895 research outputs found
Is our Sun a Singleton?
Most stars are formed in a cluster or association, where the number density
of stars can be high. This means that a large fraction of initially-single
stars will undergo close encounters with other stars and/or exchange into
binaries. We describe how such close encounters and exchange encounters can
affect the properties of a planetary system around a single star. We define a
singleton as a single star which has never suffered close encounters with other
stars or spent time within a binary system. It may be that planetary systems
similar to our own solar system can only survive around singletons. Close
encounters or the presence of a stellar companion will perturb the planetary
system, often leaving planets on tighter and more eccentric orbits. Thus
planetary systems which initially resembled our own solar system may later more
closely resemble some of the observed exoplanet systems.Comment: 2 pages, 1 figure. To be published in the proceedings of IAUS246
"Dynamical Evolution of Dense Stellar Systems". Editors: E. Vesperini (Chief
Editor), M. Giersz, A. Sill
Comparing HARPS and Kepler surveys: The alignment of multiple-planet systems
Aims. We study a subset of the planetary population characterized both by
HARPS and Kepler surveys. We compare the statistical properties of planets in
systems with m.sin i >5-10 M_Earth and R>2 R_Earth. If we assume that the
underlying population has the same characteristics, the different detection
sensitivity to the orbital inclination relative to the line of sight allows us
to probe the planets' mutual inclination.
Methods. We considered the frequency of systems with one, two and three
planets as dictated by HARPS data. We used Kepler's planetary period and host
mass and radii distributions (corrected from detection bias) to model planetary
systems in a simple yet physically plausible way. We then varied the mutual
inclination between planets in a system according to different prescriptions
(completely aligned, Rayleigh distributions and isotropic) and compared the
transit frequencies with one, two or three planets with those measured by
Kepler.
Results. The results show that the two datasets are compatible, a remarkable
result especially because there are no tunable knobs other than the assumed
inclination distribution. For m.sin i cutoffs of 7-10 M_Earth, which are those
expected to correspond to the radius cutoff of 2 R_Earth, we conclude that the
results are better described by a Rayleigh distribution with mode of 1 deg or
smaller. We show that the best-fit scenario only becomes a Rayleigh
distribution with mode of 5 deg if we assume a rather extreme mass-radius
relationship for the planetary population.
Conclusions. These results have important consequences for our understanding
of the role of several proposed formation and evolution mechanisms. They
confirm that planets are likely to have been formed in a disk and show that
most planetary systems evolve quietly without strong angular momentum exchanges
(abridged).Comment: 10 pages, 6 figures, 4 tables, accepted for publication in Astronomy
& Astrophysic
Debris discs in binaries: a numerical study
Debris disc analysis and modelling provide crucial information about the
structure and the processes at play in extrasolar planetary systems. In binary
systems, this issue is more complex because the disc should in addition respond
to the companion star's perturbations. We explore the dynamical evolution of a
collisionally active debris disc for different initial parent body populations,
diverse binary configurations and optical depths. We focus on the radial extent
and size distribution of the disc at a stationary state. We numerically follow
the evolution of massless small grains, initially produced from a
circumprimary disc of parent bodies following a size distribution in ds . Grains are submitted to both stars' gravity as well as
radiation pressure. In addition, particles are assigned an empirically derived
collisional lifetime. For all the binary configurations the disc extends far
beyond the critical semimajor axis for orbital stability. This is due
to the steady production of small grains, placed on eccentric orbits reaching
beyond by radiation pressure. The amount of matter beyond acrit
depends on the balance between collisional production and dynamical removal
rates: it increases for more massive discs as well as for eccentric binaries.
Another important effect is that, in the dynamically stable region, the disc is
depleted from its smallest grains. Both results could lead to observable
signatures. We have shown that a companion star can never fully truncate a
collisionally active disc. For eccentric companions, grains in the unstable
regions can significantly contribute to the thermal emission in the mid-IR.
