20,065 research outputs found
A distribution model for the aerial application of granular agricultural particles
A model is developed to predict the shape of the distribution of granular agricultural particles applied by aircraft. The particle is assumed to have a random size and shape and the model includes the effect of air resistance, distributor geometry and aircraft wake. General requirements for the maintenance of similarity of the distribution for scale model tests are derived and are addressed to the problem of a nongeneral drag law. It is shown that if the mean and variance of the particle diameter and density are scaled according to the scaling laws governing the system, the shape of the distribution will be preserved. Distributions are calculated numerically and show the effect of a random initial lateral position, particle size and drag coefficient. A listing of the computer code is included
Star formation history of Canis Major OB1 - II. A bimodal X-ray population revealed by XMM-Newton
The Canis Major OB1 Association has an intriguing scenario of star formation,
especially in the Canis Major R1 (CMa R1) region traditionally assigned to a
reflection nebula, but in reality an ionized region. This work is focused on
the young stellar population associated to CMa R1, for which our previous
results from ROSAT, optical and near-infrared data had revealed two stellar
groups with different ages, suggesting a possible mixing of populations
originated from distinct star-formation episodes. The X-ray data allow the
detected sources to be characterized according to hardness ratios, light curves
and spectra. Estimates of mass and age were obtained from the 2MASS catalogue,
and used to define a complete subsample of stellar counterparts, for
statistical purposes. A catalogue of 387 XMM-Newton sources is provided, 78%
being confirmed as members or probable members of the CMa R1 association.
Flares were observed for 13 sources, and the spectra of 21 bright sources could
be fitted by a thermal plasma model. Mean values of fits parameters were used
to estimate X-ray luminosities. We found a minimum value of log(L[erg/s]) =
29.43, indicating that our sample of low-mass stars (M 0.5
M), being faint X-ray emitters, is incomplete. Among the 250 objects
selected as our complete subsample (defining our best sample), 171 are found to
the East of the cloud, near Z CMa and dense molecular gas, 50% of them being
young ( 10 Myr). The opposite happens to the
West, near GU CMa, in areas lacking molecular gas: among 79 objects, 30% are
young and 50% are older. These findings confirm that a first episode of
distributed star formation occurred in the whole studied region ~10 Myr ago and
dispersed the molecular gas, while a second, localized episode (< 5 Myr) took
place in the regions where molecular gas is still present.Comment: 38 pages, 21 figures, accepted for A&
Spectroscopic characterization of X-ray emitting young stars associated with the Sh 2-296 nebula
We studied a sample of stars associated with the Sh 2-296 nebula, part of the
reflection nebulae complex in the region of Canis Major (CMa R1). Our sample
corresponds to optical counterparts of X-ray sources detected from observations
with the XMM-Newton satellite, which revealed dozens of possible low-mass young
stars not yet known in this region.
A sample of 58 young star candidates were selected based on optical spectral
features, mainly H{\alpha} and lithium lines, observed with multi-objects
spectroscopy performed by the Gemini South telescope. Among the candidates, we
find 41 confirmed T Tauri and 15 very likely young stars. Based on the
H{\alpha} emission, the T Tauri stars were distinguished between classical
(17%) and weak-lined (83%), but no significant difference was found in the age
and mass distribution of these two classes.
The characterization of the sample was complemented by near- and mid-infrared
data, providing an estimate of ages and masses from the comparison with
pre-main-sequence evolutionary models. While half of the young stars have an
age of 1-2 Myrs or less, only a small fraction (~25%) shows evidence of IR
excess revealing the presence of circumstellar discs. This low fraction is
quite rare compared to most young star-forming regions, suggesting that some
external factor has accelerated the disc dissipation
Nonlinear modeling of the cyclic response of RC columns
Cyclic load reversals (like those induced by earthquakes) result in accelerated bond degradation, leading to significant bar slippage. The bond-slip mechanism is reported to be one of the most common causes of damage and even collapse of existing RC structures subjected to earthquake loading. RC structures with plain reinforcing bars, designed and built prior to the enforcement of the modern seismic-oriented design philosophies, are particularly sensitive to bond degradation. However, perfect bond conditions are typically assumed in the numerical analysis of RC structures. This paper describes the numerical modeling of the cyclic response of two RC columns, one built with deformed bars and the other with plain bars and structural detailing similar to that typically adopted in pre-1970s structures. For each column, different modeling strategies to simulate the column response were tested. Models were built using the OpenSees and the SeismoStruct platforms, and calibrated with the available tests results. Within each platform, different types of nonlinear elements were used to represent the columns. Bond-slip effects were included in the OpenSees models resorting to a simple modeling strategy. The models and the parameters adopted are presented and discussed. Comparison is established between the most relevant experimental results and the corresponding results provided by the numerical models. Conclusions are drawn about the capacity of the tested models to simulate the columns response and about the influence of considering or not considering the effects of bars slippage
Impacts of in vivo and in vitro exposures to tamoxifen: comparative effects on human cells and marine organisms
Tamoxifen (TAM) is a first generation-SERM administered for hormone receptor-positive (HER+) breast cancer in both pre- and post-menopausal patients and may undergo metabolic activation in organisms that share similar receptors and thus face comparable mechanisms of response. The present study aimed to assess whether environmental trace concentrations of TAM are bioavailable to the filter feeder M. galloprovincialis (100 ng L-1) and to the deposit feeder N. diversicolor (0.5, 10, 25 and 100 ng L-1) after 14 days of exposure. Behavioural impairment (burrowing kinetic), neurotoxicity (AChE activity), endocrine disruption by alkali-labile phosphate (ALP) content, oxidative stress (SOD, CAT, GPXs activities), biotransformation (GST activity), oxidative damage (LPO) and genotoxicity (DNA damage) were assessed. Moreover, this study also pertained to compare TAM cytotoxicity effects to mussels and targeted human (i.e. immortalized retinal pigment epithelium - RPE; and human transformed endothelial cells - HeLa) cell lines, in a range of concentrations from 0.5 ng L-1 to 50 μg L-1. In polychaetes N. diversicolor, TAM exerted remarkable oxidative stress and damage at the lowest concentration (0.5 ng L-1), whereas significant genotoxicity was reported at the highest exposure level (100 ng L-1). In mussels M. galloprovincialis, 100 ng L-1 TAM caused endocrine disruption in males, neurotoxicity, and an induction in GST activity and LPO byproducts in gills, corroborating in genotoxicity over the exposure days. Although cytotoxicity assays conducted with mussel haemocytes following in vivo exposure was not effective, in vitro exposure showed to be a feasible alternative, with comparable sensitivity to human cell line (HeLa).info:eu-repo/semantics/publishedVersio
Modeling the input history of programs for improved instruction-memory performance
When a program is loaded into memory for execution, the relative position of
its basic blocks is crucial, since loading basic blocks that are unlikely to be
executed first places them high in the instruction-memory hierarchy only to be
dislodged as the execution goes on. In this paper we study the use of Bayesian
networks as models of the input history of a program. The main point is the
creation of a probabilistic model that persists as the program is run on
different inputs and at each new input refines its own parameters in order to
reflect the program's input history more accurately. As the model is thus
tuned, it causes basic blocks to be reordered so that, upon arrival of the next
input for execution, loading the basic blocks into memory automatically takes
into account the input history of the program. We report on extensive
experiments, whose results demonstrate the efficacy of the overall approach in
progressively lowering the execution times of a program on identical inputs
placed randomly in a sequence of varied inputs. We provide results on selected
SPEC CINT2000 programs and also evaluate our approach as compared to the gcc
level-3 optimization and to Pettis-Hansen reordering
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