1,404 research outputs found
The Milky Way's external disc constrained by 2MASS star counts
Context. Thanks to recent large scale surveys in the near infrared such as
2MASS, the galactic plane that most suffers from extinction is revealed and its
overall structure can be studied. Aims. This work aims at constraining the
structure of the Milky Way external disc as seen in 2MASS data, and in
particular the warp. Methods. We use the Two Micron All Sky Survey (hereafter
2MASS) along with the Stellar Population Synthesis Model of the Galaxy,
developed in Besancon, to constrain the external disc parameters such as its
scale length, its cutoff radius, and the slope of the warp. In order to
properly interpret the observations, the simulated stars are reddened using a
three dimensional extinction map. The shape of the stellar warp is then
compared with previous results and with similar structures in gas and dust.
Results. We find new constraints on the stellar disc, which is shown to be
asymmetrical, similar to observations of HI. The positive longitude side is
found to be easily modelled with a S shape warp but with a slope significantly
smaller than the slope seen in the HI warp. At negative longitudes, the disc
presents peculiarities which are not well reproduced by any simple model.
Finally, comparing with the warp seen in the dust, it seems to follow a slope
intermediate between the gas and the stars.Comment: 9 pages. Accepted for publication in Astronomy and Astrophysic
A power comparison of generalized additive models and the spatial scan statistic in a case-control setting
<p>Abstract</p> <p>Background</p> <p>A common, important problem in spatial epidemiology is measuring and identifying variation in disease risk across a study region. In application of statistical methods, the problem has two parts. First, spatial variation in risk must be detected across the study region and, second, areas of increased or decreased risk must be correctly identified. The location of such areas may give clues to environmental sources of exposure and disease etiology. One statistical method applicable in spatial epidemiologic settings is a generalized additive model (GAM) which can be applied with a bivariate LOESS smoother to account for geographic location as a possible predictor of disease status. A natural hypothesis when applying this method is whether residential location of subjects is associated with the outcome, i.e. is the smoothing term necessary? Permutation tests are a reasonable hypothesis testing method and provide adequate power under a simple alternative hypothesis. These tests have yet to be compared to other spatial statistics.</p> <p>Results</p> <p>This research uses simulated point data generated under three alternative hypotheses to evaluate the properties of the permutation methods and compare them to the popular spatial scan statistic in a case-control setting. Case 1 was a single circular cluster centered in a circular study region. The spatial scan statistic had the highest power though the GAM method estimates did not fall far behind. Case 2 was a single point source located at the center of a circular cluster and Case 3 was a line source at the center of the horizontal axis of a square study region. Each had linearly decreasing logodds with distance from the point. The GAM methods outperformed the scan statistic in Cases 2 and 3. Comparing sensitivity, measured as the proportion of the exposure source correctly identified as high or low risk, the GAM methods outperformed the scan statistic in all three Cases.</p> <p>Conclusions</p> <p>The GAM permutation testing methods provide a regression-based alternative to the spatial scan statistic. Across all hypotheses examined in this research, the GAM methods had competing or greater power estimates and sensitivities exceeding that of the spatial scan statistic.</p
A multilevel non-hierarchical study of birth weight and socioeconomic status
<p>Abstract</p> <p>Background</p> <p>It is unclear whether the socioeconomic status (SES) of the community of residence has a substantial association with infant birth weight. We used multilevel models to examine associations of birth weight with family- and community-level SES in the Cape Cod Family Health Study. Data were collected retrospectively on births to women between 1969 and 1983 living on Cape Cod, Massachusetts. The sample included siblings born in different residences with differing community-level SES.</p> <p>Methods</p> <p>We used cross-classified models to account for multiple levels of correlation in a non-hierarchical data structure. We accounted for clustering at family- and community-levels. Models included extensive individual- and family-level covariates. SES variables of interest were maternal education; paternal occupation; percent adults living in poverty; percent adults with a four year college degree; community mean family income; and percent adult unemployment.</p> <p>Results</p> <p>Residual correlation was detected at the family- but not the community-level. Substantial effects sizes were observed for family-level SES while smaller magnitudes were observed for community-level SES. Overall, higher SES corresponded to increased birth weight though neither family- nor community-level variables had significant associations with the outcome. In a model applied to a reduced sample that included a single child per family, enforcing a hierarchical data structure, paternal occupation was found to have a significant association with birth weight (p = 0.033). Larger effect sizes for community SES appeared in models applied to the full sample that contained limited covariates, such as those typically found on birth certificates.</p> <p>Conclusions</p> <p>Cross-classified models allowed us to include more than one child per family even when families moved between births. There was evidence of mild associations between family SES and birth weight. Stronger associations between paternal occupation and birth weight were observed in models applied to reduced samples with hierarchical data structures, illustrating consequences of excluding observations from the cross-classified analysis. Models with limited covariates showed associations of birth weight with community SES. In models adjusting for a complete set of individual- and family-level covariates, community SES was not as important.</p
