 3.7 billion and 4.6 billion miles from the star. As the larger objects bump into each other, they release fluffy particles that are propelled outward by the intense pressure from starlight.

Credit: NASA, ESA, J. R. Graham and P. Kalas (University of California, Berkeley), and B. Matthews (Hertzberg Institute of Astrophysics)

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Hubble captured this striking image of AU Microscopii.

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AU Microscopii is 33 light-years away in the constellation Microscopium
Position (J2000): R.A. 20h 45m 09s.53 Dec. -31° 20' 27".2

HST/ACS Coronagraphic Imaging of the AU Microscopium Debris Disk,
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Discovery of a Large Dust Disk Around the Nearby Star AU Microscopium.
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Title: The Signature of Primordial Grain Growth in the Polarised Light of the AU Mic Debris Disk
Authors: James R. Graham, Paul G. Kalas (Berkeley), Brenda C. Matthews (HIA)

We have used the Hubble Space Telescope/ACS coronagraph to make polarization maps of the AU Mic debris disk. The fractional linear polarization rises monotonically from about 0.05 to 0.4 between 20 and 80 AU. The polarization is perpendicular to the disk, indicating that the scattered light originates from micron sized grains in an optically thin disk. Disk models, which simultaneously fit the surface brightness and polarisation, show that the inner disk (< 40-50 AU) is depleted of micron-sized dust by a factor of more than 300, which means that the disk is collision dominated. The grains have high maximum linear polarization and strong forward scattering. Spherical grains composed of conventional materials cannot reproduce these optical properties. A Mie/Maxwell-Garnett analysis implicates highly porous (91-94%) particles. In the inner Solar System, porous particles form in cometary dust, where the sublimation of ices leaves a "bird's nest" of refractory organic and silicate material. In AU Mic, the grain porosity may be primordial, because the dust "birth ring" lies beyond the ice sublimation point. The observed porosities span the range of values implied by laboratory studies of particle coagulation by ballistic cluster-cluster aggregation. To avoid compactification, the upper size limit for the parent bodies is in the decimeter range, in agreement with theoretical predictions based on collisional lifetime arguments. Consequently, AU Mic may exhibit the signature of the primordial agglomeration process whereby interstellar grains first assembled to form macroscopic objects.

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