ABSTRACT: Formation of bodies near the deuterium-burning limit is considered by
detailed numerical simulations according to the core-nucleated giant planet
accretion scenario. The objects, with heavy-element cores in the range 5-30
Mearth, are assumed to accrete gas up to final masses of 10-15 Jupiter masses
(Mjup). After the formation process, which lasts 1-5 Myr and which ends with a
'cold-start', low-entropy configuration, the bodies evolve at constant mass up
to an age of several Gyr. Deuterium burning via proton capture is included in
the calculation, and we determined the mass, M50, above which more than 50% of
the initial deuterium is burned. This often-quoted borderline between giant
planets and brown dwarfs is found to depend only slightly on parameters, such
as core mass, stellar mass, formation location, solid surface density in the
protoplanetary disk, disk viscosity, and dust opacity. The values for M50 fall
in the range 11.6-13.6 Mjup, in agreement with previous determinations that do
not take the formation process into account. For a given opacity law during the
formation process, objects with higher core masses form more quickly. The
result is higher entropy in the envelope at the completion of accretion,
yielding lower values of M50. For masses above M50, during the
deuterium-burning phase, objects expand and increase in luminosity by 1 to 3
orders of magnitude. Evolutionary tracks in the luminosity-versus-time diagram
are compared with the observed position of the companion to Beta Pictoris.
ABSTRACT: We report the discovery of 87 new T dwarfs uncovered with the Wide-field
Infrared Survey Explorer (WISE) and three brown dwarfs with extremely red
near-infrared colors that exhibit characteristics of both L and T dwarfs. Two
of the new T dwarfs are likely binaries with L7+/-1 primaries and mid-type T
secondaries. In addition, our follow-up program has confirmed 10 previously
identified T dwarfs and four photometrically-selected L and T dwarf candidates
in the literature. This sample, along with the previous WISE discoveries,
triples the number of known brown dwarfs with spectral types later than T5.
Using the WISE All-Sky Source Catalog we present updated color-color and
color-type diagrams for all the WISE-discovered T and Y dwarfs. Near-infrared
spectra of the new discoveries are presented, along with spectral
classifications. To accommodate later T dwarfs we have modified the integrated
flux method of determining spectral indices to instead use the median flux.
Furthermore, a newly defined J-narrow index differentiates the early-type Y
dwarfs from late-type T dwarfs based on the J-band continuum slope. The K/J
indices for this expanded sample show that 32% of late-type T dwarfs have
suppressed K-band flux and are blue relative to the spectral standards, while
only 11% are redder than the standards. Comparison of the Y/J and K/J index to
models suggests diverse atmospheric conditions and supports the possible
re-emergence of clouds after the L/T transition. We also discuss peculiar brown
dwarfs and candidates that were found not to be substellar, including two Young
Stellar Objects and two Active Galactic Nuclei. The coolest WISE-discovered
brown dwarfs are the closest of their type and will remain the only sample of
their kind for many years to come.
ABSTRACT: As brown dwarfs cool, a variety of species condense in their atmospheres,
forming clouds. Iron and silicate clouds shape the emergent spectra of L
dwarfs, but these clouds dissipate at the L/T transition. A variety of other
condensates are expected to form in cooler T dwarf atmospheres. These include
Cr, MnS, Na2S, ZnS, and KCl, but the opacity of these optically thinner clouds
has not been included in previous atmosphere models. Here, we examine their
effect on model T and Y dwarf atmospheres. The cloud structures and opacities
are calculated using the Ackerman & Marley (2001) cloud model, which is coupled
to an atmosphere model to produce atmospheric pressure-temperature profiles in
radiative-convective equilibrium. We generate a suite of models between Teff =
400 and 1300 K, log g=4.0 and 5.5, and condensate sedimentation efficiencies
from fsed=2 to 5. Model spectra are compared to two red T dwarfs, Ross 458C and
UGPS 0722-05; models that include clouds are found to match observed spectra
significantly better than cloudless models. The emergence of sulfide clouds in
cool atmospheres, particularly Na2S, may be a more natural explanation for the
"cloudy" spectra of these objects, rather than the re-emergence of silicate
clouds that wane at the L-to-T transition. We find that sulfide clouds provide
a mechanism to match the near- and mid-infrared colors of observed T dwarfs.
Our results indicate that including the opacity of condensates in T dwarf
atmospheres is necessary to accurately determine the physical characteristics
of many of the observed objects.
ABSTRACT: The near-infrared colors of the planets directly imaged around the A star HR
8799 are much redder than most field brown dwarfs of the same effective
temperature. Previous theoretical studies of these objects have concluded that
the atmospheres of planets b, c, and d are unusually cloudy or have unusual
cloud properties. Some studies have also found that the inferred radii of some
or all of the planets disagree with expectations of standard giant planet
evolution models. Here we compare the available data to the predictions of our
own set of atmospheric and evolution models that have been extensively tested
against observations of field L and T dwarfs, including the reddest L dwarfs.
