1,590 research outputs found
The spin glass transition of the three dimensional Heisenberg spin glass
It is shown, by means of Monte Carlo simulation and Finite Size Scaling
analysis, that the Heisenberg spin glass undergoes a finite-temperature phase
transition in three dimensions. There is a single critical temperature, at
which both a spin glass and a chiral glass orderings develop. The Monte Carlo
algorithm, adapted from lattice gauge theory simulations, makes possible to
thermalize lattices of size L=32, larger than in any previous spin glass
simulation in three dimensions. High accuracy is reached thanks to the use of
the Marenostrum supercomputer. The large range of system sizes studied allow us
to consider scaling corrections.Comment: 4 pages, 4 Postscript figures, version to be published in Physical
Review Letter
VLBI for Gravity Probe B. V. Proper Motion and Parallax of the Guide Star, IM Pegasi
We present the principal astrometric results of the very-long-baseline
interferometry (VLBI) program undertaken in support of the Gravity Probe B
(GP-B) relativity mission. VLBI observations of the GP-B guide star, the RS CVn
binary IM Pegasi (HR 8703), yielded positions at 35 epochs between 1997 and
2005. We discuss the statistical assumptions behind these results and our
methods for estimating the systematic errors. We find the proper motion of IM
Peg in an extragalactic reference frame closely related to the International
Celestial Reference Frame 2 (ICRF2) to be -20.83 +- 0.03 +- 0.09 mas/yr in
right ascension and -27.27 +- 0.03 +- 0.09 mas/yr in declination. For each
component the first uncertainty is the statistical standard error and the
second is the total standard error (SE) including plausible systematic errors.
We also obtain a parallax of 10.37 +- 0.07 mas (distance: 96.4 +- 0.7 pc), for
which there is no evidence of any significant contribution of systematic error.
Our parameter estimates for the ~25-day-period orbital motion of the stellar
radio emission have SEs corresponding to ~0.10 mas on the sky in each
coordinate. The total SE of our estimate of IM Peg's proper motion is ~30%
smaller than the accuracy goal set by the GP-B project before launch: 0.14
mas/yr for each coordinate of IM Peg's proper motion. Our results ensure that
the uncertainty in IM Peg's proper motion makes only a very small contribution
to the uncertainty of the GP-B relativity tests.Comment: Accepted for publication in the Astrophysical Journal Supplement
Serie
Exploring the alpha-enhancement of metal-poor planet-hosting stars. The Kepler and HARPS samples
Recent studies showed that at low metallicities Doppler-detected
planet-hosting stars have preferably high alpha-content and belong to the thick
disk. We used the reconnaissance spectra of 87 Kepler planet candidates and
data available from the HARPS planet search survey to explore this phenomena.
Using the traditional spectroscopic abundance analysis methods we derived Ti,
Ca, and Cr abundances for the Kepler stars. In the metallicity region -0.65 <
[Fe/H] < -0.3 dex the fraction of Ti-enhanced thick-disk HARPS planet harboring
stars is 12.3 +/- 4.1 % and for their thin-disk counterparts this fraction is
2.2 +/- 1.3 %. The binomial statistics gives a probability of 0.008 that this
could have occurred by chance. Combining the two samples (HARPS + Kepler)
reinforces the significance of this result (P ~ 99.97 %). Since most of these
stars are harboring small-mass/size planets we can assume that, although
terrestrial planets can be found at low-iron regime, they are mostly enhanced
by alpha-elements. This implies that early formation of rocky planets could get
started in the Galactic thick disk, where the chemical conditions for their
formation were more favorable.Comment: 5 pages, 3 figure
Spin and chirality orderings of the one-dimensional Heisenberg spin glass with the long-range power-law interaction
The ordering of the one-dimensional Heisenberg spin glass interacting via the
long-range power-law interaction is studied by Monte Carlo simulations.
Particular attention is paid to the possible occurrence of the ``spin-chirality
decoupling'' for appropriate values of the power-law exponent \sigma. Our
result suggests that, for intermediate values of , the chiral-glass
order occurs at finite temperatures while the standard spin-glass order occurs
only at zero temperature.Comment: Proceedings of the Highly Frustrated Magnetism (HFM2006) conference.
To appear in a special issue of J. Phys. Condens. Matte
Comment on "Spin-glass transition of the three-dimensional Heisenberg spin glass" - Reply
© 2007 American Physical Society.Depto. de Física TeóricaFac. de Ciencias FísicasTRUEpu
Formation and structure of the three Neptune-mass planets system around HD69830
Since the discovery of the first giant planet outside the solar system in
1995 (Mayor & Queloz 1995), more than 180 extrasolar planets have been
discovered. With improving detection capabilities, a new class of planets with
masses 5-20 times larger than the Earth, at close distance from their parent
star is rapidly emerging. Recently, the first system of three Neptune-mass
planets has been discovered around the solar type star HD69830 (Lovis et al.
2006). Here, we present and discuss a possible formation scenario for this
planetary system based on a consistent coupling between the extended core
accretion model and evolutionary models (Alibert et al. 2005a, Baraffe et al.
