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TOPIC: Brown Dwarfs


L

Posts: 131433
Date:
TVLM 513
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A study of brown dwarfs has revealed that these failed stars can possess powerful magnetic fields and emit lighthouse beams of radio waves thousands of times brighter than any detected from the Sun. The brown dwarfs are behaving like pulsars, one of the most exotic types of object in our Universe.

Brown dwarfs tend to be seen as a bit boring the cinders of the galaxy. Our research shows that these objects can be fascinating and dynamic systems, and may be the key to unlocking this long-standing mystery of how pulsars produce radio emissions - Gregg Hallinan of the National University of Ireland, Galway.

Since the discovery of pulsars forty years ago, astronomers have been trying to understand how the rotating neutron stars produce their flashing radio signals. Although there have been many attempts to describe how they produce the extremely bright radio emissions, the vast magnetic field strengths of pulsars and the relativistic speeds involved make it extremely difficult to model. Brown dwarfs are now the second class of stellar object observed to produce this kind of powerful, amplified (coherent) radio signal at a persistent level. The emissions from the brown dwarfs appear to be very similar to those observed from pulsars, but the whole system is on a much slower and smaller scale, so it is much easier to decipher exactly what is going on. Importantly, the mechanisms for producing the radio emissions in brown dwarfs are well understood, as they are almost identical to the processes that produce radio emissions from planets.
Jupiters volcanic moon, Io, is a source of electrically charged gas that is accelerated by the planets magnetic field and causes powerful radio laser, or maser, emissions. The radiation can be so intense that Jupiter frequently outshines the Sun as a source of energy at radio wavelengths. For some time, scientists have thought that there may be similarities between this type of maser emission and pulsars lighthouse-like beams of radio waves. Observations of the brown dwarf, TVLM 513, using the Very Large Array (VLA) radio telescope, may provide the first direct evidence for that link.
The group observed the brown dwarf over a period of 10 hours at two different frequencies. In both cases, a bright flash was observed every 1.96 hours.
As yet, the processes controlling the radio flashes from TVLM 513 are still unclear. There is no evidence of a binary system, so interaction of the magnetosphere with a stellar wind from a nearby star seems an unlikely cause, nor is there any sign of an orbiting planet that could produce a scenario like that of Jupiter and Io. However, rapid rotation is also thought to be a source of electron acceleration for a component of Jupiters maser emission and this may also be the main source of TVLM 513s radio flashes.
The group is now planning a large survey of all the known brown dwarfs in the solar neighbourhood to find out how many are radio sources and how many of those are pulsing. If a large fraction of brown dwarfs are found to pulse, it could prove a key method of detection for these elusive objects.

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L

Posts: 131433
Date:
RE: Brown Dwarfs
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Title: A New Population of Young Brown Dwarfs
Authors: Kelle L. Cruz, J. Davy Kirkpatrick, Adam J. Burgasser, Dagny Looper, Subhanjoy Mohanty, Lisa Prato, Jackie Faherty, Adam Solomon

We report the discovery of a population of late-M and L field dwarfs with unusual optical and near-infrared spectral features that we attribute to low gravity -- likely uncommonly young, low-mass brown dwarfs. Many of these new-found young objects have southerly declinations and distance estimates within 60 parsecs. Intriguingly, these are the same properties of recently discovered, nearby, intermediate-age (5-50 Myr), loose associations such as Tucana/Horologium, the TW Hydrae association, and the Beta Pictoris moving group. We describe our efforts to confirm cluster membership and to further investigate this possible new young population of brown dwarfs.

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Posts: 131433
Date:
HD3651B
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Title: The Late-T Dwarf Companion to the Exoplanet Host Star HD 3651: A New Benchmark for Gravity and Metallicity Effects in Ultracool Spectra
Authors: Michael C. Liu (IfA/Hawaii), S. K. Leggett (Gemini), Kuenley Chiu (Exeter)

We present near-IR photometry and spectroscopy of HD 3651B, the low-luminosity, wide-separation (480 AU) companion to the K0V exoplanet host star HD 3651A. We find a spectral type of T7.5 0.5, a temperature of 780-840 K, and a mass of 40-72 Mjup; its bolometric luminosity and effective temperature are among the lowest measured for any brown dwarf. We estimate an age of 3-12 Gyr for the primary star HD 3651A and find that it is ~3x older than the K4V star Gl 570A (1-5 Gyr), the host star of the T7.5 dwarf Gl 570D. HD 3651B belongs to the rare class of substellar objects that are companions to main-sequence stars and thus provides a new benchmark for studying very low-temperature objects. Given their similar temperatures (within ~30 K) and metallicities (within ~0.1 dex) but different ages, a comparison of HD 3651B and GL 570D allows us to examine gravity-sensitive diagnostics in ultracool spectra. We find that the expected signature of HD 3651B's higher surface gravity due to its older age, namely a suppressed K-band flux relative to GL 570D, is not seen. Instead, the K-band flux of HD 3651B is enhanced compared to GL 570D, indicative of a younger age. Thus, the relative ages derived from interpretation of T dwarf spectra and from stellar activity indicators appear to be in discord. One likely explanation is that the K-band fluxes are also very sensitive to metallicity differences. Metallicity variations may be as important as surface gravity variations in causing spectral differences among field late-T dwarfs.

