1,271 research outputs found
Gaseous Planets, Protostars And Young Brown Dwarfs : Birth And Fate
We review recent theoretical progress aimed at understanding the formation
and the early stages of evolution of giant planets, low-mass stars and brown
dwarfs. Calculations coupling giant planet formation, within a modern version
of the core accretion model, and subsequent evolution yield consistent
determinations of the planet structure and evolution. Because of the
uncertainties in the initial conditions, however, it is not possible to say
whether young planets are faint or bright compared with low-mass young brown
dwarfs. We review the effects of irradiation and evaporation on the evolution
of short period planets and argue that substantial mass loss may have occurred
for these objects. Concerning star formation, geometrical effects in protostar
core collapse are examined by comparing 1D and 3D calculations. Spherical
collapse is shown to overestimate the core inner density and temperature and
thus to yield incorrect initial conditions for PMS or young brown dwarf
evolution. Accretion is also shown to occur over a very limited fraction of the
protostar surface. Accretion affects the evolution of young brown dwarfs and
yields more compact structures for a given mass and age, thus fainter
luminosities. This can lead to severe misinterpretations of the mass and/or age
of young accreting objects from their location in the HR diagram. We argue that
newborn stars and brown dwarfs should appear rapidly over an extended area in
the HR diagram, depending on their accretion history, rather than on a well
defined birth line. Finally, we suggest that the distinction between planets
and brown dwarfs be based on an observational diagnostic, reflecting the
different formation mechanisms between these two distinct populations, rather
than on an arbitrary, confusing definition.Comment: Invited Review, Protostars and Planets V (Hawai, October 2005
The mass-radius relationship from solar-type stars to terrestrial planets: a review
In this review, we summarize our present knowledge of the behaviour of the
mass-radius relationship from solar-type stars down to terrestrial planets,
across the regime of substellar objects, brown dwarfs and giant planets.
Particular attention is paid to the identification of the main physical
properties or mechanisms responsible for this behaviour. Indeed, understanding
the mechanical structure of an object provides valuable information about its
internal structure, composition and heat content as well as its formation
history. Although the general description of these properties is reasonably
well mastered, disagreement between theory and observation in certain cases
points to some missing physics in our present modelling of at least some of
these objects. The mass-radius relationship in the overlaping domain between
giant planets and low-mass brown dwarfs is shown to represent a powerful
diagnostic to distinguish between these two different populations and shows
once again that the present IAU distinction between these two populations at a
given mass has no valid foundation.Comment: Cool Stars, Stellar Systems and the Sun 15, invited revie
High Resolution, Differential, Near-infrared Transmission Spectroscopy of GJ 1214b
The nearby star GJ 1214 hosts a planet intermediate in radius and mass
between Earth and Neptune, resulting in some uncertainty as to its nature. We
have observed this planet, GJ 1214b, during transit with the high-resolution,
near-infrared NIRSPEC spectrograph on the Keck II telescope, in order to
characterize the planet's atmosphere. By cross-correlating the spectral changes
through transit with a suite of theoretical atmosphere models, we search for
variations associated with absorption in the planet atmosphere. Our
observations are sufficient to rule out tested model atmospheres with
wavelength-dependent transit depth variations >5e-4 over the wavelength range
2.1 - 2.4 micron. Our sensitivity is limited by variable slit loss and telluric
transmission effects.
We find no positive signatures but successfully rule out a number of
plausible atmospheric models, including the default assumption of a gaseous,
H-dominated atmosphere in chemical equilibrium. Such an atmosphere can be made
consistent if the absorption due to methane is reduced. Clouds can also render
such an atmosphere consistent with our observations, but only if they lie
higher in the atmosphere than indicated by recent optical and infrared
measurements.
When taken in concert with constraints from other groups, our results support
a consensus model in which the atmosphere of GJ 1214b contains significant H
and He, but where methane is depleted. If this depletion is the result of
photochemical processes, it may also produce a haze that suppresses spectral
features in the optical.Comment: 32 pages, 15 figures, preprint, accepted to ApJ, responded to
referee's comments. Comments welcom
Direct Imaging of Multiple Planets Orbiting the Star HR 8799
Direct imaging of exoplanetary systems is a powerful technique that can
reveal Jupiter-like planets in wide orbits, can enable detailed
characterization of planetary atmospheres, and is a key step towards imaging
Earth-like planets. Imaging detections are challenging due to the combined
effect of small angular separation and large luminosity contrast between a
planet and its host star. High-contrast observations with the Keck and Gemini
telescopes have revealed three planets orbiting the star HR 8799, with
projected separations of 24, 38, and 68 astronomical units. Multi-epoch data
show counter-clockwise orbital motion for all three imaged planets. The low
luminosity of the companions and the estimated age of the system imply
planetary masses between 5 and 13 times that of Jupiter. This system resembles
a scaled-up version of the outer portion of our Solar System.Comment: 30 pages, 5 figures, Research Article published online in Science
Express Nov 13th, 200
Privacy and Truthful Equilibrium Selection for Aggregative Games
We study a very general class of games --- multi-dimensional aggregative
games --- which in particular generalize both anonymous games and weighted
congestion games. For any such game that is also large, we solve the
equilibrium selection problem in a strong sense. In particular, we give an
efficient weak mediator: a mechanism which has only the power to listen to
reported types and provide non-binding suggested actions, such that (a) it is
an asymptotic Nash equilibrium for every player to truthfully report their type
to the mediator, and then follow its suggested action; and (b) that when
players do so, they end up coordinating on a particular asymptotic pure
strategy Nash equilibrium of the induced complete information game. In fact,
truthful reporting is an ex-post Nash equilibrium of the mediated game, so our
solution applies even in settings of incomplete information, and even when
player types are arbitrary or worst-case (i.e. not drawn from a common prior).
