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TOPIC: WD 0806-661 B


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Title: Ultraviolet Spectroscopy of Asteroid (4) Vesta
Authors: Jian-Yang Li, Dennis Bodewits, Lori M. Feaga, Wayne Landsman, Michael F. A'Hearn, Max J. Mutchler, Christopher T. Russell, Lucy A. McFadden, Carol A. Raymond

We report a comprehensive review of the UV-visible spectrum and rotational lightcurve of Vesta combining new observations by Hubble Space Telescope and Swift Gamma-ray Burst Observatory with archival International Ultraviolet Explorer observations. The geometric albedos of Vesta from 220 nm to 953 nm are derived by carefully comparing these observations from various instruments at different times and observing geometries. Vesta has a rotationally averaged geometric albedo of 0.09 at 250 nm, 0.14 at 300 nm, 0.26 at 373 nm, 0.38 at 673 nm, and 0.30 at 950 nm. The linear spectral slope as measured between 240 and 320 nm in the ultraviolet displays a sharp minimum near a sub-Earth longitude of 20, and maximum in the eastern hemisphere. This is consistent with the longitudinal distribution of the spectral slope in the visible wavelength. The photometric uncertainty in the ultraviolet is ~20%, and in the visible wavelengths it is better than 10%. The amplitude of Vesta's rotational lightcurves is ~10% throughout the range of wavelengths we observed, but is smaller at 950 nm (~6%) near the 1-\mum band center. Contrary to earlier reports, we found no evidence for any difference between the phasing of the ultraviolet and visible/near-infrared lightcurves with respect to sub-Earth longitude. Vesta's average spectrum between 220 and 950 nm can well be described by measured reflectance spectra of fine particle howardite-like materials of basaltic achondrite meteorites. Combining this with the in-phase behaviour of the ultraviolet, visible, and near-infrared lightcurves, and the spectral slopes with respect to the rotational phase, we conclude that there is no global ultraviolet/visible reversal on Vesta. Consequently, this implies a lack of global space weathering on Vesta, as previously inferred from visible-near-infrared data.

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Title: Confirmation of One of the Coldest Known Brown Dwarfs
Authors: K. L. Luhman, A. J. Burgasser, I. Labbe, D. Saumon, M. S. Marley, J. J. Bochanski, A. J. Monson, S. E. Persson

Using two epochs of 4.5um images from the Infrared Array Camera (IRAC) on board the Spitzer Space Telescope, we recently identified a common proper motion companion to the white dwarf WD 0806-661 that is a candidate for the coldest known brown dwarf. To verify its cool nature, we have obtained images of this object at 3.6um with IRAC, at J with HAWK-I on the Very Large Telescope, and in a filter covering the red half of J with FourStar on Magellan. WD 0806-661 B is detected by IRAC but not HAWK-I or FourStar. From these data we measure colours of [3.6]-[4.5]=2.770.16 and J-[4.5]>7.0 (SNR<3). Based on these colours and its absolute magnitudes, WD 0806-661 B is the coldest companion directly imaged outside of the solar system and is a contender for the coldest known brown dwarf with the Y dwarf WISEP J1828+2650. It is unclear which of these two objects is colder given the available data. A comparison of its absolute magnitude at 4.5um to the predictions of theoretical spectra and evolutionary models suggests that WD 0806-661 B has T=300-345 K.

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Spitzer Snaps a Picture of the Coolest of Companions

NASA's Spitzer Space Telescope has captured a picture of a nearby star and its orbiting companion -- whose temperature is like a hot summer day in Arizona.

"We have discovered a new record-holder for the coldest companion imaged outside of the solar system, which is nearly as cold as Earth" - Kevin Luhman, an astronomer at The Pennsylvania State University, University Park, and lead author of a pair of papers on the findings in The Astrophysical Journal.

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Record-Breaking Photo Reveals a Planet-sized Object as Cool as the Earth

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The photo of a nearby star and its orbiting companion -- whose temperature is like a hot summer day in Arizona -- will be presented by Penn State Associate Professor of Astronomy and Astrophysics Kevin Luhman during the Signposts of Planets conference at NASA's Goddard Space Flight Centre on 20 October 2011. A paper describing the discovery will be published in the Astrophysical Journal.
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WD 0806-661
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Spitzer Finds the Coldest Companion to a Star Ever Seen

Although we all want to think that the people we keep company with are "cool," when it comes to the cosmos, no one has a cooler companion than a star known as WD 0806-661. An object spotted around this star by NASA's Spitzer Space Telescope stands as the lowest-temperature companion ever directly seen beyond the solar system.
Astronomers suspect that the object, which might be as tepid as a mild summer day, is a "failed star," or brown dwarf. The cosmic body, called WD 0806-661 B rivals other recently detected free-floating brown dwarfs as the coolest extra-solar objects known.

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Newly found brown dwarf is ultra-cool

We think of stars, even the failed ones called brown dwarfs, as being lethally hot. That picture has been confounded by a newly discovered brown dwarf - at room temperature.
Like fully fledged stars, brown dwarfs form from collapsing gas clouds, but they are not massive enough to sustain nuclear reactions. Instead, they briefly shine red from the heat of formation, then fade. Still, the coolest known brown dwarfs are all hot enough to roast any spacefarers who venture too close.
Now Kevin Luhman of Pennsylvania State University in University Park and colleagues have used NASA's infrared Spitzer Space Telescope to detect the glow of what appears to be a brown dwarf at just 30 C

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Title: Discovery of a Candidate for the Coolest Known Brown Dwarf
Authors: K. L. Luhman, A. J. Burgasser, J. J. Bochanski

We have used multi-epoch images from the Infrared Array Camera on board the Spitzer Space Telescope to search for substellar companions to stars in the solar neighbourhood based on common proper motions. Through this work, we have discovered a faint companion to the white dwarf WD 0806-661. The comoving source has a projected separation of 130", corresponding to 2500 AU at the distance of the primary (19.2 pc). If it is physically associated, then its absolute magnitude at 4.5um is ~1 mag fainter than the faintest known T dwarfs, making it a strong candidate for the coolest known brown dwarf. The combination of M_4.5 and the age of the primary (1.5 Gyr) implies an effective temperature of ~300 K and a mass of ~7 M_Jup according to theoretical evolutionary models. The white dwarf's progenitor likely had a mass of ~2 M_sun, and thus could have been born with a circumstellar disk that was sufficiently massive to produce a companion with this mass. Therefore, the companion could be either a brown dwarf that formed like a binary star or a giant planet that was born within a disk and has been dynamically scattered to a larger orbit.

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