19,642 research outputs found
Star Formation Around Super-Massive Black Holes
The presence of young massive stars orbiting on eccentric rings within a few
tenths of a parsec of the supermassive black hole in the Galactic centre is
challenging for theories of star formation. The high tidal shear from the black
hole should tear apart the molecular clouds that form stars elsewhere in the
Galaxy, while transporting the stars to the Galactic centre also appears
unlikely during their stellar lifetimes. We present numerical simulations of
the infall of a giant molecular cloud that interacts with the black hole. The
transfer of energy during closest approach allows part of the cloud to become
bound to the black hole, forming an eccentric disc that quickly fragments to
form stars. Compressional heating due to the black hole raises the temperature
of the gas to 100-1000K, ensuring that the fragmentation produces relatively
high stellar masses. These stars retain the eccentricity of the disc and, for a
sufficiently massive initial cloud, produce an extremely top-heavy distribution
of stellar masses. This potentially repetitive process can therefore explain
the presence of multiple eccentric rings of young stars in the presence of a
supermassive black hole.Comment: 20 pages includingh 7 figures. "This is the author's version of the
work. It is posted here by permission of the AAAS for personal use, not for
redistribution. The definitive version was published in Science, 321, (22
August 2008), doi:10.1126/science.1160653". Reprints and animations can be
found at http://star-www.st-and.ac.uk/~iab1
The Magnetic Topology of the Weak-Lined T Tauri Star V410 - A Simultaneous Temperature and Magnetic Field Inversion
We present a detailed temperature and magnetic investigation of the T Tauri
star V410 Tau by means of a simultaneous Doppler- and Zeeman-Doppler Imaging.
Moreover we introduce a new line profile reconstruction method based on a
singular value decomposition (SVD) to extract the weak polarized line profiles.
One of the key features of the line profile reconstruction is that the SVD line
profiles are amenable to radiative transfer modeling within our Zeeman-Doppler
Imaging code iMap. The code also utilizes a new iterative regularization scheme
which is independent of any additional surface constraints. To provide more
stability a vital part of our inversion strategy is the inversion of both
Stokes I and Stokes V profiles to simultaneously reconstruct the temperature
and magnetic field surface distribution of V410 Tau. A new image-shear analysis
is also implemented to allow the search for image and line profile distortions
induced by a differential rotation of the star. The magnetic field structure we
obtain for V410 Tau shows a good spatial correlation with the surface
temperature and is dominated by a strong field within the cool polar spot. The
Zeeman-Doppler maps exhibit a large-scale organization of both polarities
around the polar cap in the form of a twisted bipolar structure. The magnetic
field reaches a value of almost 2 kG within the polar region but smaller fields
are also present down to lower latitudes. The pronounced non-axisymmetric field
structure and the non-detection of a differential rotation for V410 Tau
supports the idea of an underlying -type dynamo, which is predicted
for weak-lined T Tauri stars.Comment: Accepted for A&A, 18 pages, 10 figure
Heterogeneous responses of dorsal root ganglion neurons in neuropathies induced by peripheral nerve trauma and the antiretroviral drug stavudine
© 2014 The Authors. European Journal of Pain published by John Wiley & Sons Ltd on behalf of European Pain Federation - EFICŸ. Funding sources E.K.B. was funded by a BBSRC PhD studentship. A.N., A.S.C.R. and T.P. were funded by a Wellcome Trust Strategic Award (London Pain Consortium; ref. 083259). A.S.C.R. and W.H. were funded by the Innovative Medicines Initiative Joint Undertaking (Europain; grant agreement no. 115007). We thank Pfizer for providing stavudine. Conflicts of interest None declared. Funded by BBSRC PhD studentship Wellcome Trust Strategic Award. Grant Number: 083259 Innovative Medicines Initiative Joint Undertaking. Grant Number: 115007Peer reviewedPublisher PD
Factors influencing student nurse decisions to report poor practice witnessed while on placement
Background: While it is commonly accepted that nursing care is generally of a good standard, it would be naĂŻve to think that this is always the case. Over recent years concern about aspects of the quality of some nursing care has grown. In tandem with this, there is recognition that nurses do not always report poor practice. As future registrants, student nurses have a role to play in changing this culture. We know, however, relatively little about the factors that influence student decisions on whether or not to report. In the absence of a more nuanced understanding of this issue, we run the risk of assuming students will speak out simply because we say they should. Objectives: To explore influences on student decisions about whether or not to report poor clinical practice which is a result of deliberate action and which is witnessed while on placement. Methods: Qualitative interviews were conducted with thirteen pre-registration nursing students from the UK. Participants included both adult and mental health nurses with an age range from 20â47. Data were analysed to identify key themes. Category integrity and fit with data was confirmed by a team member following initial analysis. Results: Four themes emerged from the data. The first of these, âI had no choiceâ described the personal and ethical drivers which influenced students to report. âConsequences for selfâ and âLiving with ambiguityâ provide an account of why some students struggle to report, while âBeing preparedâ summarised arguments both for and against reporting concerns. Conclusion: While there is a drive to promote openness in health care settings and an expectation that staff will raise concerns about quality of care, the reality is that the decision to do this can be very difficult. This is certainly the case for some student nurses. Our results suggest ways in which educationalists might intervene to support students who witness poor practice to report
Exploring the Origins of Earth's Nitrogen: Astronomical Observations of Nitrogen-bearing Organics in Protostellar Environments
It is not known whether the original carriers of Earth's nitrogen were
molecular ices or refractory dust. To investigate this question, we have used
data and results of Herschel observations towards two protostellar sources: the
high-mass hot core of Orion KL, and the low-mass protostar IRAS 16293-2422.