2004,2006). We show that the innermost planet formed from an embryo having
started inside the iceline is composed essentially of a rocky core surrounded
by a tiny gaseous envelope. The two outermost planets started their formation
beyond the iceline and, as a consequence, accrete a substantial amount of water
ice during their formation. We calculate the present day thermodynamical
conditions inside these two latter planets and show that they are made of a
rocky core surrounded by a shell of fluid water and a gaseous envelope.Comment: Accepted in AA Letter
Nature of vibrational eigenmodes in topologically disordered solids
We use a local projectional analysis method to investigate the effect of
topological disorder on the vibrational dynamics in a model glass simulated by
molecular dynamics. Evidence is presented that the vibrational eigenmodes in
the glass are generically related to the corresponding eigenmodes of its
crystalline counterpart via disorder-induced level-repelling and hybridization
effects. It is argued that the effect of topological disorder in the glass on
the dynamical matrix can be simulated by introducing positional disorder in a
crystalline counterpart.Comment: 7 pages, 6 figures, PRB, to be publishe
Redistribution of Adhesive Forces through Src/FAK Drives Contact Inhibition of Locomotion in Neural Crest
Contact inhibition of locomotion is defined as the behavior of cells to cease migrating in their former direction after colliding with another cell. It has been implicated in multiple developmental processes and its absence has been linked to cancer invasion. Cellular forces are thought to govern this process; however, the exact role of traction through cell-matrix adhesions and tension through cell-cell adhesions during contact inhibition of locomotion remains unknown. Here we use neural crest cells to address this and show that cell-matrix adhesions are rapidly disassembled at the contact between two cells upon collision. This disassembly is dependent upon the formation of N-cadherin-based cell-cell adhesions and driven by Src and FAK activity. We demonstrate that the loss of cell-matrix adhesions near the contact leads to a buildup of tension across the cell-cell contact, a step that is essential to drive cell-cell separation after collision
Li depletion in solar analogues with exoplanets: Extending the sample
We want to study the effects of the formation of planets and planetary
systems on the atmospheric Li abundance of planet host stars. In this work we
present new determinations of lithium abundances for 326 Main Sequence stars
with and without planets in the T range 5600-5900 K. 277 stars
come from the HARPS sample, the remaining targets have been observed with a
variety of high resolution spectrographs. We confirm significant differences in
the Li distribution of solar twins (T = T 80 K,
log g = log g 0.2 and [Fe/H] = [Fe/H] 0.2): the
full sample of planet host stars (22) shows Li average values lower than
"single" stars with no detected planets (60). If we focus in subsamples with
narrower ranges in metallicity and age, we observe indications of a similar
result though it is not so clear for some of the studied subsamples.
Furthermore, we compare the observed spectra of several couples of stars with
very similar parameters which show different Li abundances up to 1.6 dex.
Therefore we show that neither age, nor mass nor metallicity of a parent star
is the only responsible for enhanced Li depletion in solar analogues. We
conclude that another variable must account for that difference and suggest
that this could be the presence of planets which causes additional rotationally
induced mixing in the external layers of planet host stars. Moreover, we find
indications that the amount of depletion of Li in planet host solar-type stars
is higher when the planets are more massive than Jupiter.Comment: 16 pages, accepted for publication in A&
Evidence for a spectroscopic direct detection of reflected light from 51 Peg b
The detection of reflected light from an exoplanet is a difficult technical
challenge at optical wavelengths. Even though this signal is expected to
replicate the stellar signal, not only is it several orders of magnitude
fainter, but it is also hidden among the stellar noise. We apply a variant of
the cross-correlation technique to HARPS observations of 51 Peg to detect the
reflected signal from planet 51 Peg b. Our method makes use of the
cross-correlation function of a binary mask with high-resolution spectra to
amplify the minute planetary signal that is present in the spectra by a factor
proportional to the number of spectral lines when performing the cross
correlation. The resulting cross-correlation functions are then normalized by a
stellar template to remove the stellar signal. Carefully selected sections of
the resulting normalized CCFs are stacked to increase the planetary signal
further. The recovered signal allows probing several of the planetary
properties, including its real mass and albedo. We detect evidence for the
reflected signal from planet 51 Peg b at a significance of 3\sigma_noise. The
detection of the signal permits us to infer a real mass of 0.46^+0.06_-0.01
M_Jup (assuming a stellar mass of 1.04\;M_Sun) for the planet and an orbital
inclination of 80^+10_-19 degrees. The analysis of the data also allows us to
infer a tentative value for the (radius-dependent) geometric albedo of the
planet. The results suggest that 51Peg b may be an inflated hot Jupiter with a
high albedo (e.g., an albedo of 0.5 yields a radius of 1.9 \pm 0.3 R_Jup for a
signal amplitude of 6.0\pm0.4 x 10^-5). We confirm that the method we perfected
can be used to retrieve an exoplanet's reflected signal, even with current
observing facilities. The advent of next generation of observing facilities
will yield new opportunities for this type of technique to probe deeper into
exoplanets.Comment: 9 pages, 6 figure
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