75 research outputs found
Magnetically Confined Wind Shocks in X-rays - a Review
A subset (~ 10%) of massive stars present strong, globally ordered (mostly
dipolar) magnetic fields. The trapping and channeling of their stellar winds in
closed magnetic loops leads to magnetically confined wind shocks (MCWS), with
pre-shock flow speeds that are some fraction of the wind terminal speed. These
shocks generate hot plasma, a source of X-rays. In the last decade, several
developments took place, notably the determination of the hot plasma properties
for a large sample of objects using XMM-Newton and Chandra, as well as fully
self-consistent MHD modelling and the identification of shock retreat effects
in weak winds. Despite a few exceptions, the combination of magnetic
confinement, shock retreat and rotation effects seems to be able to account for
X-ray emission in massive OB stars. Here we review these new observational and
theoretical aspects of this X-ray emission and envisage some perspectives for
the next generation of X-ray observatories.Comment: accepted for publication by Advances in Space Research (special issue
"X-ray emission from hot stars and their winds"
Dynamical Simulations of Magnetically Channeled Line-Driven Stellar Winds: II. The Effects of Field-Aligned Rotation
Building upon our previous MHD simulation study of magnetic channeling in
radiatively driven stellar winds, we examine here the additional dynamical
effects of stellar {\em rotation} in the (still) 2-D axisymmetric case of an
aligned dipole surface field. In addition to the magnetic confinement parameter
introduced in Paper I, we characterize the stellar rotation in
terms of a parameter (the ratio of the
equatorial surface rotation speed to orbital speed), examining specifically
models with moderately strong rotation 0.25 and 0.5, and comparing these
to analogous non-rotating cases. Defining the associated Alfv\'{e}n radius
R_{\rm{A}} \approx \eta_{\ast}^{1/4} \Rstar and Kepler corotation radius
R_{\rm{K}} \approx W^{-2/3} \Rstar, we find rotation effects are weak for
models with , but can be substantial and even dominant
for models with R_{\rm{A}} \gtwig R_{\rm{K}}. In particular, by extending our
simulations to magnetic confinement parameters (up to )
that are well above those () considered in Paper I, we are
able to study cases with ; we find that these do
indeed show clear formation of the {\em rigid-body} disk predicted in previous
analytic models, with however a rather complex, dynamic behavior characterized
by both episodes of downward infall and outward breakout that limit the buildup
of disk mass. Overall, the results provide an intriguing glimpse into the
complex interplay between rotation and magnetic confinement, and form the basis
for a full MHD description of the rigid-body disks expected in strongly
magnetic Bp stars like Ori E.Comment: 14 pp, visit this
http://shayol.bartol.udel.edu/massivewiki-media/publications/rotation.pdf for
full figure version of the paper. MNRAS, in pres
Dynamical Simulations of Magnetically Channeled Line-Driven Stellar Winds: III. Angular Momentum Loss and Rotational Spindown
We examine the angular momentum loss and associated rotational spindown for
magnetic hot stars with a line-driven stellar wind and a rotation-aligned
dipole magnetic field. Our analysis here is based on our previous 2-D numerical
MHD simulation study that examines the interplay among wind, field, and
rotation as a function of two dimensionless parameters, one characterizing the
wind magnetic confinement (), and the other the ratio () of stellar
rotation to critical (orbital) speed. We compare and contrast the 2-D, time
variable angular momentum loss of this dipole model of a hot-star wind with the
classical 1-D steady-state analysis by Weber and Davis (WD), who used an
idealized monopole field to model the angular momentum loss in the solar wind.
