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TOPIC: Brown Dwarfs


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Posts: 131433
Date:
GQ Lupi b
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Title: GQ Lup B Visible & Near-Infrared Photometric Analysis
Authors: C. Marois, B. Macintosh, T. Barman

Researchers have re-analysed archival HST and Subaru data of the recently discovered planetary mass companion (PMC) GQ Lup B. With these they have produce the first R and I band photometry of the companion and fit a radius and effective temperature using detailed model atmospheres. The researchers find an effective temperature of 2335 100K, and a radius of 0.38 0.05 Rsol and luminosity of log(L/Lsol) = -2.42 0.07 (at 140pc). Since they fit wavelengths that span most of the emitted radiation from GQ Lup this luminosity estimate is robust, with uncertainty dominated by the distance uncertainty (+/- 50 pc). The radius obtained for 140pc (0.38Rsol) is significantly larger than the one originally derived and larger than model predictions. The mass of the object is much more model-dependent than the radiative properties, but for the GAIA dusty models they find a mass between 9-20 MJup, in the range of the brown dwarf and PMC deuterium burning boundary. Assuming a distance of 140pc, observations fit to 1sigma the Baraffe evolution model for a ~15 MJup brown dwarf. Additionally, the F606W photometric band is significantly overluminous compared to model predictions and other brown dwarfs. Such overluminosity could be explained by a bright Halpha emission from chromospheric activity, interaction with another undetected companion, or accretion. Assuming that GQ Lup B has a bright Halpha emission line, its Halpha emission strength is 10^(-1.71 0.10) Lbol, significantly larger than field late-type dwarfs. GQ Lup B might be strongly accreting and still be in its formation phase.

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Posts: 131433
Date:
HD 3651B
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Title: The Spectrum of HD 3651B: An Extrasolar Nemesis?
Authors: Adam J. Burgasser (MIT)

I present detailed analysis of the near-infrared spectrum of HD 3651B, a faint, co-moving wide companion to the nearby planet-hosting star HD 3651. The presence of strong H2O and CH4 absorption bands confirm this source as a late T-type brown dwarf with spectral type T8. Application of the technique of Burgasser, Burrows & Kirkpatrick yields Teff = 840 80 K, log(g) = 4.9 0.2, M = 30 10 MJup and an age in the range 0.7-3.4 Gyr, making HD 3651B a slightly warmer analogy to the field T8 2MASS 0415-0935. The derived age for this companion is somewhat better constrained than estimates for its primary, which ranges from ~2 Gyr to >12 Gyr. As a widely orbiting massive object to a known planetary system that could potentially harbour terrestrial planets in its habitable zone, HD 3651B may play the role of Nemesis in this system.

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Posts: 131433
Date:
T Dwarfs
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Using infrared photographs obtained with NASA's Spitzer Space Telescope, astronomers have discovered two very cold brown dwarfs orbiting the stars HD 3651 (left) and HN Peg (right).

brown1
Expand (1.07mb, 2258 x 1128)
Credit NASA

These brown dwarfs have masses of only 20 and 50 times the mass of Jupiter and have orbits that are more than 10 times larger than Pluto's orbit. HD 3651 and HN Peg are in the Sun's neighbourhood of the Galaxy, with distances of only 36 and 60 light years from the Sun.

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Posts: 131433
Date:
RE: Brown Dwarfs
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Title: Discovery of Two T Dwarf Companions with the Spitzer Space Telescope
Authors: K. L. Luhman (Penn State), B. M. Patten (CfA), M. Marengo (CfA), M. T. Schuster (CfA), J. L. Hora (CfA), R. G. Ellis (CfA), J. R. Stauffer (SSC), S. M. Sonnett (CfA), E. Winston (CfA), R. A. Gutermuth (CfA), S. T. Megeath (Toledo), D. E. Backman (SOFIA), T. J. Henry (George State), M. W. Werner (JPL), G. G. Fazio (CfA)

