730 research outputs found
Elemental abundances and minimum mass of heavy elements in the envelope of HD 189733b
Oxygen (O) and carbon (C) have been inferred recently to be subsolar in
abundance from spectra of the atmosphere of the transiting hot Jupiter HD
189733b. Yet, the mass and radius of the planet coupled with structure models
indicate a strongly supersolar abundance of heavy elements in the interior of
this object. Here we explore the discrepancy between the large amount of heavy
elements suspected in the planet's interior and the paucity of volatiles
measured in its atmosphere. We describe the formation sequence of the icy
planetesimals formed beyond the snow line of the protoplanetary disk and
calculate the composition of ices ultimately accreted in the envelope of HD
189733b on its migration pathway. This allows us to reproduce the observed
volatile abundances by adjusting the mass of ices vaporized in the envelope.
The predicted elemental mixing ratios should be 0.15--0.3 times solar in the
envelope of HD 189733b if they are fitted to the recent O and C determinations.
However, our fit to the minimum mass of heavy elements predicted by internal
structure models gives elemental abundances that are 1.2--2.4 times oversolar
in the envelope of HD189733b. We propose that the most likely cause of this
discrepancy is irradiation from the central star leading to development of a
radiative zone in the planet's outer envelope which would induce gravitational
settling of elements. Hence, all strongly irradiated extrasolar planets should
present subsolar abundances of volatiles. We finally predict that the
abundances of nitrogen (N), sulfur (S) and phosphorus (P) are of , and relative to
H, respectively in the atmosphere of HD 189733b.Comment: Accepted for publication in Astronomy & Astrophysic
Global Models of Planet Formation and Evolution
Despite the increase in observational data on exoplanets, the processes that
lead to the formation of planets are still not well understood. But thanks to
the high number of known exoplanets, it is now possible to look at them as a
population that puts statistical constraints on theoretical models. A method
that uses these constraints is planetary population synthesis. Its key element
is a global model of planet formation and evolution that directly predicts
observable planetary properties based on properties of the natal protoplanetary
disk. To do so, global models build on many specialized models that address one
specific physical process. We thoroughly review the physics of the sub-models
included in global formation models. The sub-models can be classified as models
describing the protoplanetary disk (gas and solids), the (proto)planet (solid
core, gaseous envelope, and atmosphere), and finally the interactions
(migration and N-body interaction). We compare the approaches in different
global models and identify physical processes that require improved
descriptions in future. We then address important results of population
synthesis like the planetary mass function or the mass-radius relation. In
these results, the global effects of physical mechanisms occurring during
planet formation and evolution become apparent, and specialized models
describing them can be put to the observational test. Due to their nature as
meta models, global models depend on the development of the field of planet
formation theory as a whole. Because there are important uncertainties in this
theory, it is likely that global models will in future undergo significant
modifications. Despite this, they can already now yield many testable
predictions. With future global models addressing the geophysical
characteristics, it should eventually become possible to make predictions about
the habitability of planets.Comment: 30 pages, 16 figures. Accepted for publication in the International
Journal of Astrobiology (Cambridge University Press
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
Chemical composition of Earth-like planets
Models of planet formation are mainly focused on the accretion and dynamical
processes of the planets, neglecting their chemical composition. In this work,
we calculate the condensation sequence of the different chemical elements for a
low-mass protoplanetary disk around a solar-type star. We incorporate this
sequence of chemical elements (refractory and volatile elements) in our
semi-analytical model of planet formation which calculates the formation of a
planetary system during its gaseous phase. The results of the semi-analytical
model (final distributions of embryos and planetesimals) are used as initial
conditions to develope N-body simulations that compute the post-oligarchic
formation of terrestrial-type planets. The results of our simulations show that
the chemical composition of the planets that remain in the habitable zone has
similar characteristics to the chemical composition of the Earth. However,
exist differences that can be associated to the dynamical environment in which
they were formed.Comment: 3 pages, 4 figures - Accepted for publication in the Bolet\'in de la
Asociaci\'on Argentina de Astronom\'ia, vol.5
Giant Planet Formation, Evolution, and Internal Structure
The large number of detected giant exoplanets offers the opportunity to
improve our understanding of the formation mechanism, evolution, and interior
structure of gas giant planets. The two main models for giant planet formation
are core accretion and disk instability. There are substantial differences
between these formation models, including formation timescale, favorable
formation location, ideal disk properties for planetary formation, early
evolution, planetary composition, etc. First, we summarize the two models
including their substantial differences, advantages, and disadvantages, and
suggest how theoretical models should be connected to available (and future)
data. We next summarize current knowledge of the internal structures of solar-
and extrasolar- giant planets. Finally, we suggest the next steps to be taken