String Fields and the Standard Model
The Cremmer-Scherk mechanism is generalised in a non-Abelian context. In the
presence of the Higgs scalars of the standard model it is argued that fields
arising from the low energy effective string action may contribute to the mass
generation of the observed vector bosons that mediate the electroweak
interactions and that future analyses of experimental data should consider the
possibility of string induced radiative corrections to the Weinberg angle
coming from physics beyond the standard model.Comment: 4 pages, LATEX, no figure
The “Ten Commandments” of treating preschool children who wheeze
Wheezing in young children is problematic for most practitioners. Difficulties arise in both the diagnosis and management
of this clinical phenotype. Not all preschool children who wheeze have asthma. Therefore, we suggest that the “Ten
Commandments” of managing preschool wheezing include thinking that in very young infants (< 1 year) wheezing is likely
to be viral in origin; realising that allergy testing is mandatory to diagnose the cause of early wheezing; taking a history of
asthma and allergy in family members; noting that chronic coughing is a pointer to asthma; using the term “asthma” if that
is the diagnosis; ensuring that the environmental avoidance of triggers is addressed; using a short course of montelukast for
virus-induced wheezing episodes; avoiding steroids to treat virus-induced wheezing; treating associated nasal symptoms;
and making sure that the follow-up of children addresses the issue of stopping therapy if it is not working.www.safpj.co.zaam201
3D outer bulge structure from near infrared star counts
We attempt to study the characteristics of the different stellar populations
present in the Galactic central region. A Monte Carlo method is used to
simultaneously fit 11 thin disc and triaxial outer bulge density parameters on
(Ks,J-Ks) star count data in almost 100 windows from the DENIS near infrared
large scale survey at -8 deg < l< 12 deg and -4 deg < b < 4 deg. Various bulge
density profiles and luminosity functions were tested uing a population
synthesis scheme. The best models, selected by a maximum likelihood test, give
the following description: the outer bulge is proxy, prolate, and oriented
10.6+/-3 deg with respect to the Sun-center direction. It seems that the main
bulge population is not older than 10 Gyr, but this preliminary result needs
further work to be confirmed. A significant central hole is found in the middle
of the thin disc. We discuss these results in regard to previous findings and
the scenario of bulge formation.Comment: 17 pages, 12 figures, accepted for publication by A&
Orbital Separation Amplification in Fragile Binaries with Evolved Components
The secular stellar mass-loss causes an amplification of the orbital
separation in fragile, common proper motion, binary systems with separations of
the order of 1000 A.U. In these systems, companions evolve as two independent
coeval stars as they experience negligible mutual tidal interactions or mass
transfer. We present models for how post-main sequence mass-loss statistically
distorts the frequency distribution of separations in fragile binaries. These
models demonstrate the expected increase in orbital seapration resulting from
stellar mass-loss, as well as a perturbation of associated orbital parameters.
Comparisons between our models and observations resulting from the Luyten
survey of wide visual binaries, specifically those containing MS and
white-dwarf pairs, demonstrate a good agreement between the calculated and the
observed angular separation distribution functions.Comment: 37 pages, 13 figure
Association between Residences in U.S. Northern Latitudes and Rheumatoid Arthritis: A Spatial Analysis of the Nurses’ Health Study
Background: The etiology of rheumatoid arthritis (RA) remains largely unknown, although epidemiologic studies suggest genetic and environmental factors may play a role. Geographic variation in incident RA has been observed at the regional level. Objective: Spatial analyses are a useful tool for confirming existing exposure hypotheses or generating new ones. To further explore the association between location and RA risk, we analyzed individual-level data from U.S. women in the Nurses’ Health Study, a nationwide cohort study. Methods: Participants included 461 incident RA cases and 9,220 controls with geocoded addresses; participants were followed from 1988 to 2002. We examined spatial variation using addresses at baseline in 1988 and at the time of case diagnosis or the censoring of controls. Generalized additive models (GAMs) were used to predict a continuous risk surface by smoothing on longitude and latitude while adjusting for known risk factors. Permutation tests were conducted to evaluate the overall importance of location and to identify, within the entire study area, those locations of statistically significant risk. Results: A statistically significant area of increased RA risk was identified in the northeast United States (p-value = 0.034). Risk was generally higher at northern latitudes, and it increased slightly when we used the nurses’ 1988 locations compared with those at the time of diagnosis or censoring. Crude and adjusted models produced similar results. Conclusions: Spatial analyses suggest women living in higher latitudes may be at greater risk for RA. Further, RA risk may be greater for locations that occur earlier in residential histories. These results illustrate the usefulness of GAM methods in generating hypotheses for future investigation and supporting existing hypotheses
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