1,902 research outputs found
Planetary internal structures
This chapter reviews the most recent advancements on the topic of terrestrial
and giant planet interiors, including Solar System and extrasolar objects.
Starting from an observed mass-radius diagram for known planets in the
Universe, we will discuss the various types of planets appearing in this
diagram and describe internal structures for each type. The review will
summarize the status of theoretical and experimental works performed in the
field of equation of states (EOS) for materials relevant to planetary interiors
and will address the main theoretical and experimental uncertainties and
challenges. It will discuss the impact of new EOS on interior structures and
bulk composition determination. We will discuss important dynamical processes
which strongly impact the interior and evolutionary properties of planets (e.g
plate tectonics, semiconvection) and describe non standard models recently
suggested for our giant planets. We will address the case of short-period,
strongly irradiated exoplanets and critically analyse some of the physical
mechanisms which have been suggested to explain their anomalously large radius.Comment: 24 pages, 8 figures, Accepted for publication as a chapter in
Protostars and Planets VI, University of Arizona Press (2014), eds. H.
Beuther, R. Klessen, C. Dullemond, Th. Henning.
Atmospheres and radiating surfaces of neutron stars with strong magnetic fields
We review the current status of the theory of thermal emission from the
surface layers of neutron stars with strong magnetic fields G, including formation of the spectrum in a partially ionized
atmosphere and at a condensed surface. In particular, we describe recent
progress in modeling partially ionized atmospheres of central compact objects
in supernova remnants, which may have moderately strong fields G. Special attention is given to polarization of thermal
radiation emitted by a neutron star surface. Finally, we briefly describe
applications of the theory to observations of thermally emitting isolated
neutron stars.Comment: 27 pages, 5 figures, invited review at the conference "The Modern
Physics of Compact Stars 2015" (Yerevan, Armenia, Sept. 30 - Oct. 3, 2015),
edited by R. Avagyan, A. Saharian, and A. Sedrakian. In v.2, a citation
(Ref.114) is correcte
Opacities and spectra of hydrogen atmospheres of moderately magnetized neutron stars
There is observational evidence that central compact objects (CCOs) in
supernova remnants have moderately strong magnetic fields G.
Meanwhile, available models of partially ionized hydrogen atmospheres of
neutron stars with strong magnetic fields are restricted to
G. We extend the equation of state and radiative opacities, presented in
previous papers for 10^{12}\mbox{ G}\lesssim B \lesssim 10^{15} G, to weaker
fields. An equation of state and radiative opacities for a partially ionized
hydrogen plasma are obtained at magnetic fields , temperatures , and
densities typical for atmospheres of CCOs and other isolated neutron
stars with moderately strong magnetic fields. The first- and second-order
thermodynamic functions, monochromatic radiative opacities, and Rosseland mean
opacities are calculated and tabulated, taking account of partial ionization,
for 3\times10^{10}\mbox{ G}\lesssim B\lesssim 10^{12} G, K K, and a wide range of densities. Atmosphere models and spectra
are calculated to verify the applicability of the results and to determine the
range of magnetic fields and effective temperatures where the incomplete
ionization of the hydrogen plasma is important.Comment: 11 pages, 7 figures, accepted for publication in A&
On high proper motion white dwarfs from photographic surveys
The interpretation of high proper motion white dwarfs detected by Oppenheimer
et al (2001) was the start of a lively controversy. While the discoverers
identify a large fraction of their findings as dark halo members, others
interpret the same sample as essentially made of disc and/or thick disc stars.
We use the comprehensive description of Galactic stellar populations provided
by the "Besancon" model to produce a realistic simulation of Oppenheimer et al.
