scholarly journals Brown Dwarf Model Atmospheres Based on Multi-Dimensional Radiation Hydrodynamics

2009 ◽  
Vol 5 (H15) ◽  
pp. 756-756 ◽  
Author(s):  
France Allard ◽  
Bernd Freytag
Keyword(s):  
Cloud Model ◽  
Molecular Transport ◽  
Brown Dwarfs ◽  
Model Atmosphere ◽  
Magnetic Activity ◽  
Cloud Formation ◽  
Brown Dwarf ◽  
Low Mass Stars ◽  
The Status ◽  
Low Mass

AbstractThe atmospheres of Brown Dwarfs (BDs) are the site of molecular opacities and cloud formation, and control their cooling rate, radius and brightness evolution. Brown dwarfs evolve from stellar-like properties (magnetic activity, spots, flares, mass loss) to planet-like properties (electron degeneracy of the interior, cloud formation, dynamical molecular transport) while retaining, due to their fully convective interior, larger rotational velocities (≤ 30 km/s i.e. P < 4 hrs versus 11 hrs for Jupiter). Model atmospheres treating all this complexity are therefore essential to understand the evolution properties, and to interpret the observations of these objects. While the pure gas-phase based NextGen model atmospheres (Allard et al. 1997, Hauschildt et al. 1999) have allowed the understanding of the several populations of Very Low Mass Stars (VLMs), the AMES-Dusty models (Allard et al. 2001) based on equilibrium chemistry have reproduced some near-IR photometric properties of M and L-type brown dwarfs, and played a key role in the determination of the mass of brown dwarfs and Planetary Mass Objects (PMOs) in the eld and in young stellar clusters. In this paper, we present a new model atmosphere grid for VLMs, BDs, PMOs named BT-Settl, which includes a cloud model and dynamical molecular transport based on mixing information from 2D Radiation Hydrodynamic (RHD) simulations (Freytag et al. 2009). We also present the status of our 3D RHD simulations including rotation (Coriolis forces) of a cube on the surface of a brown dwarf. The BT-Settl model atmosphere grid will be available shortly via the Phoenix web simulator (http://phoenix.ens-lyon.fr/simulator/).

scholarly journals Models of very-low-mass stars, brown dwarfs and exoplanets

2012 ◽  
Vol 370 (1968) ◽  
pp. 2765-2777 ◽  
Author(s):  
F. Allard ◽  
D. Homeier ◽  
B. Freytag
Keyword(s):  
Extrasolar Planets ◽  
Cloud Model ◽  
Brown Dwarfs ◽  
Spectroscopic Properties ◽  
Model Atmosphere ◽  
Low Mass Stars ◽  
Hot Jupiters ◽  
Solar Type ◽  
Low Mass ◽  
First Time

Within the next few years, GAIA and several instruments aiming to image extrasolar planets will be ready. In parallel, low-mass planets are being sought around red dwarfs, which offer more favourable conditions, for both radial velocity detection and transit studies, than solar-type stars. In this paper, the authors of a model atmosphere code that has allowed the detection of water vapour in the atmosphere of hot Jupiters review recent advances in modelling the stellar to substellar transition. The revised solar oxygen abundances and cloud model allow the photometric and spectroscopic properties of this transition to be reproduced for the first time. Also presented are highlight results of a model atmosphere grid for stars, brown dwarfs and extrasolar planets.

scholarly journals The NIRSPEC Brown Dwarf Spectroscopic Survey

2003 ◽  
Vol 211 ◽  
pp. 385-388 ◽  
Author(s):  
Ian S. McLean ◽  
M. R. McGovern ◽  
Lisa Prato ◽  
Adam J. Burgasser ◽  
J. Davy Kirkpatrick
Keyword(s):  
Spectral Resolution ◽  
Brown Dwarfs ◽  
Spectral Classification ◽  
Brown Dwarf ◽  
Low Mass Stars ◽  
Spectral Signatures ◽  
The Status ◽  
Infrared Spectral ◽  
On Line ◽  
Low Mass

This paper describes the status of a new 0.9 – 2.4 μm spectroscopic survey of low-mass stars and brown dwarfs. Flux-calibrated spectra of 30 objects from M6 to T8 have been obtained with a uniform spectral resolution of R=2,000. For the J-band alone, 60 objects have been observed. In addition, we have obtained the first sequence of spectra at R=20,000 (15 km/s) showing the detailed behavior of the J-band potassium doublets over the same range in spectral type. A few sources have been monitored for radial velocity variations, but none have been detected at the ±1–2 km/s level. Preliminary infrared spectral classification indices have been derived and several young brown dwarfs show clear spectral signatures of lower gravity. Our survey will be available on-line in the future.

