5,851 research outputs found
Circumstellar rings, flat and flaring discs
Emission lines formed in the circumstellar envelopes of several type of stars
can be modeled using first principles of line formation. We present simple ways
of calculating line emission profiles formed in circumstellar envelopes having
different geometrical configurations. The fit of the observed line profiles
with the calculated ones may give first order estimates of the physical
parameters characterizing the line formation regions: opacity, size, particle
density distribution, velocity fields, excitation temperature.Comment: 3 pages ; to appear in the proceedings of the Sapporo meeting on
active OB stars ; ASP Conference Series ; eds: S. Stefl, S. Owocki and A.
Okazak
Critical study of the distribution of rotational velocities of Be stars; II: Differential rotation and some hidden effects interfering with the interpretation of the Vsin i parameter
We assume that stars may undergo surface differential rotation to study its
impact on the interpretation of and on the observed distribution
of ratios of true rotational velocities u=V/V_\rm c (V_\rm c is
the equatorial critical velocity). We discuss some phenomena affecting the
formation of spectral lines and their broadening, which can obliterate the
information carried by concerning the actual stellar rotation. We
studied the line broadening produced by several differential rotational laws,
but adopted Maunder's expression
as an attempt to account for
all of these laws with the lowest possible number of free parameters. We
studied the effect of the differential rotation parameter on the
measured parameter and on the distribution of ratios
u=V/V_\rm c. We conclude that the inferred is smaller than
implied by the actual equatorial linear rotation velocity V_\rm eq if the
stars rotate with . For a
given the deviations of are larger when . If
the studied Be stars have on average , the number of rotators with
V_\rm eq\simeq0.9V_\rm c is larger than expected from the observed
distribution ; if these stars have on average , this number
is lower than expected. We discuss seven phenomena that contribute either to
narrow or broaden spectral lines, which blur the information on the rotation
carried by and, in particular, to decide whether the Be phenomenon
mostly rely on the critical rotation. We show that two-dimensional radiation
transfer calculations are needed in rapid rotators to diagnose the stellar
rotation more reliably.Comment: To appear in A&
Effects of Pleistocene climate changes on species ranges and evolutionary processes in the Neotropical Atlantic Forest
The effects of global glaciations on the distribution of organisms is an essential element of many diversification models. However, the empirical evidence supporting this idea is mixed, in particular with respect to explaining tropical forest evolution. In the present study, we evaluated the impacts of range shifts associated with Pleistocene global glacial cycles on the evolution of tropical forests. In particular, we tested the predictions: (1) that population genetic structure increases with fragmentation variation between the present and the Last Glacial Maximum (LGM) and also (2) with geographical range instability; and (3) that genetic diversity increases with range stability and (4) decreases with fragmentation variation between periods. To address our predictions, we studied population genetic structures and modelled present and past distributions of 15 Atlantic Forest (AF) endemic birds. Afterwards, we evaluated the relationship of population genetic parameters with metrics of species range shifts between the present and the LGM. We found that geographical ranges of AF birds changed in concert with Pleistocene glacial cycles but, unexpectedly, our findings suggest the novel idea that ranges during glacial maxima were slightly larger on average, as well as equally fragmented and displaced from the interglacial ranges. Our findings suggest that range shifts over the late Pleistocene impacted on the diversification of forest organisms, although they did not show that those range shifts had a strong effect. We found that a combination of fragmentation variation across time, small current range size, and range stability increased population genetic structure. However, neither fragmentation, nor range stability affected genetic diversity. Our study showed that evolutionary responses to range shifts across AF birds have a high variance, which could explain the mixed support given by single-species studies to the action of Pleistocene range shifts on population evolution.Fil: Cabanne, Gustavo Sebastián. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Museo Argentino de Ciencias Naturales "Bernardino Rivadavia"; ArgentinaFil: Calderón, Pablo Luciano Sebastian. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Museo Argentino de Ciencias Naturales "Bernardino Rivadavia"; ArgentinaFil: Trujillo Arias, Natalia. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Museo Argentino de Ciencias Naturales "Bernardino Rivadavia"; ArgentinaFil: Flores, Pamela. Universidad Nacional de General Sarmiento; ArgentinaFil: Pessoa, Rodrigo. Universidade Estadual de Montes Claros; BrasilFil: d'Horta, Fernando M.. Instituto Nacional de Pesquisas da Amazônia; BrasilFil: Miyaki, Cristina Y.. Universidade de Sao Paulo; Brasi
Factorizations of one dimensional classical systems
A class of one dimensional classical systems is characterized from an