data, including all observational selections and calibration biases. The
conclusion is unambiguous: Thick disc white dwarfs resulting from ordinary
hypotheses on the local density and kinematics are sufficient to explain the
observed objects, there is no need for halo white dwarfs. This conclusion is
robust to reasonable changes in model ingredients. The main cause of the
misinterpretation seems to be that the velocity distribution of a proper motion
selected star sample is severely biased in favour of high velocities. This has
been neglected in previous analyses. Obviously this does not prove that no such
objects like halo white dwarfs can exist, but Oppenheimer et al. observations
drive their possible contribution in the dark matter halo down to an extremely
low fraction.Comment: 4 pages, 1 figure, A&A Letters, accepte
Dark halo baryons not in ancient halo white dwarfs
Having ruled out the possibility that stellar objects are the main
contributor of the dark matter embedding galaxies, microlensing experiments
cannot exclude the hypothesis that a significant fraction of the Milky Way dark
halo might be made of MACHOs with masses in the range 0.5-0.8 \msun. Ancient
white dwarfs are generally considered the most plausible candidates for such
MACHOs. We report the results of a search for such white dwarfs in a proper
motion survey covering a 0.16 sqd field at three epochs at high galactic
latitude, and 0.938 sqd at two epochs at intermediate galactic latitude (VIRMOS
survey), using the CFH telescope. Both surveys are complete to I = 23, with
detection efficiency fading to 0 at I = 24.2. Proper motion data are suitable
to separate unambiguously halo white dwarfs identified by belonging to a non
rotating system. No candidates were found within the colour-magnitude-proper
motion volume where such objects can be safely discriminated from any standard
population as well as from possible artefacts. In the same volume, we estimate
the maximum white dwarf halo fraction compatible with this observation at
different significance levels if the halo is at least 14 gigayears old and
under different ad hoc initial mass functions. Our data alone rules out a halo
fraction greater than 14% at 95% confidence level. Combined with two previous
investigations exploring comparable volumes pushes the limit below 4 % (95%
confidence level) or below 1.3% (64% confidence), this implies that if baryonic
dark matter is present in galaxy halos, it is not, or it is only marginally in
the form of faint hydrogen white dwarfs.Comment: accepted in Astronomy and Astrophysics (19-05-2004
The Evolution of L and T Dwarfs in Color-Magnitude Diagrams
We present new evolution sequences for very low mass stars, brown dwarfs and
giant planets and use them to explore a variety of influences on the evolution
of these objects. We compare our results with previous work and discuss the
causes of the differences and argue for the importance of the surface boundary
condition provided by atmosphere models including clouds.
The L- to T-type ultracool dwarf transition can be accommodated within the
Ackerman & Marley (2001) cloud model by varying the cloud sedimentation
parameter. We develop a simple model for the evolution across the L/T
transition. By combining the evolution calculation and our atmosphere models,
we generate colors and magnitudes of synthetic populations of ultracool dwarfs
in the field and in galactic clusters. We focus on near infrared color-
magnitude diagrams (CMDs) and on the nature of the ``second parameter'' that is
responsible for the scatter of colors along the Teff sequence. Variations in
metallicity and cloud parameters, unresolved binaries and possibly a relatively
young population all play a role in defining the spread of brown dwarfs along
the cooling sequence. We find that the transition from cloudy L dwarfs to
cloudless T dwarfs slows down the evolution and causes a pile up of substellar
objects in the transition region, in contradiction with previous studies. We
apply the same model to the Pleiades brown dwarf sequence. Taken at face value,
the Pleiades data suggest that the L/T transition occurs at lower Teff for
lower gravity objects. The simulated populations of brown dwarfs also reveal
that the phase of deuterium burning produces a distinctive feature in CMDs that
should be detectable in ~50-100 Myr old clusters.Comment: Accepted for publication in the ApJ. 52 pages including 20 figure
Analysis of signalling pathways using continuous time Markov chains
We describe a quantitative modelling and analysis approach for signal transduction networks.
We illustrate the approach with an example, the RKIP inhibited ERK pathway [CSK+03]. Our models are high level descriptions of continuous time Markov chains: proteins are modelled by synchronous processes and reactions by transitions. Concentrations are modelled by discrete, abstract quantities. The main advantage of our approach is that using a (continuous time) stochastic logic and the PRISM model checker, we can perform quantitative analysis such as what is the probability that if a concentration reaches a certain level, it will remain at that level thereafter? or how does varying a given reaction rate affect that probability? We also perform standard simulations and compare our results with a traditional ordinary differential equation model. An interesting result is that for the example pathway, only a small number of discrete data values is required to render the simulations practically indistinguishable
Molecular line opacity of LiCl in the mid-infrared spectra of brown dwarfs
We present a complete line list for the X 1Sigma+ electronic ground state of
LiCl computed using fully quantum-mechanical techniques. This list includes
transition energies and oscillator strengths in the spectral region
0.3-39,640.7 cm-1 for all allowed rovibrational transitions in absorption
within the electronic ground state. The calculations were performed using an
accurate hybrid potential constructed from a spectral inversion fit of
experimental data and from recent multi-reference single- and double-excitation
configuration interaction calculations. The line list was incorporated into the
stellar atmosphere code PHOENIX to compute spectra for a range of young to old
T dwarf models. The possibility of observing a signature of LiCl in absorption
near 15.8 microns is addressed and the proposal to use this feature to estimate
the total lithium elemental abundance for these cool objects is discussed.Comment: 8 pages, 2 figures, 1 table. Accepted for publication in ApJ 613,
Sept. 20 200
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