We achieve this by giving an efficient differentially private algorithm for
computing a Nash equilibrium in such games. The rates of convergence to
equilibrium in all of our results are inverse polynomial in the number of
players . We also apply our main results to a multi-dimensional market game.
Our results can be viewed as giving, for a rich class of games, a more robust
version of the Revelation Principle, in that we work with weaker informational
assumptions (no common prior), yet provide a stronger solution concept (ex-post
Nash versus Bayes Nash equilibrium). In comparison to previous work, our main
conceptual contribution is showing that weak mediators are a game theoretic
object that exist in a wide variety of games -- previously, they were only
known to exist in traffic routing games
Discovery of a 66 mas Ultracool Binary with Laser Guide Star Adaptive Optics
We present the discovery of 2MASS J21321145+1341584AB as a closely separated
(0.066") very low-mass field dwarf binary resolved in the near-infrared by the
Keck II Telescope using laser guide star adaptive optics. Physical association
is deduced from the angular proximity of the components and constraints on
their common proper motion. We have obtained a near-infrared spectrum of the
binary and find that it is best described by an L5+/-0.5 primary and an
L7.5+/-0.5 secondary. Model-dependent masses predict that the two components
straddle the hydrogen burning limit threshold with the primary likely stellar
and the secondary likely substellar. The properties of this sytem - close
projected separation (1.8+/-0.3 AU) and near unity mass ratio - are consistent
with previous results for very low-mass field binaries. The relatively short
estimated orbital period of this system (~7-12 yr) makes it a good target for
dynamical mass measurements. Interestingly, the system's angular separation is
the tightest yet for any very low-mass binary published from a ground-based
telescope and is the tightest binary discovered with laser guide star adaptive
optics to date.Comment: 10 pages, 3 figures; accepted for publication to A
Structure and evolution of the first CoRoT exoplanets: Probing the Brown Dwarf/Planet overlapping mass regime
We present detailed structure and evolution calculations for the first
transiting extrasolar planets discovered by the space-based CoRoT mission.
Comparisons between theoretical and observed radii provide information on the
internal composition of the CoRoT objects. We distinguish three different
categories of planets emerging from these discoveries and from previous
ground-based surveys: (i) planets explained by standard planetary models
including irradiation, (ii) abnormally bloated planets and (iii) massive
objects belonging to the overlapping mass regime between planets and brown
dwarfs. For the second category, we show that tidal heating can explain the
relevant CoRoT objects, providing non-zero eccentricities. We stress that the
usual assumption of a quick circularization of the orbit by tides, as usually
done in transit light curve analysis, is not justified a priori, as suggested
recently by Levrard et al. (2009), and that eccentricity analysis should be
carefully redone for some observations. Finally, special attention is devoted
to CoRoT-3b and to the identification of its very nature: giant planet or brown
dwarf ? The radius determination of this object confirms the theoretical
mass-radius predictions for gaseous bodies in the substellar regime but, given
the present observational uncertainties, does not allow an unambiguous
identification of its very nature. This opens the avenue, however, to an
observational identification of these two distinct astrophysical populations,
brown dwarfs and giant planets, in their overlapping mass range, as done for
the case of the 8 Jupiter-mass object Hat-P-2b. (abridged)Comment: 6 pages, 5 figures, accepted for publication in Astronomy and
Astrophysic
The physics of extrasolar gaseous planets : from theory to observable signatures
We review our present understanding of the physical properties of substellar
objects, brown dwarfs and irradiated or non-irradiated gaseous exoplanets. This
includes a description of their internal properties, mechanical structure and
heat content, their atmospheric properties, thermal profile and emergent
spectrum, and their evolution, in particular as irradiated companions of a
close parent star. The general theory can be used to make predictions in term
of detectability for the future observational projects. Special attention is
devoted to the evolution of the two presently detected transit planets,
HD209458B and OGLE-TR-56B. For this latter, we present a consistent evolution
for its recently revised mass and show that we reproduce the observed radius
within its error bars. We briefly discuss differences between brown dwarfs and
gaseous planets, both in terms of mass function and formation process. We
outline several arguments to show that the minimum mass for deuterium burning,
recently adopted officially as the limit to distinguish the two types of
objects, is unlikely to play any specific role in star formation, so that such
a limit is of purely semantic nature and is not supported by a physical
justification.Comment: 21 pages, 3 figure
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