1,214 research outputs found
Relativistic Lagrange Formulation
It is well-known that the equations for a simple fluid can be cast into what
is called their Lagrange formulation. We introduce a notion of a generalized
Lagrange formulation, which is applicable to a wide variety of systems of
partial differential equations. These include numerous systems of physical
interest, in particular, those for various material media in general
relativity. There is proved a key theorem, to the effect that, if the original
(Euler) system admits an initial-value formulation, then so does its
generalized Lagrange formulation.Comment: 34 pages, no figures, accepted in J. Math. Phy
Massive spheroids can form in single minor mergers
Accepted for publication in MNRAS, 12 pages, 6 figuresUnderstanding how rotationally supported discs transform into dispersion-dominated spheroids is central to our comprehension of galaxy evolution. Morphological transformation is largely merger-driven. While major mergers can efficiently create spheroids, recent work has highlighted the significant role of other processes, like minor mergers, in driving morphological change. Given their rich merger histories, spheroids typically exhibit large fractions of âex situâ stellar mass, i.e. mass that is accreted, via mergers, from external objects. This is particularly true for the most massive galaxies, whose stellar masses typically cannot be attained without a large number of mergers. Here, we explore an unusual population of extremely massive (M â > 10 11M) spheroids, in the Horizon-AGN simulation, which exhibit anomalously low ex situ mass fractions, indicating that they form without recourse to significant merging. These systems form in a single minor-merger event (with typical merger mass ratios of 0.11â0.33), with a specific orbital configuration, where the satellite orbit is virtually co-planar with the disc of the massive galaxy. The merger triggers a catastrophic change in morphology, over only a few hundred Myr, coupled with strong in situ star formation. While this channel produces a minority (âŒ5 per cent) of such galaxies, our study demonstrates that the formation of at least some of the most massive spheroids need not involve major mergers â or any significant merging at all â contrary to what is classically believed.Peer reviewedFinal Accepted Versio
Propagators in Lagrangian space
It has been found recently that propagators, e.g. the cross-correlation
spectra of the cosmic fields with the initial density field, decay
exponentially at large-k in an Eulerian description of the dynamics. We explore
here similar quantities defined for a Lagrangian space description. We find
that propagators in Lagrangian space do not exhibit the same properties: they
are found not to be monotonic functions of time, and to track back the linear
growth rate at late time (but with a renormalized amplitude). These results
have been obtained with a novel method which we describe alongside. It allows
the formal resummation of the same set of diagrams as those that led to the
known results in Eulerian space. We provide a tentative explanation for the
marked differences seen between the Eulerian and the Lagrangian cases, and we
point out the role played by the vorticity degrees of freedom that are specific
to the Lagrangian formalism. This provides us with new insights into the
late-time behavior of the propagators.Comment: 14 pages, 5 figure
Periaxonal K+ regulation in the small squid Alloteuthis. Studies on isolated and in situ axons
A novel giant axon preparation from the squid Alloteuthis is described. Properties of in situ and isolated axons are similar. Periaxonal K+ accumulation is a function of the physiological state of the animal and of the axon and its sheathing layers. Carefully dissected isolated axons, and axons in situ in a healthy mantle, show much less K+ accumulation than previously reported in squid. It is suggested that the Schwann cells are involved in the observed K+ regulation
Evidence for a meteoritic origin of the September 15, 2007, Carancas crater
On September 15th, 2007, around 11:45 local time in Peru, near the Bolivian border, the
atmospheric entry of a meteoroid produced bright lights in the sky and intense detonations. Soon after,
a crater was discovered south of Lake Titicaca. These events have been detected by the Bolivian
seismic network and two infrasound arrays operating for the Comprehensive Nuclear-Test-Ban Treaty
Organization, situated at about 80 and 1620 km from the crater. The localization and origin time
computed with the seismic records are consistent with the reported impact. The entry elevation and
azimuthal angles of the trajectory are estimated from the observed signal time sequences and backazimuths.
