277 research outputs found

### Asymptotic geometry of negatively curved manifolds of finite volume

We study the asymptotic behaviour of simply connected, Riemannian manifolds
$X$ of strictly negative curvature admitting a non-uniform lattice $\Gamma$. If
the quotient manifold $\bar X= \Gamma \backslash X$ is asymptotically
$1/4$-pinched, we prove that $\Gamma$ is divergent and $U\bar X$ has finite
Bowen-Margulis measure (which is then ergodic and totally conservative with
respect to the geodesic flow); moreover, we show that, in this case, the volume
growth of balls $B(x,R)$ in $X$ is asymptotically equivalent to a purely
exponential function $c(x)e^{\delta R}$, where $\delta$ is the topological
entropy of the geodesic flow of $\bar X$. \linebreak This generalizes Margulis'
celebrated theorem to negatively curved spaces of finite volume. In contrast,
we exhibit examples of lattices $\Gamma$ in negatively curved spaces $X$ (not
asymptotically $1/4$-pinched) where, depending on the critical exponent of the
parabolic subgroups and on the finiteness of the Bowen-Margulis measure, the
growth function is exponential, lower-exponential or even upper-exponential.Comment: 25 p. This paper replaces arXiv:1503.03971, withdrawn by the authors
due to the Theorem 1.1 whose statement is far from the main subject of the
paper; for the sake of clearness, this new version concentrates only on the
question of volume growth (theorems 1.2, 1.3 and 1.4). Theorem 1.1 of
arXiv:1503.03971 is now the subject of another paper (Signed only by 2
authors Sambusetti and Peign\'e) focused on this rigidity problem with a much
better presentation of the context and another rigidity resul

### Impact de pollutions ponctuelles sur les phytocénoses des rivières acides à neutres du Limousin (Massif Central, France)

L'impact des pollutions ponctuelles sur les phytocénoses aquatiques est étudié autour des rejets de 12 agglomérations dont 9 sont équipées d'une station d'épuration. Un échantillonnage systématique avec segmentation du cours d'eau autour de chaque rejet est réalisé. Sur chaque secteur, des relevés de végétation sont pratiqués au niveau de faciès d'écoulements homogènes dont on caractérise le milieu physique parallèlement à une analyse physicochimique de l'eau.L'ensemble des rejets provoque globalement une élévation de la conductivité, des teneurs en ammonium, nitrates et orthophosphates.Cela ce traduit par la régression de la phytocénose à Callitriche hamulata et Myriophyllum alterniflorum, par le développement de Ranunculus peltatus, Callitriche platycarpa et d'espèces cryptogames telles que Leptodyctium riparium, ou Melosira sp.Une Analyse en Composantes Principales menée sur l'ensemble des données permet d'opposer des phytocénoses propres aux secteurs amonts (Scapania undulata, Chiloscyphus polyanthus) à d'autres situées au niveau de rejets (Callitriche platycarpa, Leptodictyum riparium, Melosira sp.,).Une Analyse Canonique de Correspondances valide le déterminisme de la qualité physicochimique de l'eau sur la végétation. La conductivité, les teneurs en ammonium, nitrates et orthophosphates deviennent prépondérants par rapport aux facteurs du milieu physique classiquement discriminants dans l'installation des phytocénoses dans les rivières limousines.The impact of located pollution on aquatic phytocénoses is studied around 12 cities discharge. Nine of them are fitted out purification plant.The sampling method is based on consecutive segments from upstream to downstream. On each sector, vegetation records are realized in homogeneous water runoff facies, which are characterized by physical factors as well as water value measures.The whole discharge leads globally to an increase of conductivity, ammonium amount, nitrates and orthophosphates. The consequence of that is a decrease of Callitriche hamulata and Myriophyllum alterniflorum phytocénoses, a development of Ranunculus peltatus, Callitriche platycarpa and cryptogams species like Leptodictyum riparium or Melosira sp.A Component Principal Analysis applied on data, distinguishes phytocénoses belonging to upstream sectors (Scapania undulata, Chiloscyphus polyanthus) from the ones of discharges (Callitriche platycarpa, Leptodictyum riparium, Melosira sp.).A Canonical Correspondence Analysis validates the impact of physico-chemical water quality on vegetation. Conductivity, ammonium amount, nitrates and orthophosphates become more preponderant in comparison with physical environments usually discriminant for phytocénoses installation in Limousin rivers

