398 research outputs found
The effect of the Coriolis force on Kelvin-Helmholtz-driven mixing in protoplanetary disks
We study the stability of proto-planetary disks with vertical velocity
gradients in their equilibrium rotation rates; such gradients are expected to
develop when dust settles into the midplane. Using a linear stability analysis
of a simple three-layer model, we show that the onset of instability occurs at
a larger value of the Richardson number, and therefore for a thicker layer,
when the effects of Coriolis forces are included. This analysis also shows that
even-symmetry (midplane-crossing) modes develop faster than odd-symmetry ones.
These conclusions are corroborated by a large number of nonlinear numerical
simulations with two different parameterized prescriptions for the initial
(continuous) dust distributions. Based on these numerical experiments, the
Richardson number required for marginal stability is more than an order of
magnitude larger than the traditional 1/4 value. The dominant modes that grow
have horizontal wavelengths of several initial dust scale heights, and in
nonlinear stages mix solids fairly homogeneously over a comparable vertical
range. We conclude that gravitational instability may be more difficult to
achieve than previously thought, and that the vertical distribution of matter
within the dust layer is likely globally, rather than locally, determined.Comment: Accepted for publication in Ap
InfluĂŞncia de estresse por dĂ©ficit hĂdrico em acessos do gĂŞnero Paspalum.
Editado por Ana Rita de araĂşjo Nogueira, Simone Cristina MĂ©o Niciur
Contribution of vascular endothelial growth factor to the Nottingham prognostic index in node-negative breast cancer
The prognostic contribution of intratumour VEGF, the most important factor in tumour-induced angiogenesis, to NPI was evaluated by using flexible modelling in a series of 226 N-primary breast cancer patients in which steroid receptors and cell proliferation were also accounted for. VEGF provided an additional prognostic contribution to NPI mainly within ER-poor tumours. © 2001 Cancer Research Campaignhttp://www.bjcancer.co
Weed flora in rice areas under distinct cropping systems, herbicide and irrigation managements
We aimed to evaluate the incidence of weeds in the pre-planting of the summer crop as a function of planting system, herbicide use and irrigation management. The experiment was installed in field conditions, in RBD and 3 x 2 factorial scheme with eight replications. Treatments consisted in submitting rice to three management factors: water management – continuously flooded or intermittend irrigation (Factor A), coupled to the application (traditional control) or not (semi-ecological system) of herbicides (Factor B), and planting system – conventional soil tillage, minimum tillage and no till systems (Factor C). One year after rice cultivation, preceding the planting of the next cropping season, phytosociological evaluations of the weed communities were carried out. We assessed the overall infestation and weed species composition, which were classified by their respective density, frequency and dominance. We also estimated the diversity coefficients of Simpson and Shannon Weiner, and the sustainability coefficient of Shannon; treatments were also grouped by similarity in weed composition. In flood-irrigated rice, no till provides the lowest levels of weed infestation and, together with the conventional cropping system, results in values closer to the ecological sustainability; The application of herbicides in flooded rice crops reduces weed infestation, increases diversity and equalizes the ecological sustainability, compared to areas without the application of weed management methods. However, chemical control leads to the selection of resistant or tolerant species to herbicides, such as Polypogon sp.; Both continuous and intermittent water management systems did not cause changes in the level of infestation, composition or diversity coefficients
Vascular endothelial growth factor in node-positive breast cancer patients treated with adjuvant tamoxifen
In 212 postmenopausal women with node-positive oestrogen receptor-positive (ER(LBA)) breast cancer subjected to radical surgery and adjuvant tamoxifen, the risk of 6-year relapse increased with increasing values of intratumoral vascular endothelial growth factor (VEGF) in patients whose tumours had a low/intermediate ER(LBA) content compared to patients with high-ER(LBA) tumours. These findings indicate that tumour progression, activated or sustained by high VEGF levels, may be counteracted in high-ER(LBA) cancers by tamoxifen, which in contrast fails to contrast the metastatic potential in low-ER(LBA) tumours
Manejo de plantas daninhas em arroz irrigado pelo método intermitente de irrigação.
Este trabalho teve por objetivo avaliar as espĂ©cies de plantas que se estabelecem e a eficiĂŞncia do controle de arroz-daninho e capim-arroz em lavoura de arroz irrigado por inundação intermitente, comparativamente Ă inundação contĂnua, bem como o reflexo desses mĂ©todos de irrigação sobre o desempenho produtivo da lavoura de arroz.ODS
The activity of Main Belt comets
Main Belt comets represent a recently discovered class of objects. They are
quite intriguing because, while having a Tisserand invariant value higher than
3, are showing cometary activity. We study the activity of the Main Belt comets
making the assumption that they are icy-bodies and that the activity has been
triggered by an impact. We determine the characteristics of this activity and
if the nowadays impact rate in the Main Asteroid Belt is compatible with the
hypothesis of an activity triggered by a recent impact. Due to the fact that
the Main Belt comets can be considered as a kind of comets, we apply a thermal
evolution model developed for icy bodies in order to simulate their activity.
We also apply a model to derive the impact rate, with respect to the size of
the impactor, in the Main Belt. We demonstrate that a stable activity can
result from a recent impact, able to expose ice-rich layers, and that the
impact rate in the Main Belt is compatible with this explanation.Comment: 9 pages, 7 figure
Characterization of the weed flora in rice areas under distinct cropping systems and herbicide managements.
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