Unlike some previous studies we require mutually consistent choices for
effective temperature, gravity, cloud properties, and planetary radius. This
procedure thus yields plausible values for the masses, effective temperatures,
and cloud properties of all three planets. We find that the cloud properties of
the HR 8799 planets are not unusual but rather follow previously recognized
trends, including a gravity dependence on the temperature of the L to T
spectral transition--some reasons for which we discuss. We find the inferred
mass of planet b is highly sensitive to whether or not we include the H and K
band spectrum in our analysis. Solutions for planets c and d are consistent
with the generally accepted constraints on the age of the primary star and
orbital dynamics. We also confirm that, like in L and T dwarfs and solar system
giant planets, non-equilibrium chemistry driven by atmospheric mixing is also
important for these objects. Given the preponderance of data suggesting that
the L to T spectral type transition is gravity dependent, we present an
exploratory evolution calculation that accounts for this effect. Finally we
recompute the the bolometric luminosity of all three planets.
ABSTRACT: We present multiple-epoch photometric monitoring in the $J$, $H$, and $K_s$
bands of the T1.5 dwarf 2MASS J21392676+0220226 (2M2139), revealing persistent,
periodic ($P=7.721\pm$0.005 hr) variability with a peak-to-peak amplitude as
high as 26% in the $J$-band. The light curve shape varies on a timescale of
days, suggesting that evolving atmospheric cloud features are responsible.
Using interpolations between model atmospheres with differing cloud thicknesses
to represent a heterogeneous surface, we find that the multi-wavelength
variations and the near-infrared spectrum of 2M2139 can be reproduced by either
(1)cool, thick cloud features sitting above a thinner cloud layer, or (2)warm
regions of low condensate opacity in an otherwise cloudy atmosphere, possibly
indicating the presence of holes or breaks in the cloud layer. We find that
temperature contrasts between thick and thin cloud patches must be greater than
175 K and as high as 425 K. We also consider whether the observed variability
could arise from an interacting binary system, but this scenario is ruled out.
2M2139 joins the T2.5 dwarf SIMP0136 discovered by Artigau and coworkers as the
second L/T transition brown dwarf to display large-amplitude variability on
rotational timescales, suggesting that the fragmentation of dust clouds at the
L/T transition may contribute to the abrupt decline in condensate opacity and
$J$-band brightening observed to occur over this regime.
ABSTRACT: Condensate clouds are a salient feature of L dwarf atmospheres, but have been assumed to play little role in shaping the spectra of the coldest T-type brown dwarfs. Here we report evidence of condensate opacity in the near-infrared spectrum of the brown dwarf candidate Ross 458C, obtained with the Folded-Port Infrared Echellette (FIRE) spectrograph at the Magellan Telescopes. These data verify the low-temperature nature of this source, indicating a T8 spectral classification, log10 L bol/L ☉ = –5.62 ± 0.03, T eff = 650 ± 25 K, and a mass at or below the deuterium burning limit. The data also reveal enhanced emission at the K band associated with youth (low surface gravity) and supersolar metallicity, reflecting the properties of the Ross 458 system (age = 150-800 Myr, [Fe/H] = +0.2 to +0.3). We present fits of FIRE data for Ross 458C, the T9 dwarf ULAS J133553.45+113005.2, and the blue T7.5 dwarf SDSS J141624.08+134826.7B, to cloudless and cloudy spectral models from Saumon & Marley. For Ross 458C, we confirm a low surface gravity and supersolar metallicity, while the temperature differs depending on the presence (635+25 –35 K) or absence (760+70 –45 K) of cloud extinction. ULAS J1335+1130 and SDSS J1416+1348B have similar temperatures (595+25 –45 K), but distinct surface gravities (log g = 4.0-4.5 cgs versus 5.0-5.5 cgs) and metallicities ([M/H] +0.2 versus –0.2). In all three cases, cloudy models provide better fits to the spectral data, significantly so for Ross 458C. These results indicate that clouds are an important opacity source in the spectra of young cold T dwarfs and should be considered when characterizing planetary-mass objects in young clusters and directly imaged exoplanets. The characteristics of Ross 458C suggest that it could itself be regarded as a planet, albeit one whose cosmogony does not conform with current planet formation theories.