Despite the differences, we find that the total angular momentum loss averaged over both solid angle and time follows closely the general WD
scaling , where is the
mass loss rate, is the stellar angular velocity, and is a
characteristic Alfv\'{e}n radius. However, a key distinction here is that for a
dipole field, this Alfv\'{e}n radius has a strong-field scaling , instead of the scaling for a monopole field. This leads to a slower stellar
spindown time that in the dipole case scales as , where is the
characteristic mass loss time, and is the dimensionless factor for stellar
moment of inertia. The full numerical scaling relation we cite gives typical
spindown times of order 1 Myr for several known magnetic massive stars.Comment: 13 pages, 7 figures, accepted for publication in MNRAS. MNRAS in
pres
Powerful Winds from Low-Mass Stars: V374 Peg
The rapid rotation (P=0.44 d) of the M dwarf V374Peg (M4) along with its
intense magnetic field point toward magneto-centrifugal acceleration of a
coronal wind. In this work, we investigate the structure of the wind of V374Peg
by means of 3D magnetohydrodynamical (MHD) numerical simulations. For the first
time, an observationally derived surface magnetic field map is implemented in
MHD models of stellar winds for a low mass star. We show that the wind of
V374Peg deviates greatly from a low-velocity, low-mass-loss rate solar-type
wind. We find general scaling relations for the terminal velocities, mass-loss
rates, and spin-down times of highly magnetized M dwarfs. In particular, for
V374Peg, our models show that terminal velocities across a range of stellar
latitudes reach ~(1500-2300) n_{12}^{-1/2} km/s, where n_{12} is the coronal
wind base density in units of 10^{12} cm^{-3}, while the mass-loss rates are
about 4 x 10^{-10} n_{12}^{1/2} Msun/yr. We also evaluate the angular-momentum
loss of V374Peg, which presents a rotational braking timescale ~28
n_{12}^{-1/2} Myr. Compared to observationally derived values from period
distributions of stars in open clusters, this suggests that V374Peg may have
low coronal base densities (< 10^{11} cm^{-3}). We show that the wind ram
pressure of V374Peg is about 5 orders of magnitude larger than for the solar
wind. Nevertheless, a small planetary magnetic field intensity (~ 0.1G) is able
to shield a planet orbiting at 1 AU against the erosive effects of the stellar
wind. However, planets orbiting inside the habitable zone of V374Peg, where the
wind ram pressure is higher, might be facing a more significant atmospheric
erosion. In that case, higher planetary magnetic fields of, at least, about
half the magnetic field intensity of Jupiter, are required to protect the
planet's atmosphere.Comment: 13 pages, 5 figures, 1 table. MNRAS in pres
Accretion, Outflows, and Winds of Magnetized Stars
Many types of stars have strong magnetic fields that can dynamically
influence the flow of circumstellar matter. In stars with accretion disks, the
stellar magnetic field can truncate the inner disk and determine the paths that
matter can take to flow onto the star. These paths are different in stars with
different magnetospheres and periods of rotation. External field lines of the
magnetosphere may inflate and produce favorable conditions for outflows from
the disk-magnetosphere boundary. Outflows can be particularly strong in the
propeller regime, wherein a star rotates more rapidly than the inner disk.
Outflows may also form at the disk-magnetosphere boundary of slowly rotating
stars, if the magnetosphere is compressed by the accreting matter. In isolated,
strongly magnetized stars, the magnetic field can influence formation and/or
propagation of stellar wind outflows. Winds from low-mass, solar-type stars may
be either thermally or magnetically driven, while winds from massive, luminous
O and B type stars are radiatively driven. In all of these cases, the magnetic
field influences matter flow from the stars and determines many observational
properties. In this chapter we review recent studies of accretion, outflows,
and winds of magnetized stars with a focus on three main topics: (1) accretion
onto magnetized stars; (2) outflows from the disk-magnetosphere boundary; and
(3) winds from isolated massive magnetized stars. We show results obtained from
global magnetohydrodynamic simulations and, in a number of cases compare global
simulations with observations.Comment: 60 pages, 44 figure
Thousands of Rab GTPases for the Cell Biologist
Rab proteins are small GTPases that act as essential regulators of vesicular trafficking. 44 subfamilies are known in humans, performing specific sets of functions at distinct subcellular localisations and tissues. Rab function is conserved even amongst distant orthologs. Hence, the annotation of Rabs yields functional predictions about the cell biology of trafficking. So far, annotating Rabs has been a laborious manual task not feasible for current and future genomic output of deep sequencing technologies. We developed, validated and benchmarked the Rabifier, an automated bioinformatic pipeline for the identification and classification of Rabs, which achieves up to 90% classification accuracy. We cataloged roughly 8.000 Rabs from 247 genomes covering the entire eukaryotic tree. The full Rab database and a web tool implementing the pipeline are publicly available at www.RabDB.org. For the first time, we describe and analyse the evolution of Rabs in a dataset covering the whole eukaryotic phylogeny. We found a highly dynamic family undergoing frequent taxon-specific expansions and losses. We dated the origin of human subfamilies using phylogenetic profiling, which enlarged the Rab repertoire of the Last Eukaryotic Common Ancestor with Rab14, 32 and RabL4. Furthermore, a detailed analysis of the Choanoflagellate Monosiga brevicollis Rab family pinpointed the changes that accompanied the emergence of Metazoan multicellularity, mainly an important expansion and specialisation of the secretory pathway. Lastly, we experimentally establish tissue specificity in expression of mouse Rabs and show that neo-functionalisation best explains the emergence of new human Rab subfamilies. With the Rabifier and RabDB, we provide tools that easily allows non-bioinformaticians to integrate thousands of Rabs in their analyses. RabDB is designed to enable the cell biology community to keep pace with the increasing number of fully-sequenced genomes and change the scale at which we perform comparative analysis in cell biology
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