We report the discovery of T dwarf companions to the nearby stars HN Peg (G0V, 18.4 pc, ~0.3 Gyr) and HD 3651 (K0V, 11.1 pc, ~7 Gyr). During an ongoing survey of 5'x5' fields surrounding stars in the solar neighbourhood with IRAC aboard the Spitzer Space Telescope, we identified these companions as candidate T dwarfs based on their mid-IR colors. Using near-IR spectra obtained with SpeX at the NASA IRTF, we confirm the presence of methane absorption that characterizes T dwarfs and measure spectral types of T2.5 0.5 and T7.5 0.5 for HN Peg B and HD 3651 B, respectively. By comparing our Spitzer data to images from 2MASS obtained several years earlier, we find that the proper motions of HN Peg B and HD 3651 B are consistent with those of the primaries, confirming their companionship. HN Peg B and HD 3651 B have angular separations of 43.2" and 42.9" from their primaries, which correspond to projected physical separations of 795 and 476 AU, respectively. A comparison of their luminosities to the values predicted by theoretical evolutionary models implies masses of 0.0210.009 and 0.0510.014 Msun for HN Peg B and HD 3651 B. In addition, the models imply an effective temperature for HN Peg B that is significantly lower than the values derived for other T dwarfs at similar spectral types, which is the same behaviour reported by Metchev & Hillenbrand for the young late-L dwarf HD 203030 B. Thus, the temperature of the L/T transition appears to depend on surface gravity. Meanwhile, HD 3651 B is the first substellar companion directly imaged around a star that is known to harbour a close-in planet from RV surveys. The discovery of this companion supports the notion that the high eccentricities of close-in planets like the one near HD 3651 may be the result of perturbations by low-mass companions at wide separations.

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Posts: 131433
Date:
IBIS J013656.57+093347.3
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Title: Discovery of the brightest T dwarf in the northern hemisphere
Authors: Etienne Artigau (1 and 2), Rene Doyon (1), David Lafreniere (1), Daniel Nadeau (1), Jasmin Robert (1), Loic Albert (3) ((1) Universite de Montreal, (2) Gemini Observatory Southern Operations Center, (3) Canada-France-Hawaii Telescope Corporation)

We report the discovery of a bright (H=12.77) brown dwarf designated IBIS J013656.57+093347.3. The discovery was made as part of the Infrared Brown dwarf Identification Survey (IBIS), which uses proper motion and near-infrared/optical photometry to identify brown dwarf candidates. A low resolution (R~40) spectrum of this brown dwarf covering the 0.88-2.35 microns wavelength interval is presented. Analysis of the spectrum indicates a spectral type of T2.5 0.5. A photometric distance of 6.4 0.3 pc is estimated assuming it is a single object. Current observations rule out a binary of mass ratio ~1 and separation >5 AU. IBIS0136 is the brightest T dwarf in the northern hemisphere and is surpassed only by Eps Indi Bab over the whole sky. It is thus an excellent candidate for detailed studies and should become a benchmark object for the early-T spectral class.

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Posts: 131433
Date:
9 New Companions
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Title: Discovery of 9 New Companions to Nearby Young M Stars with the Altair AO System
Authors: Sebastian Daemgen, Nick Siegler, I. Neill Reid, Laird M. Close

We present results of a high-resolution, near-infrared survey of 41 nearby, young (<~300 Myr) M0-M5.0 dwarfs using the Altair natural guide star adaptive optics system at the Gemini North telescope. Twelve of the objects appear to be binaries, 7 of which are reported here for the first time. One triple system was discovered. Statistical properties are studied and compared with earlier (F to K) and later (>= M6 very low-mass, VLM) populations. We find that the separation distribution of the binaries in this sample peaks at 13+14-9 AU, which is consistent with previous measurements of early-M binaries. Hence, early-M binaries seem to occur in--on average--tighter systems than G binaries. At the same time they are significantly wider than field VLM binary stars. The distribution of mass ratios q of primary and secondary stars was found to show an intermediate distribution between the strongly q-->1 peaked distribution of field VLM systems and the almost flat distribution of earlier-type stars. Consequently, we show evidence for relatively young, early-M binaries representing a transition between the well known earlier star distributions and the recently examined field VLM population characteristics. Despite the fact that this survey was dedicated to the search for faint brown dwarf and planetary mass companions, all planetary mass candidates were background objects. We exclude the existence of physical companions with masses greater than 10 Jupiter masses (M_Jup) at separations of >~40 AU and masses greater than 24 M_Jup for separations >~10 AU around 37 of the 41 observed objects.