From the crater diameter and the airwave amplitudes, the kinetic energy, mass and
explosive energy are calculated. Using the estimated velocity of the meteoroid and similarity criteria
between orbital elements, an association with possible parent asteroids is attempted. The favorable
setting of this event provides a unique opportunity to evaluate physical and kinematic parameters of
the object that generated the first actual terrestrial meteorite impact seismically recorded
Relaxation times of unstable states in systems with long range interactions
We consider several models with long-range interactions evolving via
Hamiltonian dynamics. The microcanonical dynamics of the basic Hamiltonian Mean
Field (HMF) model and perturbed HMF models with either global anisotropy or an
on-site potential are studied both analytically and numerically. We find that
in the magnetic phase, the initial zero magnetization state remains stable
above a critical energy and is unstable below it. In the dynamically stable
state, these models exhibit relaxation time scales that increase algebraically
with the number of particles, indicating the robustness of the
quasistationary state seen in previous studies. In the unstable state, the
corresponding time scale increases logarithmically in .Comment: Minor change
Algebraic Correlation Function and Anomalous Diffusion in the HMF model
In the quasi-stationary states of the Hamiltonian Mean-Field model, we
numerically compute correlation functions of momenta and diffusion of angles
with homogeneous initial conditions. This is an example, in a N-body
Hamiltonian system, of anomalous transport properties characterized by non
exponential relaxations and long-range temporal correlations. Kinetic theory
predicts a striking transition between weak anomalous diffusion and strong
anomalous diffusion. The numerical results are in excellent agreement with the
quantitative predictions of the anomalous transport exponents. Noteworthy, also
at statistical equilibrium, the system exhibits long-range temporal
correlations: the correlation function is inversely proportional to time with a
logarithmic correction instead of the usually expected exponential decay,
leading to weak anomalous transport properties
Galaxy merger histories and the role of merging in driving star formation at z>1
This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society. © 2015 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.We use Horizon-AGN, a hydrodynamical cosmological simulation, to explore the role of mergers in the evolution of massive (M > 10^10 MSun) galaxies around the epoch of peak cosmic star formation (1zR(4:1 3 are 'blue' (i.e. have significant associated star formation), the proportion of 'red' mergers increases rapidly at ztodays stellar mass was formed.Peer reviewe
Seismic diagnostics for transport of angular momentum in stars 1. Rotational splittings from the PMS to the RGB
Rotational splittings are currently measured for several main sequence stars
and a large number of red giants with the space mission Kepler. This will
provide stringent constraints on rotation profiles. Our aim is to obtain
seismic constraints on the internal transport and surface loss of angular
momentum of oscillating solar-like stars. To this end, we study the evolution
of rotational splittings from the pre-main sequence to the red-giant branch for
stochastically excited oscillation modes. We modified the evolutionary code
CESAM2K to take rotationally induced transport in radiative zones into account.
Linear rotational splittings were computed for a sequence of
models. Rotation profiles were derived from our evolutionary models and
eigenfunctions from linear adiabatic oscillation calculations. We find that
transport by meridional circulation and shear turbulence yields far too high a
core rotation rate for red-giant models compared with recent seismic
observations. We discuss several uncertainties in the physical description of
stars that could have an impact on the rotation profiles. For instance, we find
that the Goldreich-Schubert-Fricke instability does not extract enough angular
momentum from the core to account for the discrepancy. In contrast, an increase
of the horizontal turbulent viscosity by 2 orders of magnitude is able to
significantly decrease the central rotation rate on the red-giant branch. Our
results indicate that it is possible that the prescription for the horizontal
turbulent viscosity largely underestimates its actual value or else a mechanism
not included in current stellar models of low mass stars is needed to slow down
the rotation in the radiative core of red-giant stars.Comment: 15 pages, 13 figures, accepted for publication in A&
Intrinsic alignment of simulated galaxies in the cosmic web: implications for weak lensing surveys
The intrinsic alignment of galaxy shapes (by means of their angular momentum) and their cross-correlation with the surrounding dark matter tidal field are investigated using the 160000, z=1.2 synthetic galaxies extracted from the high-resolution cosmological hydrodynamical simulation horizon-agn. One- and two-point statistics of the spin of the stellar component are measured as a function of mass and colour. For the low-mass galaxies, this spin is locally aligned with the tidal field âfilamentary' direction while, for the high-mass galaxies, it is perpendicular to both filaments and walls. The bluest galaxies of our synthetic catalogue are more strongly correlated with the surrounding tidal field than the reddest galaxies, and this correlation extends up toâŒ10hâ 1âMpc comoving distance. We also report a correlation of the projected ellipticities of blue, intermediate-mass galaxies on a similar scale at a level of 10â4 which could be a concern for cosmic shear measurements. We do not report any measurable intrinsic alignments of the reddest galaxies of our sample. This work is a first step towards the use of very realistic catalogue of synthetic galaxies to evaluate the contamination of weak lensing measurement by the intrinsic galactic alignment
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