Discs with sharp outer edges, especially bright ones such as HR4796A, are
probably shaped by other mechanisms.Comment: accepted for publication in A&
Long-time discrete particle effects versus kinetic theory in the self-consistent single-wave model
The influence of the finite number N of particles coupled to a monochromatic
wave in a collisionless plasma is investigated. For growth as well as damping
of the wave, discrete particle numerical simulations show an N-dependent long
time behavior resulting from the dynamics of individual particles. This
behavior differs from the one due to the numerical errors incurred by Vlasov
approaches. Trapping oscillations are crucial to long time dynamics, as the
wave oscillations are controlled by the particle distribution inhomogeneities
and the pulsating separatrix crossings drive the relaxation towards thermal
equilibrium.Comment: 11 pages incl. 13 figs. Phys. Rev. E, in pres
WASP-23b: a transiting hot Jupiter around a K dwarf and its Rossiter-McLaughlin effect
We report the discovery of a new transiting planet in the Southern
Hemisphere. It has been found by the WASP-south transit survey and confirmed
photometrically and spectroscopically by the 1.2m Swiss Euler telescope, LCOGT
2m Faulkes South Telescope, the 60 cm TRAPPIST telescope and the ESO 3.6m
telescope. The orbital period of the planet is 2.94 days. We find it is a gas
giant with a mass of 0.88 \pm 0.10 Mj and a radius estimated at 0.96 \pm 0.05
Rj . We have also obtained spectra during transit with the HARPS spectrograph
and detect the Rossiter-McLaughlin effect despite its small amplitude. Because
of the low signal to noise of the effect and of a small impact parameter we
cannot place a constraint on the projected spin-orbit angle. We find two
confiicting values for the stellar rotation. Our determination, via spectral
line broadening gives v sin I = 2.2 \pm 0.3 km/s, while another method, based
on the activity level using the index log R'HK, gives an equatorial rotation
velocity of only v = 1.35 \pm 0.20 km/s. Using these as priors in our analysis,
the planet could either be misaligned or aligned. This should send strong
warnings regarding the use of such priors. There is no evidence for
eccentricity nor of any radial velocity drift with time.Comment: 13 pages, 8 figures, 7 tables, accepted for publication in A&
The EBLM Project I-Physical and orbital parameters, including spin-orbit angles, of two low-mass eclipsing binaries on opposite sides of the Brown Dwarf limit
This paper introduces a series of papers aiming to study the dozens of low
mass eclipsing binaries (EBLM), with F, G, K primaries, that have been
discovered in the course of the WASP survey. Our objects are mostly single-line
binaries whose eclipses have been detected by WASP and were initially followed
up as potential planetary transit candidates. These have bright primaries,
which facilitates spectroscopic observations during transit and allows the
study of the spin-orbit distribution of F, G, K+M eclipsing binaries through
the Rossiter-McLaughlin effect. Here we report on the spin-orbit angle of
WASP-30b, a transiting brown dwarf, and improve its orbital parameters. We also
present the mass, radius, spin-orbit angle and orbital parameters of a new
eclipsing binary, J1219-39b (1SWAPJ121921.03-395125.6, TYC 7760-484-1), which,
with a mass of 95 +/- 2 Mjup, is close to the limit between brown dwarfs and
stars. We find that both objects orbit in planes that appear aligned with their
primaries' equatorial planes. Neither primaries are synchronous. J1219-39b has
a modestly eccentric orbit and is in agreement with the theoretical
mass--radius relationship, whereas WASP-30b lies above it.Comment: 12 pages, 7 figures, data in appendices, submitted to A&A (taking in
account 1st referee report
Debris disks as signposts of terrestrial planet formation. II Dependence of exoplanet architectures on giant planet and disk properties
We present models for the formation of terrestrial planets, and the
collisional evolution of debris disks, in planetary systems that contain
multiple unstable gas giants. We previously showed that the dynamics of the
giant planets introduces a correlation between the presence of terrestrial
planets and debris disks. Here we present new simulations that show that this
connection is qualitatively robust to changes in: the mass distribution of the
giant planets, the width and mass distribution of the outer planetesimal disk,
and the presence of gas in the disk. We discuss how variations in these
parameters affect the evolution. Systems with equal-mass giant planets undergo
the most violent instabilities, and these destroy both terrestrial planets and
the outer planetesimal disks that produce debris disks. In contrast, systems