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Posts: 131433
Date:
VLM binary
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Title: New Masses and Ages for the Planetary Mass Binary Candidate Ophiuchus #11 (2MASS J16222521-2405139) and the Discovery of Another Very Wide, Low-Mass, Binary in Ophiuchus (2MASS J16233609-2402209)
Authors: Laird M. Close, Ben Zuckerman, Inseok Song, Travis Barman, Christian Marois, Emily L. Rice, Nick Siegler, Bruce Macintosh, Eric E. Becklin, Randy Campbell, James E. Lyke, Al Conrad, David Le Mignant

We imaged five objects considered by Allers et al. (2006) to be of low-mass and associated with the young star-forming clouds in Ophiuchus. We resolved their #11 (2MASS J16222521-2405139) and #16 (2MASS J16233609-2402209) into binaries. The #16 binary is an unusually wide (212 AU), very low-mass (VLM), binary composed of a ~100 Mjup primary (16A) and a ~73 MJup (16B) secondary. The #11 object is even more unusual. The binary nature of 11 was independently discovered by Jayawardhana & Ivanov (2006b) who call it Oph 162225-240515. The Oph #11 projected separation, 243 AU, is the largest known for a VLM binary and an order of magnitude larger than that of older field VLM binaries. We have obtained the first spatially resolved near-infrared (J & K) spectra and Mid-IR photometry. We estimate for 11A and 11B gravities (log(g)>3.75), ages (5 2 Myr), luminosities (log(L)=-2.77 0.10 and -2.96 0.10), and temperatures (Teff=2375 175 and 2175 175 K). We find self-consistent DUSTY evolutionary model (Chabrier et al. 2000) masses of 17.5 2.5 MJup and 15.5 2.5 MJup, respectively. Our masses are higher than the previously reported 13-15 MJup and 7-8 MJup masses for 11A and 11B (Jayawardhana & Ivanov 2006b). Hence, we find the system is unlikely a ''planetary mass binary'', but it is the second lowest mass binary known. Moreover, due to its large separation, Oph 11 appears to have a binding energy lower than any other known binary. The formation (and survival) of such a weakly bound system with Vesc~0.5 km/s may pose a serious challenge to brown dwarf/star formation theories. Oph #11 and Oph #16 belong to a new population of wide (>200 AU), young, roughly equal mass, VLM stellar and brown dwarf binaries that may become unbound over time.

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Posts: 131433
Date:
HD3651B
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Title: HD3651B: the first directly imaged brown dwarf companion of an exoplanet host star
Authors: M. Mugrauer, A. Seifahrt, R. Neuhaeuser, T. Mazeh

In the course of our ongoing multiplicity study of exoplanet host stars we detected a faint companion located at ~43arcsec (480AU physical projected separation) north-west of its primary -- the exoplanet host star HD3651 at 11pc. The companion, HD3651B, clearly shares the proper motion of the exoplanet host star in our four images, obtained with ESO/NTT and UKIRT, spanning three years in epoch difference. The magnitude of the companion is H=16.75 0.16mag, the faintest co-moving companion of an exoplanet host star imaged directly. HD3651B is not detected in the POSS-II B-, R- and I-band images, indicating that this object is fainter than ~20mag in the B- and R-band and fainter than ~19mag in the I-band. With the Hipparcos distance of HD3651 of 11pc, the absolute magnitude of HD3651B is about 16.5mag in the H band. Our H-band photometry and the Baraffe et al. (2003) evolutionary models yield a mass of HD3651B to be 20 to 60MJup for assumed ages between 1 and 10Gyr. The effective temperature ranges between 800 and 900K, consistent with a spectral type of T7 to T8. We conclude that HD3651B is a brown-dwarf companion, the first of its kind directly imaged as a companion of an exoplanet host star, and one of the faintest T dwarfs found in the solar vicinity (within 11pc).