### Kick stability in groups and dynamical systems

We consider a general construction of ``kicked systems''. Let G be a group of
measure preserving transformations of a probability space. Given its
one-parameter/cyclic subgroup (the flow), and any sequence of elements (the
kicks) we define the kicked dynamics on the space by alternately flowing with
given period, then applying a kick. Our main finding is the following stability
phenomenon: the kicked system often inherits recurrence properties of the
original flow. We present three main examples. 1) G is the torus. We show that
for generic linear flows, and any sequence of kicks, the trajectories of the
kicked system are uniformly distributed for almost all periods. 2) G is a
discrete subgroup of PSL(2,R) acting on the unit tangent bundle of a Riemann
surface. The flow is generated by a single element of G, and we take any
bounded sequence of elements of G as our kicks. We prove that the kicked system
is mixing for all sufficiently large periods if and only if the generator is of
infinite order and is not conjugate to its inverse in G. 3) G is the group of
Hamiltonian diffeomorphisms of a closed symplectic manifold. We assume that the
flow is rapidly growing in the sense of Hofer's norm, and the kicks are
bounded. We prove that for a positive proportion of the periods the kicked
system inherits a kind of energy conservation law and is thus superrecurrent.
We use tools of geometric group theory and symplectic topology.Comment: Latex, 40 pages, revised versio

### Grating formation in step flow heterogeneous growth and wavelength selection induced by confinement

Based on kinetic Monte Carlo simulations, we show that modulated wires and island gratings can be formed at vicinal surfaces. The modulation (grating) wavelength along the steps can be tailored by external conditions (coverage and temperature) and intrinsic surface properties (diffusion, terrace width) via a scaling law. Above 220 K a thermodynamic saturation value for the wavelength occurs, which depends only on the terrace width. This morphological behavior can be understood in terms of nucleation arguments applied to heteroepitaxial growth of Ag on stepped Pt(111) surfaces. (C) 2004 Elsevier B.V. All rights reserved

### On the growth of nonuniform lattices in pinched negatively curved manifolds

We study the relation between the exponential growth rate of volume in a
pinched negatively curved manifold and the critical exponent of its lattices.
These objects have a long and interesting story and are closely related to the
geometry and the dynamical properties of the geodesic flow of the manifold

### A search for late-type supergiants in the inner regions of the Milky Way

We present the results of a narrow-band infrared imaging survey of a narrow
strip (12' wide) around the galactic equator between 6 deg and 21 deg of
galactic longitude aimed at detecting field stars with strong CO absorption,
mainly late-type giants and supergiants. Our observations include follow-up low
resolution spectroscopy (R = 980) of 191 selected candidates in the H and K
bands. Most of these objects have photometric and spectroscopic characteristics
consistent with them being red giants, and some display broad, strong
absorption wings due to water vapor absorption between the H and K bands. We
also identify in our sample 18 good supergiant candidates characterized by
their lack of noticeable water absorption, strong CO bands in the H and K
windows, and HK_S photometry suggestive of high intrinsic luminosity and
extinctions reaching up to A_V ~40 mag. Another 9 additional candidates share
the same features except for weak H2O absorption, which is also observed among
some M supergiants in the solar neighbourhood. Interesting differences are
noticed when comparing our stars to a local sample of late-type giants and
supergiants, as well as to a sample of red giants in globular clusters of
moderately subsolar metallicity and to a sample of bulge stars. (...) We
propose that the systematic spectroscopic differences of our inner Galaxy stars
are due to their higher metallicities that cause deeper mixing in their
mantles, resulting in lower surface abundances of C and O and higher abundances
of CN, which contribute to the strength of the CaI and NaI features at low
resolution. Our results stress the limitations of using local stars as
templates for the study of composite cool stellar populations such as central
starbursts in galaxies (Abridged).Comment: 21 pages (including figures), A&A accepte

