1,528 research outputs found
The effect of magnetic fields on star cluster formation
We examine the effect of magnetic fields on star cluster formation by
performing simulations following the self-gravitating collapse of a turbulent
molecular cloud to form stars in ideal MHD. The collapse of the cloud is
computed for global mass-to-flux ratios of infinity, 20, 10, 5 and 3, that is
using both weak and strong magnetic fields. Whilst even at very low strengths
the magnetic field is able to significantly influence the star formation
process, for magnetic fields with plasma beta < 1 the results are substantially
different to the hydrodynamic case. In these cases we find large-scale
magnetically-supported voids imprinted in the cloud structure; anisotropic
turbulent motions and column density structure aligned with the magnetic field
lines, both of which have recently been observed in the Taurus molecular cloud.
We also find strongly suppressed accretion in the magnetised runs, leading to
up to a 75% reduction in the amount of mass converted into stars over the
course of the calculations and a more quiescent mode of star formation. There
is also some indication that the relative formation efficiency of brown dwarfs
is lower in the strongly magnetised runs due to the reduction in the importance
of protostellar ejections.Comment: 16 pages, 9 figures, 8 very pretty movies, MNRAS, accepted. Version
with high-res figures + movies available from
http://www.astro.ex.ac.uk/people/dprice/pubs/mcluster/index.htm
The Properties of Prestellar Discs in Isolated and Multiple Prestellar Systems
We present high-resolution 3D smoothed particle hydrodynamics simulations of
the formation and evolution of protostellar discs in a turbulent molecular
cloud. Using a piecewise polytropic equation of state, we perform two sets of
simulations. In both cases we find that isolated systems undergo a
fundamentally different evolution than members of binary or multiple systems.
When formed, isolated systems must accrete mass and increase their specific
angular momentum, leading to the formation of massive, extended discs, which
undergo strong gravitational instabilities and are susceptible to disc
fragmentation. Fragments with initial masses of 5.5 M_jup, 7.4 M_jup and 12
M_jup are produced in our simulations. In binaries and small clusters, we
observe that due to competition for material from the parent core, members do
not accrete significant amounts of high specific angular momentum gas relative
to isolated systems. We find that discs in multiple systems are strongly
self-gravitating but that they are stable against fragmentation due to disc
truncation and mass profile steeping by tides, accretion of high specific
angular momentum gas by other members, and angular momentum being redirected
into members' orbits. In general, we expect disc fragmentation to be less
likely in clusters and to be more a feature of isolated systems.Comment: 15 pages, 21 figures. Accepted for publication in MNRA
Antisymmetric magnetoresistance in magnetic multilayers with perpendicular anisotropy
While magnetoresistance (MR) has generally been found to be symmetric in
applied field in non-magnetic or magnetic metals, we have observed
antisymmetric MR in Co/Pt multilayers. Simultaneous domain imaging and
transport measurements show that the antisymmetric MR is due to the appearance
of domain walls that run perpendicular to both the magnetization and the
current, a geometry existing only in materials with perpendicular magnetic
anisotropy. As a result, the extraordinary Hall effect (EHE) gives rise to
circulating currents in the vicinity of the domain walls that contributes to
the MR. The antisymmetric MR and EHE have been quantitatively accounted for by
a theoretical model.Comment: 17 pages, 4 figure
STUDY OF DISPERSANT AGENTS FOR THORIUM OXIDE
S>A preliminary study of dispersing agents for thorium oxide was completed and several of the dispersants have possible uses. Also many of the industrial dispersing agents tested are not usable with thorium oxide due to induced behavior causing balling and caking. The effects of nitric acid concentration were observed to also effect each dispersing agent. (auth
The large-scale disk fraction of brown dwarfs in the Taurus cloud as measured with Spitzer
Aims. The brown dwarf (BD) formation process has not yet been completely
understood. To shed more light on the differences and similarities between star
and BD formation processes, we study and compare the disk fraction among both
kinds of objects over a large angular region in the Taurus cloud. In addition,
we examine the spatial distribution of stars and BD relative to the underlying
molecular gas Methods. In this paper, we present new and updated photometry
data from the Infrared Array Camera (IRAC) aboard the Spitzer Space Telescope
on 43 BDs in the Taurus cloud, and recalculate of the BD disk fraction in this
region. We also useed recently available CO mm data to study the spatial
distribution of stars and BDs relative to the cloud's molecular gas. Results.
