30,305 research outputs found
Spectral Variations in Early-Type Galaxies as a Function of Mass
We report on the strengths of three spectral indicators - Mg_2, Hbeta, and
Hn/Fe - in the integrated light of a sample of 100 field and cluster E/S0
galaxies. The measured indices are sensitive to age and/or and metallicity
variations within the galaxy sample. Using linear regression analysis for data
with non-uniform errors, we determine the intrinsic scatter present among the
spectral indices of our galaxy sample as a function of internal velocity
dispersion. Our analysis indicates that there is significantly more intrinsic
scatter in the two Balmer line indices than in the Mg_2 index, indicating that
the Balmer indices provide more dynamic range in determining the age of a
stellar population than does the Mg_2 index. Furthermore, the scatter is much
larger for the low velocity dispersion galaxies, indicating that star formation
has occurred more recently in the lower mass galaxies.Comment: 4 pages, 1 figure, 1 table, to appear in the Astrophysical Journal
Letter
Estimation of Delamination Crack Depth Using Passive Thermography
Passive thermography is used to monitor small increases in temperature resulting from delamination damage formation in a composite hat-stiffened panel during quasi-static loading. The heating is composed of two heat generation components. The first component is an instantaneous response due to a strain release during quasi-static loading. The second component is mechanical heating, at the interface of failure, due to fracture damage. This second component produces a transient rise in temperature that is a function of the damage depth and thermal diffusivity. The first component defines the thermal start time for the transient response. A one-dimensional thermal model is used to determine the damage depth. The results are compared to ultrasonic and X-ray CT data. The advantages and limitations of the thermal technique for damage depth detection are discussed
Time outdoors and the prevention of myopia
Recent epidemiological evidence suggests that children who spend more time outdoors are less likely to be, or to become myopic, irrespective of how much near work they do, or whether their parents are myopic. It is currently uncertain if time outdoors also blocks progression of myopia. It has been suggested that the mechanism of the protective effect of time outdoors involves light-stimulated release of dopamine from the retina, since increased dopamine release appears to inhibit increased axial elongation, which is the structural basis of myopia. This hypothesis has been supported by animal experiments which have replicated the protective effects of bright light against the development of myopia under laboratory conditions, and have shown that the effect is, at least in part, mediated by dopamine, since the D2-dopamine antagonist spiperone reduces the protective effect. There are some inconsistencies in the evidence, most notably the limited inhibition by bright light under laboratory conditions of lens-induced myopia in monkeys, but other proposed mechanisms possibly associated with time outdoors such as relaxed accommodation, more uniform dioptric space, increased pupil constriction, exposure to UV light,
changes in the spectral composition of visible light, or increased physical activity have little epidemiological
or experimental support. Irrespective of the mechanisms involved, clinical trials are now underway
to reduce the development of myopia in children by increasing the amount of time they spend outdoors. These trials would benefit from more precise definition of thresholds for protection in terms of intensity and duration of light exposures. These can be investigated in animal experiments in appropriate models, and can also be determined in epidemiological studies, although more precise measurement of exposures than those currently provided by questionnaires is desirable
Time evolution of the Partridge-Barton Model
The time evolution of the Partridge-Barton model in the presence of the
pleiotropic constraint and deleterious somatic mutations is exactly solved for
arbitrary fecundity in the context of a matricial formalism. Analytical
expressions for the time dependence of the mean survival probabilities are
derived. Using the fact that the asymptotic behavior for large time is
controlled by the largest matrix eigenvalue, we obtain the steady state values
for the mean survival probabilities and the Malthusian growth exponent. The
mean age of the population exhibits a power law decayment. Some Monte
Carlo simulations were also performed and they corroborated our theoretical
results.Comment: 10 pages, Latex, 1 postscript figure, published in Phys. Rev. E 61,
5664 (2000
Exploring Challenges in Conducting E-Mental Health Research Among Asian American Women
In this discussion paper, we explore the challenges of conducting e-mental health intervention research among Asian American women and propose a model for addressing these barriers. Based on an extensive literature review, we identify two main types of barriers to conducting e-mental health intervention research among Asian American women: recruitment barriers and adherence barriers. Recruitment barriers are further subcategorized into those related to (1) stigmatized cultural beliefs about mental illness and mental health services; (2) lack of awareness about mental health services; and (3) language barrier. As to adherence barriers, the two identified subtypes concern (1) acuity and severity of mental health condition; and (2) lack of time. In order to enhance recruitment and adherence in e-mental health intervention research among the studied population, we formulate the following three main research strategies, namely: (1) considering the cultural and social contexts of Asian American women in the development of e-mental health interventions; (2) determining appropriate program length; and (3) conducting feasibility studies to test e-mental health interventions. We suggest that nurse researchers integrate our proposed model in conducting e-mental health interventions among Asian American women. Our proposed model also implies that nurses play an important role in encouraging Asian American women’s acceptance of and adherence to e-mental health interventions. In order to overcome the obstacles to conducting e-mental health research among Asian American women, we recommend that nurses familiarize themselves with credible, relevant, and evidence-based e-mental health resources and integrate online mental health services and information within their nursing practice
A Solution of the Maxwell-Dirac Equations in 3+1 Dimensions
We investigate a class of localized, stationary, particular numerical
solutions to the Maxwell-Dirac system of classical nonlinear field equations.
The solutions are discrete energy eigenstates bound predominantly by the
self-produced electric field.Comment: 12 pages, revtex, 2 figure
HST and Spitzer point source detection and optical extinction in powerful narrow-line radio galaxies
We present the analysis of infrared HST and Spitzer data for a sample of 13
FRII radio galaxies at 0.03<z<0.11 that are classified as narrow-line radio
galaxies (NLRG). In the context of the unified schemes for active galactic
nuclei (AGN), our direct view of the AGN in NLRG is impeded by a parsec-scale
dusty torus structure. Our high resolution infrared observations provide new
information about the degree of extinction induced by the torus, and the
incidence of obscured AGN in NLRG.
We find that the point-like nucleus detection rate increases from 25 per cent
at 1.025m, to 80 per cent at 2.05m, and to 100 per cent at 8.0m.
This supports the idea that most NLRG host an obscured AGN in their centre. We
estimate the extinction from the obscuring structures using X-ray, near-IR and
mid-IR data. We find that the optical extinction derived from the 9.7m
silicate absorption feature is consistently lower than the extinction derived
using other techniques. This discrepancy challenges the assumption that all the
mid-infrared emission of NLRG is extinguished by a simple screen of dust at
larger radii. This disagreement can be explained in terms of either weakening
of the silicate absorption feature by (i) thermal mid-IR emission from the
narrow-line region, (ii) non-thermal emission from the base of the radio jets,
or (iii) by direct warm dust emission that leaks through a clumpy torus without
suffering major attenuation.Comment: 18 pages, 7 figures, 8 tables, accepted for publication in MNRA
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