scholarly journals Stellar to Substellar Model Atmospheres

2011 ◽  
Vol 7 (S282) ◽  
pp. 235-242 ◽  
Author(s):  
France Allard ◽  
Derek Homeier ◽  
Bernd Freytag
Keyword(s):  
Velocity Field ◽  
Dust Cloud ◽  
Brown Dwarfs ◽  
Model Atmosphere ◽  
General Purpose ◽  
Surface Velocity ◽  
Cloud Formation ◽  
Low Mass Stars ◽  
Spectral Transition ◽  
Low Mass

AbstractThe spectral transition from Very Low Mass stars (VLMs) to brown dwarfs (BDs) and planetary mass objects (Planemos) requires model atmospheres that can treat line, molecule, and dust-cloud formation with completeness and accuracy. One of the essential problems is the determination of the surface velocity field throughout the main sequence down to the BD and planemo mass regimes. We present local 2D and 3D radiation hydrodynamic simulations using the CO5BOLD code with binned Phoenix gas opacities, forsterite dust formation (and opacities) and rotation. The resulting velocity field vs depth and Teff has been used in the general purpose model atmosphere code Phoenix, adapted in static 1D spherical symmetry for these cool atmospheres. The result is a better understanding of the spectral transition from the stellar to substellar regimes. However, problems remain in reproducing the colors of the dustiest brown dwarfs. The global properties of rotation can change the averaged spectral properties of these objects. Our project for the period 2011-2015 is therefore to develop scaled down global 3D simulations of convection, cloud formation and rotation thanks to funding by the Agence Nationale de la Recherche in France.

scholarly journals X-Ray Detection of Brown Dwarfs with Chandra

2003 ◽  
Vol 211 ◽  
pp. 447-450 ◽  
Author(s):  
Scott J. Wolk
Keyword(s):  
Brown Dwarfs ◽  
Stellar Clusters ◽  
Brown Dwarf ◽  
Low Mass Stars ◽  
X Ray ◽  
Low Mass ◽  
Vast Range

I review recent observations of brown dwarfs by the Chandra X-ray Observatory. These observations fall in 2 categories, young stellar clusters which contain brown dwarfs and brown dwarf candidates and directed pointings at brown dwarfs and very low mass stars. Surprisingly, there are already over 60 published detections of brown dwarfs by Chandra. A review of the X–ray characteristics shows these objects are subject to flaring and their temperatures and luminosities have a vast range which is related to age.

scholarly journals The formation of brown dwarfs in discs: Physics, numerics, and observations

2010 ◽  
Vol 6 (S270) ◽  
pp. 223-226
Author(s):  
Dimitris Stamatellos ◽  
Anthony Whitworth
Keyword(s):  
Large Fraction ◽  
Brown Dwarfs ◽  
Brown Dwarf ◽  
Low Mass Stars ◽  
Hydrodynamic Simulations ◽  
Low Mass

AbstractA large fraction of brown dwarfs and low-mass stars may form by gravitational fragmentation of relatively massive (a few 0.1 M⊙) and extended (a few hundred AU) discs around Sun-like stars. We present an ensemble of radiative hydrodynamic simulations that examine the conditions for disc fragmentation. We demonstrate that this model can explain the low-mass IMF, the brown dwarf desert, and the binary properties of low-mass stars and brown dwarfs. Observing discs that are undergoing fragmentation is possible but very improbable, as the process of disc fragmentation is short lived (discs fragment within a few thousand years).

scholarly journals A Search for Lithium in Brown Dwarf Candidates

1993 ◽  
Vol 137 ◽  
pp. 171-173
Author(s):  
Rafael Rebolo ◽  
Eduardo L. Martin ◽  
Antonio Magazzu
Keyword(s):  
High Resolution ◽  
Resonance Line ◽  
Brown Dwarfs ◽  
Brown Dwarf ◽  
Low Mass Stars ◽  
Substellar Objects ◽  
Low Mass

