13,769 research outputs found
Magnetic fields from low mass stars to brown dwarfs
Magnetic fields have been detected on stars across the H-R diagram and
substellar objects either directly by their effect on the formation of spectral
lines, or through the activity phenomena they power which can be observed
across a large part of the electromagnetic spectrum. Stars show a very wide
variety of magnetic properties in terms of strength, geometry or variability.
Cool stars generate their magnetic fields by dynamo effect, and their
properties appear to correlate - to some extent - with stellar parameters such
as mass, rotation and age. With the improvements of instrumentation and data
analysis techniques, magnetic fields can now be detected and studied down to
the domain of very-low-mass stars and brown dwarfs, triggering new theoretical
works aimed, in particular, at modelling dynamo action in these objects. After
a brief discussion on the importance of magnetic field in stellar physics, the
basics of dynamo theory and magnetic field measurements are presented. The main
results stemming from observational and theoretical studies of magnetism are
then detailed in two parts: the fully-convective transition, and the very-low
mass stars and brown dwarfs domain.Comment: 30 pages, 9 figures. Notes for lectures presented at the Evry
Schatzman school on "Low-mass stars and the transition from stars to brown
dwarfs", September 2011, Roscoff, France. To appear in the EAS Conference
Series, edited by C. Charbonnel, C. Reyle, M. Schulthei
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Changes in the genetic requirements for microbial interactions with increasing community complexity.
Microbial community structure and function rely on complex interactions whose underlying molecular mechanisms are poorly understood. To investigate these interactions in a simple microbiome, we introduced E. coli into an experimental community based on a cheese rind and identified the differences in E. coli's genetic requirements for growth in interactive and non-interactive contexts using Random Barcode Transposon Sequencing (RB-TnSeq) and RNASeq. Genetic requirements varied among pairwise growth conditions and between pairwise and community conditions. Our analysis points to mechanisms by which growth conditions change as a result of increasing community complexity and suggests that growth within a community relies on a combination of pairwise and higher-order interactions. Our work provides a framework for using the model organism E. coli as a readout to investigate microbial interactions regardless of the genetic tractability of members of the studied ecosystem
The evolution of surface magnetic fields in young solar-type stars
The surface rotation rates of young solar-type stars decrease rapidly with
age from the end of the pre-main sequence though the early main sequence. This
suggests that there is also an important change in the dynamos operating in
these stars, which should be observable in their surface magnetic fields. Here
we present early results in a study aimed at observing the evolution of these
magnetic fields through this critical time period. We are observing stars in
open clusters and stellar associations to provide precise ages, and using
Zeeman Doppler Imaging to characterize the complex magnetic fields. Presented
here are results for six stars, three in the in the beta Pic association (~10
Myr old) and three in the AB Dor association (~100 Myr old).Comment: To appear in the proceedings of IAU symposium 302: Magnetic fields
throughout stellar evolution. 2 pages, 3 figure
From solar-like to anti-solar differential rotation in cool stars
Stellar differential rotation can be separated into two main regimes:
solar-like when the equator rotates faster than the poles and anti-solar when
the polar regions rotate faster than the equator. We investigate the transition
between these two regimes with 3-D numerical simulations of rotating spherical
shells. We conduct a systematic parameter study which also includes models from
different research groups. We find that the direction of the differential
rotation is governed by the contribution of the Coriolis force in the force
balance, independently of the model setup (presence of a magnetic field,
thickness of the convective layer, density stratification). Rapidly-rotating
cases with a small Rossby number yield solar-like differential rotation, while
weakly-rotating models sustain anti-solar differential rotation. Close to the
transition, the two kinds of differential rotation are two possible bistable
states. This study provides theoretical support for the existence of anti-solar
differential rotation in cool stars with large Rossby numbers.Comment: 5 pages, 6 figures, accepted for publication in MNRA
The "Binarity and Magnetic Interactions in various classes of Stars" (BinaMIcS) project
The "Binarity and Magnetic Interactions in various classes of stars"
(BinaMIcS) project is based on two large programs of spectropolarimetric
observations with ESPaDOnS at CFHT and Narval at TBL. Three samples of
spectroscopic binaries with two spectra (SB2) are observed: known cool magnetic
binaries, the few known hot magnetic binaries, and a survey sample of hot
binaries to search for additional hot magnetic binaries. The goal of BinaMIcS
is to understand the complex interplay between stellar magnetism and binarity.
To this aim, we will characterise and model the magnetic fields, magnetospheric
structure and coupling of both components of hot and cool close binary systems
over a significant range of evolutionary stages, to confront current theories
and trigger new ones. First results already provided interesting clues, e.g.
about the origin of magnetism in hot stars.Comment: 4 pages, 2 figures, proceedings of the SF2A conferenc
Measurement of the Higgs mass via the channel : e+e- -> ZH -> e+e- + X
In this communication, the mass declined for the decay channel, e+e- -> ZH ->
e+e- + X, as measured by the ILD detector was studied. The Higgs mass is
assumed to be 120 GeV and the center of mass energy is 250 GeV. For an
integrated luminosity of 250 fb-1, the accuracy of the reconstruction and the
good knowledge of the initial state allow for the measurement of the Higgs
boson mass with a precision of about 100 MeV.Comment: 7 pages, 14 figures, LCWS/ILC 2010 (International Linear Collider
Workshop 2010 LCWS10 and ILC10
A homomorphism between link and XXZ modules over the periodic Temperley-Lieb algebra
We study finite loop models on a lattice wrapped around a cylinder. A section
of the cylinder has N sites. We use a family of link modules over the periodic
Temperley-Lieb algebra EPTL_N(\beta, \alpha) introduced by Martin and Saleur,
and Graham and Lehrer. These are labeled by the numbers of sites N and of
defects d, and extend the standard modules of the original Temperley-Lieb
algebra. Beside the defining parameters \beta=u^2+u^{-2} with u=e^{i\lambda/2}
(weight of contractible loops) and \alpha (weight of non-contractible loops),
this family also depends on a twist parameter v that keeps track of how the
defects wind around the cylinder. The transfer matrix T_N(\lambda, \nu) depends
on the anisotropy \nu and the spectral parameter \lambda that fixes the model.
(The thermodynamic limit of T_N is believed to describe a conformal field
theory of central charge c=1-6\lambda^2/(\pi(\lambda-\pi)).)
The family of periodic XXZ Hamiltonians is extended to depend on this new
parameter v and the relationship between this family and the loop models is
established. The Gram determinant for the natural bilinear form on these link
modules is shown to factorize in terms of an intertwiner i_N^d between these
link representations and the eigenspaces of S^z of the XXZ models. This map is
shown to be an isomorphism for generic values of u and v and the critical
curves in the plane of these parameters for which i_N^d fails to be an
isomorphism are given.Comment: Replacement of "The Gram matrix as a connection between periodic loop
models and XXZ Hamiltonians", 31 page
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