2,345 research outputs found
Precision radial velocities of double-lined spectroscopic binaries with an iodine absorption cell
A spectroscopic technique employing an iodine absorption cell (I_2) to
superimpose a reference spectrum onto a stellar spectrum is currently the most
widely adopted approach to obtain precision radial velocities of solar-type
stars. It has been used to detect ~80 extrasolar planets out of ~130 know. Yet
in its original version, it only allows us to measure precise radial velocities
of single stars. In this paper, we present a novel method employing an I_2
absorption cell that enables us to accurately determine radial velocities of
both components of double-lined binaries. Our preliminary results based on the
data from the Keck I telescope and HIRES spectrograph demonstrate that 20-30
m/s radial velocity precision can be routinely obtained for "early" type
binaries (F3-F8). For later type binaries, the precision reaches ~10 m/s. We
discuss applications of the technique to stellar astronomy and searches for
extrasolar planets in binary systems. In particular, we combine the
interferometric data collected with the Palomar Testbed Interferometer with our
preliminary precision velocities of the spectroscopic double-lined binary HD
4676 to demonstrate that with such a combination one can routinely obtain
masses of the binary components accurate at least at the level of 1.0%.Comment: Accepted for publication in The Astrophysical Journa
On the high-density expansion for Euclidean Random Matrices
Diagrammatic techniques to compute perturbatively the spectral properties of
Euclidean Random Matrices in the high-density regime are introduced and
discussed in detail. Such techniques are developed in two alternative and very
different formulations of the mathematical problem and are shown to give
identical results up to second order in the perturbative expansion. One method,
based on writing the so-called resolvent function as a Taylor series, allows to
group the diagrams in a small number of topological classes, providing a simple
way to determine the infrared (small momenta) behavior of the theory up to
third order, which is of interest for the comparison with experiments. The
other method, which reformulates the problem as a field theory, can instead be
used to study the infrared behaviour at any perturbative order.Comment: 29 page
Extreme events driven glassy behaviour in granular media
Motivated by recent experiments on the approach to jamming of a weakly forced
granular medium using an immersed torsion oscillator [Nature 413 (2001) 407],
we propose a simple model which relates the microscopic dynamics to macroscopic
rearrangements and accounts for the following experimental facts: (1) the
control parameter is the spatial amplitude of the perturbation and not its
reduced peak acceleration; (2) a Vogel-Fulcher-Tammann-like form for the
relaxation time. The model draws a parallel between macroscopic rearrangements
in the system and extreme events whose probability of occurrence (and thus the
typical relaxation time) is estimated using extreme-value statistics. The range
of validity of this description in terms of the control parameter is discussed
as well as the existence of other regimes.Comment: 7 pages, to appear in Europhys. Let
PDGF controls contact inhibition of locomotion by regulating N-cadherin during neural crest migration
A fundamental property of neural crest (NC) migration is contact
inhibition of locomotion (CIL), a process by which cells change their
direction of migration upon cell contact. CIL has been proven to
be essential for NC migration in amphibians and zebrafish by
controlling cell polarity in a cell contact-dependent manner. Cell
contact during CIL requires the participation of the cell adhesion
molecule N-cadherin, which starts to be expressed by NC cells as a
consequence of the switch between E- and N-cadherins during
epithelial-to-mesenchymal transition (EMT). However, the mechanism
that controls the upregulation of N-cadherin remains unknown. Here,
we show that platelet-derived growth factor receptor alpha (PDGFRα)
and its ligand platelet-derived growth factor A (PDGF-A) are
co-expressed in migrating cranial NC. Inhibition of PDGF-A/PDGFRα
blocks NC migration by inhibiting N-cadherin and, consequently,
impairing CIL. Moreover, we identify phosphatidylinositol-3-kinase
(PI3K)/AKT as a downstream effector of the PDGFRα cellular
response during CIL. Our results lead us to propose PDGF-A/
PDGFRα signalling as a tissue-autonomous regulator of CIL by
controlling N-cadherin upregulation during EMT. Finally, we show that
once NC cells have undergone EMT, the same PDGF-A/PDGFRα
works as an NC chemoattractant, guiding their directional migration
A model for the doped copper oxide compounds