with low-mass giant planets efficiently produce both terrestrial planets and
debris disks. A large fraction of systems with low-mass outermost giant planets
have stable gaps between these planets that are frequently populated by
planetesimals. Planetesimal belts between outer giant planets may affect debris
disk SEDs. If Earth-mass seeds are present in outer planetesimal disks, the
disks radially spread to colder temperatures. We argue that this may explain
the very low frequency of > 1 Gyr-old solar-type stars with observed 24 micron
excesses. Among the (limited) set of configurations explored, the best
candidates for hosting terrestrial planets at ~1 AU are stars older than 0.1-1
Gyr with bright debris disks at 70 micron but with no currently-known giant
planets. These systems combine evidence for rocky building blocks, with giant
planet properties least likely to undergo destructive dynamical evolution. We
predict an anti-correlation between debris disks and eccentric giant planets,
and a positive correlation between debris disks and terrestrial planets.Comment: Astronomy and Astrophysics, in press. Movies from simulations are at
http://www.obs.u-bordeaux1.fr/e3arths/raymond/movies_debris.htm
Design and implementation of GRIP: a computerized glucose control system at a surgical intensive care unit
BACKGROUND: Tight glucose control by intensive insulin therapy has become a key part of critical care and is an important field of study in acute coronary care. A balance has to be found between frequency of measurements and the risk of hypoglycemia. Current nurse-driven protocols are paper-based and, therefore, rely on simple rules. For safety and efficiency a computer decision support system that employs complex logic may be superior to paper protocols. METHODS: We designed and implemented GRIP, a stand-alone Java computer program. Our implementation of GRIP will be released as free software. Blood glucose values measured by a point-of-care analyzer were automatically retrieved from the central laboratory database. Additional clinical information was asked from the nurse and the program subsequently advised a new insulin pump rate and glucose sampling interval. RESULTS: Implementation of the computer program was uneventful and successful. GRIP treated 179 patients for a total of 957 patient-days. Severe hypoglycemia (< 2.2 mmol/L) only occurred once due to human error. With a median (IQR) of 4.9 (4.2 – 6.2) glucose measurements per day the median percentage of time in which glucose fell in the target range was 78%. Nurses rated the program as easy to work with and as an improvement over the preceding paper protocol. They reported no increase in time spent on glucose control. CONCLUSION: A computer driven protocol is a safe and effective means of glucose control at a surgical ICU. Future improvements in the recommendation algorithm may further improve safety and efficiency
Quasispecies Theory for Horizontal Gene Transfer and Recombination
We introduce a generalization of the parallel, or Crow-Kimura, and Eigen
models of molecular evolution to represent the exchange of genetic information
between individuals in a population. We study the effect of different schemes
of genetic recombination on the steady-state mean fitness and distribution of
individuals in the population, through an analytic field theoretic mapping. We
investigate both horizontal gene transfer from a population and recombination
between pairs of individuals. Somewhat surprisingly, these nonlinear
generalizations of quasi-species theory to modern biology are analytically
solvable. For two-parent recombination, we find two selected phases, one of
which is spectrally rigid. We present exact analytical formulas for the
equilibrium mean fitness of the population, in terms of a maximum principle,
which are generally applicable to any permutation invariant replication rate
function. For smooth fitness landscapes, we show that when positive epistatic
interactions are present, recombination or horizontal gene transfer introduces
a mild load against selection. Conversely, if the fitness landscape exhibits
negative epistasis, horizontal gene transfer or recombination introduce an
advantage by enhancing selection towards the fittest genotypes. These results
prove that the mutational deterministic hypothesis holds for quasi-species
models. For the discontinuous single sharp peak fitness landscape, we show that
horizontal gene transfer has no effect on the fitness, while recombination
decreases the fitness, for both the parallel and the Eigen models. We present
numerical and analytical results as well as phase diagrams for the different
cases.Comment: 54 pages; 8 figures; 12 tables; some typos corrected; to appear in
Phys. Rev.
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