algebraic point of view. The Hamiltonians of these systems are factorized in
terms of two functions that together with the Hamiltonian itself close a
Poisson algebra. These two functions lead directly to two time-dependent
integrals of motion from which the phase motions are derived algebraically. The
systems so obtained constitute the classical analogues of the well known
factorizable one dimensional quantum mechanical systems.Comment: 19 pages, 7 figure
SU(2) Kinetic Mixing Terms and Spontaneous Symmetry Breaking
The non-abelian generalization of the Holdom model --{\it i.e.} a theory with
two gauge fields coupled to the kinetic mixing term -- is considered. Contrarily to the abelian case, the group
structure is explicitly broken to . For SU(2) this fact implies
that the residual gauge symmetry as well as the Lorentz symmetry is
spontaneusly broken. We show that this mechanism provides of masses for the
involved particles. Also, the model presents instanton solutions with a
redefined coupling constant.Comment: 9pp. typos and clarifications are adde
SrKZnMnAs: a ferromagnetic semiconductor with colossal magnetoresistance
A bulk diluted magnetic semiconductor (Sr,K)(Zn,Mn)As was
synthesized with decoupled charge and spin doping. It has a hexagonal
CaAlSi-type structure with the (Zn,Mn)As layer forming
a honeycomb-like network. Magnetization measurements show that the sample
undergoes a ferromagnetic transition with a Curie temperature of 12 K and
\revision{magnetic moment reaches about 1.5 /Mn under = 5 T
and = 2 K}. Surprisingly, a colossal negative magnetoresistance, defined as
, up to 38\% under a low field of = 0.1
T and to 99.8\% under = 5 T, was observed at = 2 K. The
colossal magnetoresistance can be explained based on the Anderson localization
theory.Comment: Accepted for publication in EP
Influence of MWCNT/surfactant dispersions on the mechanical properties of Portland cement pastes
This work studies the reinforcing effect of Multi Walled Carbon Nanotubes (MWCNT) on cement pastes. A 0.35% solid concentration of MWCNT in powder was dispersed in deionized water with sodium dodecyl sulfate (cationic surfactant), cetylpyridinium chloride (anionic surfactant) and triton X-100 (amphoteric surfactant) using an ultrasonic tip processor. Three concentrations of each surfactant (1mM, 10mM and 100mM) were tested, and all samples were sonicated until an adequate dispersion degree was obtained. Cement pastes with additions of carbon nanotubes of 0.15% by mass of cement were produced in two steps; first the dispersions of MWCNT were combined with the mixing water using an ultrasonic tip processor to guarantee homogeneity, and then cement was added and mixed until a homogeneous paste was obtained. Direct tensile strength, apparent density and open porosity of the pastes were measured after 7 days of curing. It was found that the MWCNT/surfactants dispersions decrease the mechanical properties of the cement based matrix due to an increased porosity caused by the presence of surfactants. © Published under licence by IOP Publishing Ltd
Looking for discrete UV absorption features in the early-type eclipsing binaries μ1 Scorpii and AO Cassiopeiae
A search for discrete absorption components in the ultraviolet spectra of the early-type binaries μ1 Scorpii and AO Cassiopeiae has been undertaken by analyzing material secured with the International Ultraviolet Explorer satellite during an exclusively assigned interval of nearly 50 hr. While the spectra of μ1 Sco definitely do not show the presence of such lines, the spectra of AO Cas do confirm them and permit us to draw some conclusions about where they may be formed.Facultad de Ciencias Astronómicas y Geofísica
Lipid antioxidants: free radical scavenging versus regulation of enzymatic lipid peroxidation
The essentiality of polyunsaturated lipids makes membranes susceptible to peroxidative modifications. One of the most contemporary examples includes selective peroxidation of cardiolipin in mitochondria of cells undergoing apoptosis. Cardiolipin peroxidation products are required for the mitochondrial membrane permeabilization, release of pro-apoptotic factors and completion of the cell death program. Therefore, search for effective inhibitors of cardiolipin peroxidation is critical to discovery and development of anti-apoptotic antioxidants. Mitochondria contain significant amounts of α-tocopherol, a well known scavenger of reactive free radicals. In the present study, we used an oxidative lipidomics approach to evaluate the effect of α-tocopherol and its homologues with different lengths of the side-chain such as 2,5,7,8,-tetramethyl-2(4-methylpentyl)-6-chromanol and 2,2,5,7,8-pentamethyl-6-chromanol, on oxidation of tetralinoleoyl cardiolipin induced by cytochrome c in the presence of hydrogen peroxide. Our data indicate that vitamin E homologues inhibit not only accumulation of tetralinoleoyl cardiolipin hydroperoxides but also hydroxy-derivatives of tetralinoleoyl cardiolipin formed in the enzymatic peroxidase half-reaction catalyzed by cytochrome c. This suggests that protective effects of vitamin E homologues against tetralinoleoyl cardiolipin peroxidation catalyzed by cytochrome c/hydrogen peroxide are realized largely due to their effects on the peroxidase activity of cytochrome c towards tetralinoleoyl cardiolipin rather than via their scavenging activity
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