AbstractWe are conducting a search for lithium in very low mass objects with the aim of discriminating between stellar and substellar objects. Lithium is expected to be preserved in brown dwarfs with M/M⊙ ≤0.06, while it is known to be efficiently destroyed in low mass stars. In this paper we present high resolution observations in the region of the λ 6708 Li I resonance line of 5 very low mass dwarfs. In none of them lithium is detected, implying a Li destruction in their atmospheres of about four orders of magnitude. Our results suggest that these objects are probably very low mass stars rather than brown dwarfs.

scholarly journals Searching for Planets of Brown Dwarfs

2003 ◽  
Vol 211 ◽  
pp. 225-232 ◽  
Author(s):  
Eike Guenther ◽  
Guenther Wuchterl
Keyword(s):  
High Probability ◽  
Brown Dwarfs ◽  
Surface Structures ◽  
Brown Dwarf ◽  
Low Mass Stars ◽  
Double Line ◽  
Normal Star ◽  
Spectroscopic Binary ◽  
Planetary Mass ◽  
Low Mass

Up to now, most planet search projects have concentrated on G and K stars. In order to considerably widen the view, we have stated a survey for planets of old, nearby brown dwarfs and very low-mass stars. Using UVES, we have observed 26 brown dwarfs and very low-mass stars. As it turned out these objects are very inactive and thus highly suitable for such a project. For 19 objects, we can exclude a planet with the mass of 3 MJ, and a period of 100 days or less with a probability of more than 60%. For these objects, we can also exclude Pegasi-planets with a high probability. For another 4 objects, we can exclude at least a brown dwarf companion. One object is a double line spectroscopic binary, and one object shows significant radial-velocity variations that can not be caused by a normal stellar-spot. This object either has a planetary-mass companion, or the variations are caused by surface structures that are quite different from normal star-spots.

scholarly journals Brown Dwarfs as Ejected Stellar Embryos: Observational Perspectives

2003 ◽  
Vol 211 ◽  
pp. 13-22 ◽  
Author(s):  
Bo Reipurth ◽  
Cathie Clarke
Keyword(s):  
Stochastic Process ◽  
Large Fraction ◽  
Brown Dwarfs ◽  
Circumstellar Disks ◽  
Brown Dwarf ◽  
Low Mass Stars ◽  
Multiple Systems ◽  
Observational Test ◽  
Dynamical Simulations ◽  
Low Mass

We discuss a scenario in which brown dwarfs are formed like stars, except that their full collapse phases are interrupted through dynamical interactions in small multiple systems, leading to the ejection of the lightest member. This disintegration is a stochastic process, often resulting in the expulsion of newborn low mass stars, but when it occurs early enough the ejected stellar embryo will be a substellar object. This process may be so common at early ages that a large fraction of the ubiquitous brown dwarfs could have formed in this manner. Detailed gas dynamical simulations are required in order to better understand the details of the decay of small newborn multiple systems. We discuss the observational consequences of the ejection hypothesis, noting especially the importance of binaries with brown dwarf components as an observational test. Finally, we note that brown dwarfs that have recently been ejected may be so disturbed, by infall from the collapsing core and also by heavy accretion from perturbed circumstellar disks, that traditional spectral and luminosity criteria may fail to identify their substellar nature.

A proto-brown dwarf candidate in rho Ophiuchus

2021 ◽  
Vol 32 ◽  
Author(s):  
Phan Bao Ngoc
Keyword(s):  
Formation Process ◽  
Brown Dwarfs ◽  
Small Scale ◽  
The Sun ◽  
Formation Processes ◽  
Brown Dwarf ◽  
Low Mass Stars ◽  
Star Forming ◽  
Low Mass ◽  
Rho Ophiuchus

Brown dwarfs with masses below 0.075 solar masses are thought to form like low-mass stars (e.g., the Sun).However, it is still unclear how the physical formation processes occurin brown dwarfs at the ealiest stages (i.e., proto-brown dwarfs) of their formation.Up to date, only a few proto-brown dwarfs have been detected.The detection of proto-brown dwarfs offers us excellent benchmarks to studythe formation process of brown dwarfs, and thus understand their formation mechanism.In this paper, we present our identification of a proto-brown dwarf candidate in the star-forming regionrho Ophiuchus.The candidate shows a small-scale bipolar molecular outlfow that is similar to the outflows observed inother young brown dwarfs. The detection of this candidateprovides us with additional important implications for the formation mechanism of brown dwarfs.

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