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Posts: 131433
Date:
WD 0137-349
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The lives of stars like the sun follow a predictable course: billions of years spent pumping out energy as hydrogen fuses into helium in a superhot core followed by an expansive phase when the fuel runs out. Such older sunlike stars become red giants, swelling to consume the hydrogen in the outer reaches of their atmospheres until that too is consumed and the swollen star's outer layer is expelled as a nebula. This leaves behind a small, so-called white dwarf consisting of the residual helium core. Now scientists have discovered an unusual binary star system consisting of a brown dwarf (a pseudostar 55 times the size of Jupiter but still too small to reliably fuse hydrogen) and a white dwarf.

"Such a system must have had a very troubled history. Its existence proves that the brown dwarf came out almost unaltered from an episode in which it was swallowed by a red giant" - Pierre Maxted of Keele University in the United Kingdom who led the observations.

The brown dwarf may have even gained mass from the common envelope that existed during the system's (known as WD 0137-349) red giant phase (depicted in the image at right with the red centre showing the future white dwarf and a black dot representing the brown dwarf). But it also lost some distance from its stellar neighbour. During its engulfment, the brown dwarf spiralled ever closer to the helium core until now the two orbit each other every 116 minutes; the brown dwarf travels at speeds of 800,000 kilometres per hour. Despite its name, the brown dwarf only survived thanks to its size.

"Had the companion been less than 20 Jupiter masses, it would have evaporated during (the red giant) phase" - Pierre Maxted .

But size will not save it for long in stellar terms; in roughly 1.4 billion years, the orbital period will decline to as little as one hour at which point the compact but powerful white dwarf will begin to suction material from the facing side of the brown dwarf. Such systems are known as cataclysmic variables and, as the white dwarf acquires new hydrogen, it will slowly acquire the mass for a spectacular nuclear chain reaction: a nova. So the brown dwarf may have survived the red giant but the white dwarf will consume it in a future thermonuclear explosion. The paper presenting the research appears in today's Nature.


Title: Survival of a brown dwarf after engulfment by a red giant star
Authors: P.F.L. Maxted (1), R. Napiwotzki (2), P.D. Dobbie (3), M.R. Burleigh (3) ((1)Keele University, UK, (2) University of Hertfordshire, UK, (3) University of Leicester, UK)

High resolution optical spectroscopy of the white dwarf WD0137-349 reveals a weak Halpha line in emission, due to a low mass companion in a close orbit. Using this emission feature, and the narrow NLTE core of the white dwarf's Halpha absorption line, we measure the orbital period at 116 minutes, the separation at 0.65Rsun, and the mass ratio m2/m1 = K1/K2 = 0.1340.006. From the mass of the white dwarf (0.390.035Msun) we derive a mass for the companion of 0.0530.006Msun. This is well below the limit of about 0.075Msun commonly used to distinguish stars from brown dwarfs. The observed infrared flux distribution of WD0137-349 is also consistent with a model of an old brown dwarf spectral type mid T or slightly earlier. The system is detached, and the brown dwarf must have survived a previous phase of common envelope evolution, providing a key observational test for models of this interaction.

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Posts: 131433
Date:
DEN0255-4700
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As part of an ongoing search for Earth's nearest stellar neighbours, astronomers have determined the distance to a stellar-like body known as DEN 0255-477 and discovered that it is the nearest-known L dwarf. This body is now also the faintest object outside our solar system for which its intrinsic visual brightness has been measured.