The Astrophysical Journal 11/2010; 725(2):1405. · 6.02 Impact Factor
ABSTRACT: One of the mechanisms suggested for the L to T dwarf spectral type transition is the appearance of relatively cloud-free regions across the disk of brown dwarfs as they cool. The existence of partly cloudy regions has been supported by evidence for variability in dwarfs in the late L to early T spectral range, but no self-consistent atmosphere models of such partly cloudy objects have yet been constructed. Here we present a new approach for consistently modeling partly cloudy brown dwarfs and giant planets. We find that even a small fraction of cloud holes dramatically alter the atmospheric thermal profile, spectra, and photometric colors of a given object. With decreasing cloudiness objects briskly become bluer in J - K and brighten in J band, as is observed at the L/T transition. Model spectra of partly cloudy objects are similar to our models with globally homogenous, but thinner, clouds. Hence spectra alone may not be sufficient to distinguish partial cloudiness although variability and polarization measurements are potential observational signatures. Finally we note that partial cloud cover may be an alternative explanation for the blue L dwarfs. Comment: 14 pages, 3 figures, Ap. J. Let. in press
ABSTRACT: Clouds of metal-bearing condensates play a critical role in shaping the emergent spectral energy distributions of the coolest classes of low-mass stars and brown dwarfs, L and T dwarfs. Because condensate clouds in planetary atmospheres show distinct horizontal structure, we have explored a model for partly cloudy atmospheres in brown dwarfs. Our model successfully reproduces the colors and magnitudes of both L and T dwarfs for the first time, including the unexpected brightening of the early- and mid-type T dwarfs at the J band, provided that clouds are rapidly removed from the photosphere at Teff ≈ 1200 K. The clearing of cloud layers also explains the surprising persistence and strengthening of gaseous FeH bands in early- and mid-type T dwarfs. The breakup of cloud layers is likely driven by convection in the troposphere, analogous to phenomena observed on Jupiter. Our results demonstrate that planetary-like atmospheric dynamics must be considered when examining the evolution of free-floating brown dwarfs.
The Astrophysical Journal 12/2008; 571(2):L151. · 6.02 Impact Factor
ABSTRACT: We examine the spectra and infrared colors of the cool methane-dominated atmospheres at Teff < 1400 K expected for young gas giant planets. We couple these spectral calculations to an updated version of the Marley et al. (2007) giant planet thermal evolution models that include formation by core accretion-gas capture. These relatively cool "young Jupiters" can be 1-6 magnitudes fainter than predicted by standard cooling tracks that include a traditional initial condition, which may provide a diagnostic of formation. If correct, this would make true Jupiter-like planets much more difficult to detect at young ages than previously thought. Since Jupiter and Saturn are of distinctly super-solar composition, we examine emitted spectra for model planets at both solar metallicity and a metallicity of 5 times solar. These metal-enhanced young Jupiters have lower pressure photospheres than field brown dwarfs of the same effective temperatures arising from both lower surface gravities and enhanced atmospheric opacity. We highlight several diagnostics for enhanced metallicity. A stronger CO absorption band at 4.5 $\mu$m for the warmest objects is predicted. At all temperatures, enhanced flux in $K$ band is expected due to reduced collisional induced absorption by H$_2$. This leads to correspondingly redder near infrared colors, which are redder than solar metallicity models with the same surface gravity by up to 0.7 in $J-K$ and 1.5 in $H-K$. Molecular absorption band depths increase as well, most significantly for the coolest objects. We also qualitatively assess the changes to emitted spectra due to nonequilibrium chemistry. Comment: Accepted to ApJ. Most figures in color
The Astrophysical Journal 05/2008; · 6.02 Impact Factor
ABSTRACT: An equation of state for hydrogen which predicts a molecular-metallic phase transition at finite temperatures has become available recently. The effect of this phase transition on the cooling histories of these two giant planets and of substellar brown dwarfs is studied. The phase transition alters the present age of Jupiter and of Saturn by a few percent. Interestingly, the cooling of brown dwarfs is most strongly affected at the time when the interior adiabat crosses the critical point of the phase transition.
ABSTRACT: Recently, a new equation of state for hydrogen which predicts a
molecular-metallic phase transition at finite temperature has become
available. It is combined with a helium equation of state, and the
resulting thermodynamic description of H/He mixtures is used to compute
interior models of Jupiter and Saturn, subject to the constraints of the
measured gravitational harmonics of both planets. The inferred heavy
element abundance distribution in their interior and the possible
consequences on their formation are discussed. In particular, the
Z-element enhancement and smaller core in Saturn relative to Jupiter, a
conclusion of this study, may indicate a depletion of water ice in the
Jupiter formation zone.
The Astrophysical Journal 05/1992; 391:817-826. · 6.02 Impact Factor
ABSTRACT: Determinations of the luminosity and atmospheric properties of the T8 brown dwarf 2MASS J09393548 - 2448279 are presented, based on Spitzer IRAC and IRS observations and ground-based astrometry. We find log_(10)(L_(bol)/L⊙) = -5.69 ± 0.03 for this source, comparable to the current low-luminosity record holder 2MASS J04151954 - 0935066. However, modeling of near- and mid-infrared spectral data indicates an effective temperature of 600 ± 35 K, roughly 100 K cooler than 2M0415. These parameters require a highly inflated radius for 2M0939 (R ≈ 0.13 R⊙) which cannot be reconciled with brown dwarf structure models. However, if this source is an unresolved, equal-mass binary, then the reduced luminosity of each component (L_(bol) ≈ 10^-6 L⊙) can be brought into agreement with the inferred atmospheric parameters for an age of 0.4-12 Gyr and component masses of 0.01-0.05 M⊙. This hypothesis can be tested through future high-resolution imaging and/or spectroscopic observations.