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Coldest Brown Dwarfs Blur Lines between Stars and Planets

Astronomers are constantly on the hunt for ever-colder star-like bodies, and two years ago a new class of objects was discovered by researchers using NASA's WISE space telescope. However, until now no one has known exactly how cool their surfaces really are - some evidence suggested they could be room temperature.
A new study shows that while these brown dwarfs, sometimes called failed stars, are indeed the coldest known free-floating celestial bodies, they are warmer than previously thought with temperatures about 250-350 degrees Fahrenheit.
To reach such low surface temperatures after cooling for billions of years means that these objects can only have about 5 to 20 times the mass of Jupiter. Unlike the Sun, these objects' only source of energy is from their gravitational contraction, which depends directly on their mass.

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Even Brown Dwarfs May Grow Rocky Planets

Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) have for the first time found that the outer region of a dusty disc encircling a brown dwarf contains millimetre-sized solid grains like those found in denser discs around newborn stars. The surprising finding challenges theories of how rocky, Earth-scale planets form, and suggests that rocky planets may be even more common in the Universe than expected.
Rocky planets are thought to form through the random collision and sticking together of what are initially microscopic particles in the disc of material around a star. These tiny grains, known as cosmic dust, are similar to very fine soot or sand. However, in the outer regions around a brown dwarf - a star-like object, but one too small to shine brightly like a star - astronomers expected that grains could not grow because the discs were too sparse, and particles would be moving too fast to stick together after colliding. Also, prevailing theories say that any grains that manage to form should move quickly towards the central brown dwarf, disappearing from the outer parts of the disc where they could be detected. 

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Title: Deep search for companions to probable young brown dwarfs
Authors: G. Chauvin, J. Faherty, A. Boccaletti, K. Cruz, A.-M. Lagrange, B. Zuckerman, M. S. Bessell, J.-L. Beuzit, M. Bonnefoy, C. Dumas, P. Lowrance, D. Mouillet, I. Song

We have obtained high contrast images of four nearby, faint, and very low mass objects 2MASSJ04351455-1414468, SDSSJ044337.61+000205.1, 2MASSJ06085283-2753583 and 2MASSJ06524851-5741376 (here after 2MASS0435-14, SDSS0443+00, 2MASS0608-27 and 2MASS0652-57), identified in the field as probable isolated young brown dwarfs. Our goal was to search for binary companions down to the planetary mass regime. We used the NAOS-CONICA adaptive optics instrument (NACO) and its unique capability to sense the wavefront in the near-infrared to acquire sharp images of the four systems in Ks, with a field of view of 28"*28". Additional J and L' imaging and follow-up observations at a second epoch were obtained for 2MASS0652-57. With a typical contrast DKs= 4.0-7.0 mag, our observations are sensitive down to the planetary mass regime considering a minimum age of 10 to 120 Myr for these systems. No additional point sources are detected in the environment of 2MASS0435-14, SDSS0443+00 and 2MASS0608-27 between 0.1-12" (i.e about 2 to 250 AU at 20 pc). 2MASS0652-57 is resolved as a \sim230 mas binary. Follow-up observations reject a background contaminate, resolve the orbital motion of the pair, and confirm with high confidence that the system is physically bound. The J, Ks and L' photometry suggest a q\sim0.7-0.8 mass ratio binary with a probable semi-major axis of 5-6 AU. Among the four systems, 2MASS0652-57 is probably the less constrained in terms of age determination. Further analysis would be necessary to confirm its youth. It would then be interesting to determine its orbital and physical properties to derive the system's dynamical mass and to test evolutionary model predictions.

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 WISE Finds Few Brown Dwarfs Close to Home

Astronomers are getting to know the neighbours better. Our sun resides within a spiral arm of our Milky Way galaxy about two-thirds of the way out from the center. It lives in a fairly calm, suburb-like area with an average number of stellar residents. Recently, NASA's Wide-field Infrared Survey Explorer, or WISE, has been turning up a new crowd of stars close to home: the coldest of the brown dwarf family of "failed" stars.
Now, just as scientists are "meeting and greeting" the new neighbours, WISE has a surprise in store: there are far fewer brown dwarfs around us than predicted.