The new record holder is nearly 100 million times fainter than the Sun. It is located only 16.2 light-years (4.97 parsecs) from Earth, making DEN 0255-4700 the 48th nearest known system of stars or brown dwarfs. This very reddish object is a third closer to Earth than the next known L dwarf, which is 24 light-years away. The discovery by a team led by Edgardo Costa and Rene Mendez of the Universidad de Chile in Santiago will be published in the September 2006 issue of the Astronomical Journal.
The Sun's new neighbour has a formal spectral type of L7.5 V, indicative of a cool object with a temperature of only 2,600 degrees F (1700 K). This is rather cooler than the least massive stars, which have temperatures of 4,000 F (2500 K). A spectrum of DEN 0255-4700 shows that it has exotic elements such as potassium, rubidium, and caesium in its outer atmosphere, mixed with superheated water.
L dwarfs are among the coolest and lowest-mass stellar-like objects known. DEN 0255-4700 is probably not a star at all, but rather a brown dwarf. The primary attribute that separates stars from brown dwarfs is mass, and the mass of DEN 0255-4700 is likely to be below the limit of 80 Jupiter masses required to fuse hydrogen into helium.
DEN 0255-477 was originally identified as an interesting object by Martin et al. in 1999, but a measure of its distance and of its intrinsic visual brightness was not available until the present study, which used two telescopes in Chile.
The distance to DEN 0255-4700 was measured via a classic trigonometric parallax technique using the 1.5-metre telescope at the National Science Foundation's Cerro Tololo Inter-American Observatory (CTIO) in the Chilean Andes. The visual (V-band) magnitude of the object was measured with the European Southern Observatory's 3.6-meter telescope in Chile.
The combination of the two pieces of information yields an absolute magnitude for DEN 0255-4700 of 24.4, nearly 20 magnitudes fainter in intrinsic brightness than the Sun.
The parallax technique for measuring the distance to a star takes advantage of the Earth's changing position in the cosmos as it orbits the Sun each year. The apparent back and forth motion of a nearby star during this year reflects the motion of the Earth around the Sun, much like how your finger jumps back and forth in front of your eyes if you blink one eye, then the other.
From Earth, nearby stars appear to make tiny ellipses in the sky because the Earth does not jump from one side of its orbit to another, but slides smoothly around the Sun. The extreme points of the Earth in its orbit are much like the positions of your eyes in your head, and the breadth of the motion in your finger depends on how close you hold it to your eyes when nearer, it seems to jump more, relative to distant background objects.

The team of astronomers, including Costa, Mendez, and collaborators at Georgia State University in Atlanta and the University of Virginia in Charlottesville, measured the size of the ellipse traced by DEN 0255-4700 to be so large that it must be only 16 light-years away. Over a period of more than three years, the team took pictures of DEN 0255-4700 among a set of distant background stars and monitored its wobbling.
The long-term parallax program, known as the CTIO Parallax Investigation (CTIOPI), has been sponsored by Fondecyt in Chile, and the National Optical Astronomy Observatory (NOAO) in the United States.
The purpose of CTIOPI is to discover and characterise overlooked stars and brown dwarfs in the vicinity the Sun. Objects are scrutinised by measuring their positions (and wobbles), their brightnesses and colours, and by taking spectroscopic fingerprints to examine their atmospheric composition. The estimated "missing" population of solar neighbourhood members is expected to be composed primarily of very low mass stars with spectral type M (known as red dwarfs), and objects of spectral types L (like DEN 0255-4700) and T, many of which are actually brown dwarfs with too little mass to start long-term thermonuclear reactions. Such objects shine feebly, glowing only because of energy leaking out since their gravitational formation, many billions of years ago.

The Chilean members of CTIOPI have been supported by the Fondo Nacional de Investigacion Cientifica y Tecnologica and the Chilean Centro de Astrofisica FONDAP. The team from the United States has been supported by NASA's Space Interferometry Mission, and the National Science Foundation.

Cerro Tololo Inter-American Observatory, based in La Serena, Chile, is part of the National Optical Astronomy Observatory, Tucson, Arizona, which is operated by the Association of Universities for Research in Astronomy Inc. (AURA), under a cooperative agreement with the National Science Foundation.

www.noao.edu

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