### The long Galactic bar as seen by UKIDSS Galactic Plane Survey

Over the last decade there have been a series of results supporting the
hypothesis of the existence of a long thin bar in the Milky Way with a
half-length of 4.5 kpc and a position angle of around 45 deg. This is
apparently a very different structure from the triaxial bulge of the Galaxy,
which is thicker and shorter and dominates the star counts at |l|<10 deg. In
this paper, we analyse the stellar distribution in the inner Galaxy to see if
there is clear evidence for two triaxial or bar-like structures in the Milky
Way.
By using the red-clump population as a tracer of Galactic structure, we
determine the apparent morphology of the inner Galaxy. Deeper and higher
spatial resolution NIR photometry from the UKIDSS Galactic Plane Survey allows
us to use in-plane data even at the innermost Galactic longitudes, a region
where the source confusion is a dominant effect that makes it impossible to use
other NIR databases such as 2MASS or TCS-CAIN. We show that results previously
obtained with using the red-clump giants are confirmed with the in-plane data
from UKIDSS GPS. There are two different structures coexisting in the inner
Galactic plane: one with a position angle of 23.60+-2.19 deg that can be traced
from the Galactic Centre up to l=10 deg (the Galactic bulge), and other with a
larger position angle of 42.44+-2.14 deg, that ends around l=28 deg (the long
Galactic bar).Comment: (8 pages, 14 figures, accepted for publication in A&A

### The EROS2 search for microlensing events towards the spiral arms: the complete seven season results

The EROS-2 project has been designed to search for microlensing events
towards any dense stellar field. The densest parts of the Galactic spiral arms
have been monitored to maximize the microlensing signal expected from the stars
of the Galactic disk and bulge. 12.9 million stars have been monitored during 7
seasons towards 4 directions in the Galactic plane, away from the Galactic
center. A total of 27 microlensing event candidates have been found. Estimates
of the optical depths from the 22 best events are provided. A first order
interpretation shows that simple Galactic models with a standard disk and an
elongated bulge are in agreement with our observations. We find that the
average microlensing optical depth towards the complete EROS-cataloged stars of
the spiral arms is $\bar{\tau} =0.51\pm .13\times 10^{-6}$, a number that is
stable when the selection criteria are moderately varied. As the EROS catalog
is almost complete up to $I_C=18.5$, the optical depth estimated for the
sub-sample of bright target stars with $I_C<18.5$ ($\bar{\tau}=0.39\pm
>.11\times 10^{-6}$) is easier to interpret. The set of microlensing events
that we have observed is consistent with a simple Galactic model. A more
precise interpretation would require either a better knowledge of the distance
distribution of the target stars, or a simulation based on a Galactic model.
For this purpose, we define and discuss the concept of optical depth for a
given catalog or for a limiting magnitude.Comment: 22 pages submitted to Astronomy & Astrophysic

### MOB1 Mediated Phospho-recognition in the Core Mammalian Hippo Pathway.

The Hippo tumor suppressor pathway regulates organ size and tissue homoeostasis in response to diverse signaling inputs. The core of the pathway consists of a short kinase cascade: MST1 and MST2 phosphorylate and activate LATS1 and LATS2, which in turn phosphorylate and inactivate key transcriptional coactivators, YAP1 and TAZ (gene WWTR1). The MOB1 adapter protein regulates both phosphorylation reactions firstly by concurrently binding to the upstream MST and downstream LATS kinases to enable the trans phosphorylation reaction, and secondly by allosterically activating the catalytic function of LATS1 and LATS2 to directly stimulate phosphorylation of YAP and TAZ. Studies of yeast Mob1 and human MOB1 revealed that the ability to recognize phosphopeptide sequences in their interactors, Nud1 and MST2 respectively, was critical to their roles in regulating the Mitotic Exit Network in yeast and the Hippo pathway in metazoans. However, the underlying rules of phosphopeptide recognition by human MOB1, the implications of binding specificity for Hippo pathway signaling, and the generality of phosphopeptide binding function to other human MOB family members remained elusive.Employing proteomics, peptide arrays and biochemical analyses, we systematically examine the phosphopeptide binding specificity of MOB1 and find it to be highly complementary to the substrate phosphorylation specificity of MST1 and MST2. We demonstrate that autophosphorylation of MST1 and MST2 on several threonine residues provides multiple MOB1 binding sites with varying binding affinities which in turn contribute to a redundancy of MST1-MOB1 protein interactions in cells. The crystal structures of MOB1A in complex with two favored phosphopeptide sites in MST1 allow for a full description of the MOB1A phosphopeptide-binding consensus. Lastly, we show that the phosphopeptide binding properties of MOB1A are conserved in all but one of the seven MOB family members in humans, thus providing a starting point for uncovering their elusive cellular functions

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