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Title: A search for pre-substellar cores and proto-brown dwarf candidates in Taurus: multiwavelength analysis in the B213-L1495 clouds
Authors: Aina Palau, I. de Gregorio-Monsalvo, O. Morata, D. Stamatellos, N. Huélamo, C. Eiroa, A. Bayo, M. Morales-Calderón, H. Bouy, A. Ribas, D. Asmus, D. Barrado

In an attempt to study whether the formation of brown dwarfs (BDs) takes place as a scaled-down version of low-mass stars, we conducted IRAM30m/MAMBO-II observations at 1.2 mm in a sample of 12 proto-BD candidates selected from Spitzer/IRAC data in the B213-L1495 clouds in Taurus. Subsequent observations with the CSO at 350 micron, VLA at 3.6 and 6 cm, and IRAM30m/EMIR in the 12CO(1-0), 13CO(1-0), and N2H+(1-0) transitions were carried out toward the two most promising Spitzer/IRAC source(s), J042118 and J041757. J042118 is associated with a compact (<10 arcsec or <1400 AU) and faint source at 350 micron, while J041757 is associated with a partially resolved (~16 arcsec or ~2000 AU) and stronger source emitting at centimetre wavelengths with a flat spectral index. The corresponding masses of the dust condensations are ~1 and ~5 Jupiter masses for J042118 and J041757, respectively. In addition, about 40 arcsec to the northeast of J041757 we detect a strong and extended submillimetre source, J041757-NE, which is not associated with NIR/FIR emission down to our detection limits, but is clearly detected in 13CO and N2H+ at ~7 km/s, and for which we estimated a total mass of ~75 Jupiter masses, high enough to be gravitationally bound. In summary, our observational strategy has allowed us to find in B213-L1495 two proto-BD candidates and one pre-substellar core candidate, whose properties seem to be consistent with a scaled-down version of low-mass stars.

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 Astronomers Find Possible Secret of the Origin of Brown Dwarfs

The origin of brown dwarfs is one of the great unsolved mysteries facing astrophysicists today. In a new study published in The Astrophysical Journal, Westerns Shantanu Basu and University of Viennas Eduard Vorobyov present a new model of brown dwarf formation that unites the best parts of existing theories and has far-reaching implications for understanding the population of low mass objects in the universe.
Brown dwarfs are astronomical objects that have too little mass to be called stars and too much mass to be called planets. Only a theoretical concept until discovered in the mid-1990s, several hundred brown dwarfs have now been identified through infrared telescopes and surveys.

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Title: The First Hundred Brown Dwarfs Discovered by the Wide-field Infrared Survey Explorer (WISE)
Authors: J. Davy Kirkpatrick, Michael C. Cushing, Christopher R. Gelino, Roger L. Griffith, Michael F. Skrutskie, Kenneth A. Marsh, Edward L. Wright, Amanda K. Mainzer, Peter R. Eisenhardt, Ian S. McLean, Maggie A. Thompson, James M. Bauer, Dominic J. Benford, Carrie R. Bridge, Sean E. Lake, Sara M. Petty, S. Adam Stanford, Chao-Wei Tsai, Vanessa Bailey, Charles A. Beichman, John J. Bochanski, Adam J. Burgasser, Peter L. Capak, Kelle L. Cruz, Philip M. Hinz, Jeyhan S. Kartaltepe, Russell P. Knox, Swarnima Manohar, Daniel Masters, Maria Morales-Calderon, Lisa A. Prato, Timothy J. Rodigas, Mara Salvato, Steven D. Schurr, Nicholas Z. Scoville, Robert A. Simcoe, Karl R. Stapelfeldt, Daniel Stern, Nathan D. Stock, William D. Vacca

