21,441 research outputs found
The microlensing rate and distribution of free-floating planets towards the Galactic bulge
Ground-based optical microlensing surveys have provided tantalising, if
inconclusive, evidence for a significant population of free-floating planets
(FFPs). Both ground and space-based facilities are being used and developed
which will be able to probe the distrubution of FFPs with much better
sensitivity. It is vital also to develop a high-precision microlensing
simulation framework to evaluate the completeness of such surveys. We present
the first signal-to-noise limited calculations of the FFP microlensing rate
using the Besancon Galactic model. The microlensing distribution towards the
Galactic centre is simulated for wide-area ground-based optical surveys such as
OGLE or MOA, a wide-area ground-based near-IR survey, and a targeted
space-based near-IR survey which could be undertaken with Euclid or WFIRST. We
present a calculation framework for the computation of the optical and
near-infrared microlensing rate and optical depth for simulated stellar
catalogues which are signal-to-noise limited, and take account of extinction,
unresolved stellar background light and finite source size effects, which can
be significant for FFPs. We find that the global ground-based I-band yield over
a central 200 deg^2 region covering the Galactic centre ranges from 20
Earth-mass FFPs year^-1 up to 3,500 year^-1 for Jupiter FFPs in the limit of
100% detection efficiency, and almost an order of magnitude larger for a K-band
survey. For ground-based surveys we find that the inclusion of finite source
and the unresolved background reveals a mass-dependent variation in the spatial
distribution of FFPs. For a space-based H-band covering 2 deg^2, the yield
depends on the target field but maximizes close to the Galactic centre with
around 76 Earth through to 1,700 Jupiter FFPs year^-1. For near-IR space-based
surveys the spatial distribution of FFPs is found to be largely insensitive to
the FFP mass scale.Comment: 14 pages, submitted to A&A and accepte
Estimation from quantized Gaussian measurements: when and how to use dither
Subtractive dither is a powerful method for removing the signal dependence of quantization noise for coarsely quantized signals. However, estimation from dithered measurements often naively applies the sample mean or midrange, even when the total noise is not well described with a Gaussian or uniform distribution. We show that the generalized Gaussian distribution approximately describes subtractively dithered, quantized samples of a Gaussian signal. Furthermore, a generalized Gaussian fit leads to simple estimators based on order statistics that match the performance of more complicated maximum likelihood estimators requiring iterative solvers. The order statistics-based estimators outperform both the sample mean and midrange for nontrivial sums of Gaussian and uniform noise. Additional analysis of the generalized Gaussian approximation yields rules of thumb for determining when and how to apply dither to quantized measurements. Specifically, we find subtractive dither to be beneficial when the ratio between the Gaussian standard deviation and quantization interval length is roughly less than one-third. When that ratio is also greater than 0.822/K^0.930 for the number of measurements K > 20, estimators we present are more efficient than the midrange.https://arxiv.org/abs/1811.06856Accepted manuscrip
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
Accretion by the Galaxy
Cosmology requires at least half of the baryons in the Universe to be in the
intergalactic medium, much of which is believed to form hot coronae around
galaxies. Star-forming galaxies must be accreting from their coronae. HI
observations of external galaxies show that they have HI halos associated with
star formation. These halos are naturally modelled as ensembles of clouds
driven up by supernova bubbles. These models can fit the data successfully only
if clouds exchange mass and momentum with the corona. As a cloud orbits, it is
ablated and forms a turbulent wake where cold high-metallicity gas mixes with
hot coronal gas causing the prompt cooling of the latter. As a consequence the
total mass of HI increases. This model has recently been used to model the
Leiden-Argentina-Bonn survey of Galactic HI. The values of the model's
parameters that are required to model NGC 891, NGC 2403 and our Galaxy show a
remarkable degree of consistency, despite the very different natures of the two
external galaxies and the dramatic difference in the nature of the data for our
Galaxy and the external galaxies. The parameter values are also consistent with
hydrodynamical simulations of the ablation of individual clouds. The model
predicts that a galaxy that loses its cool-gas disc for instance through a
major merger cannot reform it from its corona; it can return to steady star
formation only if it can capture a large body of cool gas, for example by
accreting a gas-rich dwarf. Thus the model explains how major mergers can make
galaxies "red and dead."Comment: Invited review at "Assembling the Puzzle of the Milky Way", Grand
Bornand, April 2011; 6 page
Kinematic groups across the MW disc: insights from models and from the RAVE catalogue
With the advent of the Gaia data, the unprecedented kinematic census of great
part of the Milky Way disc will allow us to characterise the local kinematic
groups and new groups in different disc neighbourhoods. First, we show here
that the models predict a stellar kinematic response to the spiral arms and bar
strongly dependent on disc position. For example, we find that the kinematic
groups induced by the spiral arm models change significantly if one moves only
~ 0.6 kpc in galactocentric radius, but ~ 2 kpc in azimuth. There are more and
stronger groups as one approaches the spiral arms. Depending on the spiral
pattern speed, the kinematic imprints are more intense in nearby vicinities or
far from the Sun. Secondly, we present a preliminary study of the kinematic
groups observed by RAVE. This sample will allow us, for the first time, to
study the dependence on Galactic position of the (thin and thick) disc moving
groups. In the solar neighbourhood, we find the same kinematics groups as
detected in previous surveys, but now with better statistics and over a larger
spatial volume around the Sun. This indicates that these structures are indeed
large scale kinematic features.Comment: 4 pages, 3 figures, to appear in the proceedings of "Assembling the
Puzzle of the Milky Way", Le Grand Bornand (April 17-22, 2011), C. Reyle, A.
Robin, M. Schultheis (eds.
A review of Allen, A. and Ainley, P. (2013): The Great Reversal: young people, education and employment in a declining economy
The visibility of the Galactic bulge in optical surveys. Application to the Gaia mission
The bulge is a region of the Galaxy which is of tremendous interest for
understanding Galaxy formation. However, measuring photometry and kinematics in
it raises several inherent issues, like high extinction in the visible and
severe crowding. Here we attempt to estimate the problem of the visibility of
the bulge at optical wavelengths, where large CCD mosaics allow to easily cover
wide regions from the ground, and where future astrometric missions are
planned. Assuming the Besancon Galaxy model and high resolution extinction
maps, we estimate the stellar density as a function of longitude, latitude and
apparent magnitude and we deduce the possibility of reaching and measuring
bulge stars. The method is applied to three Gaia instruments, the BBP and MBP
photometers, and the RVS spectrograph. We conclude that, while in the BBP most
of the bulge will be accessible, in the MBP there will be a small but
significant number of regions where bulge stars will be detected and accurately
measured in crowded fields. Assuming that the RVS spectra may be extracted in
moderately crowded fields, the bulge will be accessible in most regions apart
from the strongly absorbed inner plane regions, because of high extinction, and
in low extinction windows like the Baades's window where the crowding is too
severe.Comment: 11 pages, 9 figures, accepted for publication in A&A, latex using A&A
macro
The Galactic bulge as seen in optical surveys
The bulge is a region of the Galaxy of tremendous interest for understanding
galaxy formation. However measuring photometry and kinematics in it raises
several inherent issues, such as severe crowding and high extinction in the
visible. Using the Besancon Galaxy model and a 3D extinction map, we estimate
the stellar density as a function of longitude, latitude and apparent magnitude
and we deduce the possibility of reaching and measuring bulge stars with Gaia.
We also present an ongoing analysis of the bulge using the Canada-France-Hawaii
Telescope.Comment: In SF2A-2008: Proceedings of the Annual meeting of the French Society
of Astronomy and Astrophysic
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