We find that the disk fraction among BDs in the Taurus cloud is 41 \pm 12%, a
value statistically consistent with the one among TTS (58 \pm 9%). We find that
BDs in transition from a state where they have a disk to a diskless state are
rare, and we study one isolated example of a transitional disk with an inner
radius of \approx 0.1 AU (CFHT BD Tau 12, found via its relatively small mid-IR
excess compared to most members of Taurus that have disks. We find that BDs are
statistically found in regions of similar molecular gas surface density to
those associated with stars. Furthermore, we find that the gas column density
distribution is almost identical for stellar and substellar objects with and
without disks.Comment: 8 page, 6 figures, Accepted in Astronomy & Astrophysics
On the circum(sub)stellar environment of brown dwarfs in Taurus
Aims : We want to investigate whether brown dwarfs (BDs) form like stars or
are ejected embryos. We study the presence of disks around BDs in the Taurus
cloud, and discuss implications for substellar formation models. Methods : We
use photometric measurements from the visible to the far infrared to determine
the spectral energy distributions (SEDs) of Taurus BDs. Results: We use Spitzer
color indices, Halpha as an accretion indicator, and models fit to the SEDs in
order to estimate physical parameters of the disks around these BDs. We study
the spatial distribution of BDs with and without disks across the Taurus
aggregates, and we find that BDs with and without disks are not distributed
regularly across the Taurus cloud. Conclusions: We find that 48%+/- 14% of
Taurus BDs have a circumstellar disk signature, a ratio similar to recent
results from previous authors in other regions. We fit the SEDs and find that
none of the disks around BDs in Taurus can be fitted convincingly with a
flaring index beta = 0, indicating that heating by the central object is
efficient and that the disks we observe retain a significant amount of gas. We
find that BDs with disks are proportionally more numerous in the northern
Taurus filament, possibly the youngest filament. We do not find such a clear
segregation for classical T Tauri stars (CTTS) and weak-lined T Tauri stars
(WTTS), suggesting that, in addition to the effects of evolution, any
segregation effects could be related to the mass of the object. A by-product of
our study is to propose a recalibration of the Barrado y Navascues & Martin
(2003) accretion limit in the substellar domain. The global shape of the limit
fits our data points if it is raised by a factor 1.25-1.30.Comment: 11 pages, 5 figures, A&A accepte
Simulations of the grand design galaxy M51: a case study for analysing tidally induced spiral structure
We present hydrodynamical models of the grand design spiral M51 (NGC 5194),
and its interaction with its companion NGC 5195. Despite the simplicity of our
models, our simulations capture the present day spiral structure of M51
remarkably well, and even reproduce details such as a kink along one spiral
arm, and spiral arm bifurcations. We investigate the offset between the stellar
and gaseous spiral arms, and find at most times (including the present day)
there is no offset between the stars and gas to within our error bars. We also
compare our simulations with recent observational analysis of M51. We compute
the pattern speed versus radius, and like the observations, find no single
global pattern speed. We also show that the spiral arms cannot be fitted well
by logarithmic spirals. We interpret these findings as evidence that M51 does
not exhibit a quasi-steady density wave, as would be predicted by density wave
theory. The internal structure of M51 derives from the complicated and
dynamical interaction with its companion, resulting in spiral arms showing
considerable structure in the form of short-lived kinks and bifurcations.
Rather than trying to model such galaxies in terms of global spiral modes with
fixed pattern speeds, it is more realistic to start from a picture in which the
spiral arms, while not being simple material arms, are the result of tidally
induced kinematic density `waves' or density patterns, which wind up slowly
over time.Comment: 23 pages, 20 figures, accepted for publication in MNRA
DETERMINATION OF Li IN AQUEOUS SOLUTION BY NEUTRON ACTIVATION ANALYSIS
A method for determining the concentration of Li/sup 6/ in aqueous solution was tested using the nuclear reactions Li/sup 6/(n, alpha )H and O/sup 16/(H/sup 3/,n)F/sup 18/. Annihilation gamma radiation of induced 1.87-hr F/ sup 18/ radioactivity was counted with a welltype scintillation counter, and the radioactivity per millimole of lithium was found to be independent of lithium concentration below about 0.2 moles/liter. The sensitivity limit for detecting lithium is less than 0.1 micromole (0.0075 micromole Li/sup 6/). (auth
GPI-anchor signal sequence influences PrPC sorting, shedding and signalling, and impacts on different pathomechanistic aspects of prion disease in mice
The cellular prion protein (PrPC) is a cell surface glycoprotein attached to the membrane by a glycosylphosphatidylinositol (GPI)-anchor and plays a critical role in transmissible, neurodegenerative and fatal prion diseases. Alterations in membrane attachment influence PrPC-associated signaling, and the development of prion disease, yet our knowledge of the role of the GPI-anchor in localization, processing, and function of PrPC in vivo is limited We exchanged the PrPC GPI-anchor signal sequence of for that of Thy-1 (PrPCGPIThy-1) in cells and mice. We show that this modifies the GPI-anchor composition, which then lacks sialic acid, and that PrPCGPIThy-1 is preferentially localized in axons and is less prone to proteolytic shedding when compared to PrPC. Interestingly, after prion infection, mice expressing PrPCGPIThy-1 show a significant delay to terminal disease, a decrease of microglia/astrocyte activation, and altered MAPK signaling when compared to wild-type mice. Our results are the first to demonstrate in vivo, that the GPI-anchor signal sequence plays a fundamental role in the GPI-anchor composition, dictating the subcellular localization of a given protein and, in the case of PrPC, influencing the development of prion disease
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