We present ground-based spectroscopic verification of six Y dwarfs (see Cushing et al), eighty-nine T dwarfs, eight L dwarfs, and one M dwarf identified by the Wide-field Infrared Survey Explorer (WISE). Eighty of these are cold brown dwarfs with spectral types greater than or equal to T6, six of which have been announced earlier in Mainzer et al and Burgasser et al. We present colour-colour and colour-type diagrams showing the locus of M, L, T, and Y dwarfs in WISE colour space. Near-infrared classifications as late as early Y are presented and objects with peculiar spectra are discussed. After deriving an absolute WISE 4.6 um (W2) magnitude vs. spectral type relation, we estimate spectrophotometric distances to our discoveries. We also use available astrometric measurements to provide preliminary trigonometric parallaxes to four our discoveries, which have types of L9 pec (red), T8, T9, and Y0; all of these lie within 10 pc of the Sun. The Y0 dwarf, WISE 1541-2250, is the closest at 2.8 (+1.3,-0.6) pc; if this 2.8 pc value persists after continued monitoring, WISE 1541-2250 will become the seventh closest stellar system to the Sun. Another ten objects, with types between T6 and >Y0, have spectrophotometric distance estimates also placing them within 10 pc. The closest of these, the T6 dwarf WISE 1506+7027, is believed to fall at a distance of roughly 4.9 pc. WISE multi-epoch positions supplemented with positional info primarily from Spitzer/IRAC allow us to calculate proper motions and tangential velocities for roughly one half of the new discoveries. This work represents the first step by WISE to complete a full-sky, volume-limited census of late-T and Y dwarfs. Using early results from this census, we present preliminary, lower limits to the space density of these objects and discuss constraints on both the functional form of the mass function and the low-mass limit of star formation.

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Title: A possible dividing line between massive planets and brown-dwarf companions
Authors: Johannes Sahlmann, Damien Segransan, Didier Queloz, Stephane Udry

Brown dwarfs are intermediate objects between planets and stars. The lower end of the brown-dwarf mass range overlaps with the one of massive planets and therefore the distinction between planets and brown-dwarf companions may require to trace the individual formation process. We present results on new potential brown-dwarf companions of Sun-like stars, which were discovered using CORALIE radial-velocity measurements. By combining the spectroscopic orbits and Hipparcos astrometric measurements, we have determined the orbit inclinations and therefore the companion masses for many of these systems. This has revealed a mass range between 25 and 45 Jupiter masses almost void of objects, suggesting a possible dividing line between massive planets and sub-stellar companions.

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Title: A Spitzer Search For Planetary-Mass Brown Dwarfs With Circumstellar Disks: Candidate Selection
Authors: Paul M. Harvey, Daniel T. Jaffe, Katelyn Allers, Michael Liu

We report on initial results from a Spitzer program to search for very low-mass brown dwarfs in Ophiuchus. This program is an extension of an earlier study by Allers et al. which had resulted in an extraordinary success rate, 18 confirmed out of 19 candidates. Their program combined near-infrared and Spitzer photometry to identify objects with very cool photospheres together with circumstellar disk emission to indicate youth. Our new program has obtained deep IRAC photometry of a 0.5 deg2 field that was part of the original Allers et al. study. We report 18 new candidates whose luminosities extend down to 10-4 L\cdot which suggests masses down to ~ 2 MJ if confirmed. We describe our selection techniques, likely contamination issues, and follow-on photometry and spectroscopy that are in progress.

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The Coolest Stars Come Out of the Dark

Astronomers have uncovered what appear to be 14 of the coldest stars known in our universe. These failed stars, called brown dwarfs, are so cold and faint that they'd be impossible to see with current visible-light telescopes. Spitzer's infrared vision was able to pick out their feeble glow, much as a firefighter uses infrared goggles to find hot spots buried underneath a dark forest floor.
The brown dwarfs join only a handful of similar objects previously discovered. The new objects are between the temperatures of about 450 Kelvin to 600 Kelvin (350 to 620 degrees Fahrenheit). As far as stars go, this is bitter cold -- as cold, in some cases, as planets around other stars.
These cool orbs have remained elusive for years, but will soon start coming out of the dark in droves. NASA's Wide-field Infrared Survey Explorer (WISE) mission, which is up scanning the entire sky now in infrared wavelengths, is expected to find hundreds of objects of a similarly chilly disposition, if not even colder. WISE is searching a volume of space 40 times larger than that sampled in the recent Spitzer study, which concentrated on a region in the constellation Bootes. The Spitzer mission is designed to look at targeted patches of sky in detail, while WISE is combing the whole sky.

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