Towards Orion KL, our analysis of the molecular inventory of Crockett et al.
(2014) indicates that HCN is the organic molecule that contains by far the most
nitrogen, carrying of nitrogen-in-organics. Following this
evidence, we explore HCN towards IRAS 16293-2422, which we consider a solar
analog. Towards IRAS 16293-2422, we have reduced and analyzed Herschel spectra
of HCN, and fit these observations against "jump" abundance models of IRAS
16293-2422's protostellar envelope. We find an inner-envelope HCN abundance
and an outer-envelope HCN
abundance . We also find the
sublimation temperature of HCN to be ~K; this
measured enables us to predict an HCN binding energy
~K. Based on a comparison of the HCN/H2O ratio
in these protostars to N/H2O ratios in comets, we find that HCN (and, by
extension, other organics) in these protostars is incapable of providing the
total bulk N/H2O in comets. We suggest that refractory dust, not molecular
ices, was the bulk provider of nitrogen to comets. However, interstellar dust
is not known to have 15N enrichment, while high 15N enrichment is seen in both
nitrogen-bearing ices and in cometary nitrogen. This may indicate that these
15N-enriched ices were an important contributor to the nitrogen in
planetesimals and likely to the Earth.Comment: Accepted to ApJ; 21 pages, 4 figure
Apollo experience report: The cryogenic storage system
A review of the design, development, and flight history of the Apollo cryogenic storage system and of selected components within the system is presented. Discussions are presented on the development history of the pressure vessels, heaters, insulation, and selected components. Flight experience and operational difficulties are reported in detail to provide definition of the problems and applicable corrective actions
Detecting gravitational waves from inspiraling binaries with a network of detectors : coherent versus coincident strategies
We compare two strategies of multi-detector detection of compact binary
inspiral signals, namely, the coincidence and the coherent. For simplicity we
consider here two identical detectors having the same power spectral density of
noise, that of initial LIGO, located in the same place and having the same
orientation. We consider the cases of independent noise as well as that of
correlated noise. The coincident strategy involves separately making two
candidate event lists, one for each detector, and from these choosing those
pairs of events from the two lists which lie within a suitable parameter
window, which then are called as coincidence detections. The coherent strategy
on the other hand involves combining the data phase coherently, so as to obtain
a single network statistic which is then compared with a single threshold. Here
we attempt to shed light on the question as to which strategy is better. We
compare the performances of the two methods by plotting the Receiver Operating
Characteristics (ROC) for the two strategies. Several of the results are
obtained analytically in order to gain insight. Further we perform numerical
simulations in order to determine certain parameters in the analytic formulae
and thus obtain the final complete results. We consider here several cases from
the relatively simple to the astrophysically more relevant in order to
establish our results. The bottom line is that the coherent strategy although
more computationally expensive in general than the coincidence strategy, is
superior to the coincidence strategy - considerably less false dismissal
probability for the same false alarm probability in the viable false alarm
regime.Comment: 18 pages, 10 figures, typo correcte
Enhanced Coherence of Antinodal Quasiparticles in a Dirty d-wave Superconductor
Recent ARPES experiments show a narrow quasiparticle peak at the gap edge
along the antinodal [1,0]-direction for the overdoped cuprate superconductors.
We show that within weak coupling BCS theory for a d-wave superconductor the
s-wave single-impurity scattering cross section vanishes for energies of the
gap edge. This coherence effect occurs through multiple scattering off the
impurity. For small impurity concentrations the spectral function has a
pronounced increase of the (scattering) lifetime for antinodal quasiparticles
but shows a very broad peak in the nodal direction, in qualitative agreement
with experiment and in strong contrast to the behavior observed in underdoped
cuprates.Comment: 4 pages, 3 figures, submitte
Astrometric signatures of self-gravitating protoplanetary discs
We use high resolution numerical simulations to study whether gravitational
instabilities within circumstellar discs can produce astrometrically detectable
motion of the central star. For discs with masses of M_disc = 0.1 M_star, which
are permanantly stable against fragmentation, we find that the magnitude of the
astrometric signal depends upon the efficiency of disc cooling. Short cooling
times produce prominent filamentary spiral structures in the disc, and lead to
stellar motions that are potentially observable with future high precision
astrometric experiments. For a disc that is marginally unstable within radii of
\~10 au, we estimate astrometric displacements of 10-100 microarcsec on decade
timescales for a star at a distance of 100 pc. The predicted displacement is
suppressed by a factor of several in more stable discs in which the cooling
time exceeds the local dynamical time by an order of magnitude. We find that
the largest contribution comes from material in the outer regions of the disc
and hence, in the most pessimistic scenario, the stellar motions caused by the
disc could confuse astrometric searches for low mass planets orbiting at large
radii. They are, however, unlikely to present any complications in searches for
embedded planets orbiting at small radii, relative to the disc size, or Jupiter
mass planets or greater orbiting at large radii.Comment: 6 pages, 9 figures, accepted for publication in MNRA
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