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L

Posts: 131433
Date:
WD 0137-349
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Near Infrared Spectra Constrain an Extraordinary Pair of Cosmic Dwarves
discovered and characterized an extremely rare stellar system composed of a white dwarf and brown dwarf pair. Named WD 0137-349, this pairing is the only known close, detached binary system with a white dwarf and a confirmed substellar companion. The system is exceptionally intriguing given the measured orbital period of P = 116 minutes. Such a short period implies that the stars orbit within one solar radius from one another. This is close enough that the brown dwarf must have survived the period of common envelope evolution where it was completely engulfed by the atmosphere of the red giant progenitor of the white dwarf.
Earlier work by Maxted et al. had already established a mass of 0.39 0.035 Msun for the white dwarf and of 0.053 0.006 Msun for the companion. The derived mass of the companion is well below the 0.075 Msun threshold that separates stars from brown dwarfs. The original discovery of the system was part of the VLT 'SN Ia Progenitor Survey' led by R. Napiwotzki.
The Maxted et al. paper noted that the only way to accurately determine the spectral type of the brown dwarf was through near-infrared spectroscopy. To this end the team obtained a near infrared spectrum using the GNIRS instrument on the Gemini South Telescope. Figure 2 shows that the spectrum is dominated by the hot 16,500 K white dwarf in the optical, however, excess emission is seen at all wavelengths longer than ~1.95 microns. The team fit this excess emission using a variety of known brown dwarf spectra as templates to determine the spectral type of the companion. The best fit is provided by a spectral type L8 brown dwarf, making the WD 0137-349 object the lowest mass and lowest temperature brown dwarf companion to a white dwarf as yet discovered. For a cooling age of about one billion years, the temperature of the brown dwarf companion is between 1300 K and 1400 K.

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L

Posts: 131433
Date:
RE: Brown Dwarfs
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A team of US astronomers led by Sebastian Daemgen (University of Arizona) has just released the results of a search for low-mass brown dwarf or planetary companions around young early-type M dwarf stars within 20 parsecs of the Sun.

The 41 stars in the survey have age estimates of < 300 million years, comparable to that of the Pleiades. Within the sample 13 objects are found to host fairly bright companions, eight of these (seven binaries and one triplet) are new discoveries. Understanding binarity or multiplicity rates in low-mass stellar systems could provide important clues on the formation of low-mass objects including planetary systems and their population of planets.
The survey was conducted using The Gemini North adaptive optics system ALTAIR in its natural guide star (NGS) mode. The strategy of the survey was to target young stars in order to increase the probability of finding intrinsically faint companions, such as large self-luminous Jovian planets. The team was able set a detection limit for faint companions of ΔKs ~ 7.8 magnitudes at separations greater than 0.5 arcsecond from the primary, and ΔKs >10 beyond 1.0 arcsecond separation.
The masses of the individual stars in the tiny M dwarf pairs vary between a bit less than 0.10 to 0.60 solar masses. The orbital periods of the pairs range from 6 to 477 years. The most likely separation between the components was found to be ~13 astronomical units (AU), which is less than the median value of the more massive solar type binary distribution. Yet, it is wider than that of the very low-mass (VLM) stars as determined by previous Gemini and Hubble Space Telescope studies.
No faint physical companions were found, despite the ability to detect them. Hence, the team was able to exclude the presence of companions more massive than 10 MJup at > 40 AU or heavier than 23 MJup at > 10 AU. All planetary mass candidates were confirmed to be background objects.

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Posts: 131433
Date:
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Title: On the Formation of Brown Dwarfs
Authors: Ing-Guey Jiang (1), G. Laughlin (2), D.N.C. Lin (2), ((1) National Tsing-Hua University, Taiwan, (2) UC Santa Cruz, U.S.A.)

The observational properties of brown dwarfs pose challenges to the theory of star formation. Because their mass is much smaller than the typical Jeans mass of interstellar clouds, brown dwarfs are most likely formed through secondary fragmentation processes, rather than through the direct collapse of a molecular cloud core. In order to prevent substantial post-formation mass accretion, young brown dwarfs must leave the high density formation regions in which they form. We propose here that brown dwarfs are formed in the optically thin outer regions of circumbinary disks. Through post-formation dynamical interaction with their host binary stars, young brown dwarfs are either scattered to large distance or removed, with modest speed, from their cradles.