We present a relativistic spin-fermion model for the cuprates, in which both
the charge and spin degrees of freedom are treated dynamically. The spin-charge
coupling parameter is associated with the doping fraction. The model is able to
account for the various phases of the cuprates and their properties, not only
at low and intermediate doping but also for (highly) over-doped compounds. In
particular, we acquire a qualitative understanding of high-T_c
superconductivity through Bose-Einstein condensation of bound charge pairs. The
mechanism that binds these pairs does not require a Fermi sea.Comment: 9 pages, 2 postscript figures. Version accepted for publication in
Europhys. Let
The correspondence between long-range and short-range spin glasses
We compare the critical behavior of the short-range Ising spin glass with a
spin glass with long-range interactions which fall off as a power sigma of the
distance. We show that there is a value of sigma of the long-range model for
which the critical behavior is very similar to that of the short-range model in
four dimensions. We also study a value of sigma for which we find the critical
behavior to be compatible with that of the three dimensional model, though we
have much less precision than in the four-dimensional case.Comment: 15 pages, 10 figure
No planet for HD 166435
The G0V star HD166435 has been observed by the fiber-fed spectrograph ELODIE
as one of the targets in the large extra-solar planet survey that we are
conducting at the Observatory of Haute-Provence. We detected coherent,
low-amplitude, radial-velocity variations with a period of 3.7987days,
suggesting a possible close-in planetary companion. Subsequently, we initiated
a series of high-precision photometric observations to search for possible
planetary transits and an additional series of CaII H and K observations to
measure the level of surface magnetic activity and to look for possible
rotational modulation. Surprisingly, we found the star to be photometrically
variable and magnetically active. A detailed study of the phase stability of
the radial-velocity signal revealed that the radial-velocity variability
remains coherent only for durations of about 30days. Analysis of the time
variation of the spectroscopic line profiles using line bisectors revealed a
correlation between radial velocity and line-bisector orientation. All of these
observations, along with a one-quarter cycle phase shift between the
photometric and the radial-velocity variationss, are well explained by the
presence of dark photospheric spots on HD166435. We conclude that the
radial-velocity variations are not due to gravitational interaction with an
orbiting planet but, instead, originate from line-profile changes stemming from
star spots on the surface of the star. The quasi-coherence of the
radial-velocity signal over more than two years, which allowed a fair fit with
a binary model, makes the stability of this star unusual among other active
stars. It suggests a stable magnetic field orientation where spots are always
generated at about the same location on the surface of the star.Comment: 9 pages, 8 figures, Accepted for publication in A&
Stellar Oscillations Network Group
Stellar Oscillations Network Group (SONG) is an initiative aimed at designing
and building a network of 1m-class telescopes dedicated to asteroseismology and
planet hunting. SONG will have 8 identical telescope nodes each equipped with a
high-resolution spectrograph and an iodine cell for obtaining precision radial
velocities and a CCD camera for guiding and imaging purposes. The main
asteroseismology targets for the network are the brightest (V<6) stars. In
order to improve performance and reduce maintenance costs the instrumentation
will only have very few modes of operation. In this contribution we describe
the motivations for establishing a network, the basic outline of SONG and the
expected performance.Comment: Proc. Vienna Workshop on the Future of Asteroseismology, 20 - 22
September 2006. Comm. in Asteroseismology, Vol. 150, in the pres
The dynamical structure factor in topologically disordered systems
A computation of the dynamical structure factor of topologically disordered
systems, where the disorder can be described in terms of euclidean random
matrices, is presented. Among others, structural glasses and supercooled
liquids belong to that class of systems. The computation describes their
relevant spectral features in the region of the high frequency sound. The
analytical results are tested with numerical simulations and are found to be in
very good agreement with them. Our results may explain the findings of
inelastic X-ray scattering experiments in various glassy systems.Comment: Version to be published in J. Chem. Phy
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