in giant planet exploration.Comment: Accepted for publication as a chapter in Protostars and Planets VI,
to be published in 2014 by University of Arizona Pres
The formation of Jupiter by hybrid pebble-planetesimal accretion
The standard model for giant planet formation is based on the accretion of
solids by a growing planetary embryo, followed by rapid gas accretion once the
planet exceeds a so-called critical mass. The dominant size of the accreted
solids (cm-size particles named pebbles or km to hundred km-size bodies named
planetesimals) is, however, unknown. Recently, high-precision measurements of
isotopes in meteorites provided evidence for the existence of two reservoirs in
the early Solar System. These reservoirs remained separated from ~1 until ~ 3
Myr after the beginning of the Solar System's formation. This separation is
interpreted as resulting from Jupiter growing and becoming a barrier for
material transport. In this framework, Jupiter reached ~20 Earth masses within
~1 Myr and slowly grew to ~50 Earth masses in the subsequent 2 Myr before
reaching its present-day mass. The evidence that Jupiter slowed down its growth
after reaching 20 Earth masses for at least 2 Myr is puzzling because a planet
of this mass is expected to trigger fast runaway gas accretion. Here, we use
theoretical models to describe the conditions allowing for such a slow
accretion and show that Jupiter grew in three distinct phases. First, rapid
pebble accretion brought the major part of Jupiter's core mass. Second, slow
planetesimal accretion provided the energy required to hinder runaway gas
accretion during 2 Myr. Third, runaway gas accretion proceeded. Both pebbles
and planetesimals therefore have an important role in Jupiter's formation.Comment: Published in Nature Astronomy on August 27, 201
A Myc-regulated transcriptional network controls B-cell fate in response to BCR triggering
BACKGROUND: The B cell antigen receptor (BCR) is a signaling complex that mediates the differentiation of stage-specific cell fate decisions in B lymphocytes. While several studies have shown differences in signal transduction components as being key to contrasting phenotypic outcomes, little is known about the differential BCR-triggered gene transcription downstream of the signaling cascades. RESULTS: Here we define the transcriptional changes that underlie BCR-induced apoptosis and proliferation of immature and mature B cells, respectively. Comparative genome-wide expression profiling identified 24 genes that discriminated between the early responses of the two cell types to BCR stimulation. Using mice with a conditional Myc-deletion, we validated the microarray data by demonstrating that Myc is critical to promoting BCR-triggered B-cell proliferation. We further investigated the Myc-dependent molecular mechanisms and found that Myc promotes a BCR-dependent clonal expansion of mature B cells by inducing proliferation and inhibiting differentiation. CONCLUSION: This work provides the first comprehensive analysis of the early transcriptional events that lead to either deletion or clonal expansion of B cells upon antigen recognition, and demonstrates that Myc functions as the hub of a transcriptional network that control B-cell fate in the periphery
Beat Cepheids as Probes of Stellar and Galactic Metallicity
The mere location of a Beat Cepheid model in a Period Ratio vs. Period
diagram (Petersen diagram) puts very tight constraints on its metallicity Z.
The Beat Cepheid Peterson diagrams are revisited with linear nonadiabatic
turbulent convective models, and their accuracy as a probe for stellar
metallicity is evaluated. They are shown to be largely independent of the
helium content Y, and they are also only weakly dependent on the
mass-luminosity relation that is used in their construction. However, they are
found to show sensitivity to the relative abundances of the elements that are
lumped into the metallicity parameter Z. Rotation is estimated to have but a
small effect on the 'pulsation metallicities'. A composite Petersen diagram is
presented that allows one to read off upper and lower limits on the metallicity
Z from the measured period P0 and period ratio P1/P0.Comment: 9 pages, 12 color figures (black and white version available from 1st
author's website). With minor revisions. to appear in Ap
Classical Cepheid Pulsation Models: IX. New Input Physics
We constructed several sequences of classical Cepheid envelope models at
solar chemical composition () to investigate the dependence of
the pulsation properties predicted by linear and nonlinear hydrodynamical
models on input physics. To study the dependence on the equation of state (EOS)
we performed several numerical experiments by using the simplified analytical
EOS originally developed by Stellingwerf and the recent analytical EOS
developed by Irwin. Current findings suggest that the pulsation amplitudes as
well as the topology of the instability strip marginally depend on the adopted
EOS.
We also investigated the dependence of observables predicted by theoretical
models on the mass-luminosity (ML) relation and on the spatial resolution
across the Hydrogen and the Helium partial ionization regions. We found that
nonlinear models are marginally affected by these physical and numerical
assumptions. In particular, the difference between new and old models in the
location as well as in the temperature width of the instability strip is on
average smaller than 200 K. However, the spatial resolution somehow affects the
pulsation properties. The new fine models predict a period at the center of the
Hertzsprung Progression (9.84 days) that reasonably agree with
empirical data based on light curves ( days;
\citealt{mbm92}) and on radial velocity curves ( days;
\citealt{mall00}), and improve previous predictions by Bono, Castellani, and
Marconi (2000, hereinafter BCM00).Comment: 35 pages, 7 figures. Accepted for publication in the Astrophysical
Journa
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