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Posts: 131433
Date:
HD 3651B
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Title: The Physical Properties 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. These data confirm the companion as a brown dwarf with spectral type T8, consistent with the analysis of Luhman et al. Application of the semi-empirical technique of Burgasser, Burrows & Kirkpatrick indicates that HD 3651B has Teff = 790 30 K and log g = 5.0 0.3 for a metallicity of (M/H) = 0.12 0.04, consistent with a mass M = 0.033 0.013 M_sun and an age of 0.7-4.7 Gyr. The surface gravity, mass and age estimates of this source are all highly sensitive to the assumed metallicity; however, a supersolar metallicity is deduced by direct comparison of spectral models to the observed absolute fluxes. The age of HD 3651B is somewhat better constrained than that of the primary, with estimates for the latter ranging over ~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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L

Posts: 131433
Date:
L Dwarf/T Dwarf Transition
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Title: Binaries and the L Dwarf/T Dwarf Transition
Authors: Adam J. Burgasser (MIT)

High-resolution imaging has revealed an unusually high binary fraction amongst objects spanning the transition between the L dwarf and T dwarf spectral classes. In an attempt to reproduce and unravel the origins of this apparent binary excess, I present a series of Monte Carlo mass function and multiplicity simulations of field brown dwarfs in the vicinity of the Sun. These simulations are based on the solar metallicity brown dwarf evolutionary models and incorporate empirical luminosity and absolute magnitude scales, measured multiplicity statistics and observed spectral templates in the construction and classification of composite binary spectra. In addition to providing predictions on the number and surface density distributions of L and T dwarfs for volume-limited and magnitude-limited samples, these simulations successfully reproduce the observed binary fraction distribution assuming an intrinsic (resolved) binary fraction of 11(+6)(-3)% (95% confidence interval), consistent with prior determinations. However, the true binary fraction may be as high as 40% if, as suggested by Liu et al., a significant fraction of L/T transition objects (~66%) are tightly-bound, unresolved multiples. The simulations presented here demonstrate that the binary excess amongst L/T transition objects arises primarily from the flattening of the luminosity scale over these spectral types and is not inherently the result of selection effects incurred in current magnitude-limited imaging samples. Indeed, the existence of a binary excess can be seen as further evidence that brown dwarfs traverse the L/T transition rapidly, possibly driven by a nonequilibrium submergence of photospheric condensates.

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Posts: 131433
Date:
RE: Brown Dwarfs
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Title: A New Brown Dwarf Desert? A Scarcity of Wide Ultracool Binaries
Authors: Peter R. Allen, David W. Koerner, Michael W. McElwain, Kelle L. Cruz, I. Neill Reid

We present the results of a deep-imaging search for wide companions to low-mass stars and brown dwarfs using NSFCam on IRTF. We searched a sample of 132 M7-L8 dwarfs to magnitude limits of J ~ 20.5 and K ~ 18.5, corresponding to secondary-primary mass ratios of ~ 0.5. No companions were found with separations between 2" to 31" (~40 AU to ~1000 AU). This null result implies a wide companion frequency below 2.3% at the 95% confidence level within the sensitivity limits of the survey. Preliminary modelling efforts indicate that we could have detected 85% of companions more massive than 0.05 solar masses and 50% above 0.03 solar masses.

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Posts: 131433
Date:
HD3651
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First Directly Imaged Brown Dwarf Companion to an Exoplanet Host Star
Astronomers have detected a new faint companion to the star HD 3651, already known to host a planet. This companion, a brown dwarf, is the faintest known companion of an exoplanet host star imaged directly and one of the faintest T dwarfs detected in the Solar neighbourhood so far. The detection yields important information on the conditions under which planets form.

"Such a system is an interesting example that might prove that planet and brown dwarf can form around the same star" - Markus Mugrauer, lead author of the paper presenting the discovery.

HD 3651 b

HD 3651 is a star slightly less massive than the Sun, located 36 light-years away in the constellation Pisces (the "Fish"). For several years, it has been known to harbour a planet less massive than Saturn, sitting closer to its parent star than Mercury is from the Sun: the planet accomplishes a full orbit in 62 days.
Mugrauer and his colleagues first spotted the faint companion in 2003 on images from the 3.8-m United Kingdom Infrared Telescope (UKIRT) in Hawaii. Observations in 2004 and 2006 using ESO's 3.6 m New Technology Telescope (NTT) at La Silla provided the crucial confirmation that the speck of light is not a spurious background star, but indeed a true companion. The newly found companion, HD 3651B, is 16 times further away from HD 3651 than Neptune is from the Sun.

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