Mycoplasma-like diseases

Since 1967, many diseases have been associated with mycoplasma- like microorganisms (MLM), especially when symptoms have been characterized by general plant chlorosis, stunting, excessive proliferation of branches (witches' broom), bud proliferation and disorders of floral organs (phyllody and virescence). Many causal agents are transmitted naturally by leafhoppers to various hosts. Various MLM are known to infect beans, such as legume little-leaf transmitted by Orosius argentatus, witches'-broom, phyllody and virescence. In Colombia, a MLM was detected and is known as machismo or amachamiento, transmitted by Scaphytopius fuliginosus. The symptoms of infection generaly become apparent during flowering and pod development. Control measures include adequate crop rotation and elimination of weed hosts, and the use of insecticides. The symptoms and damage caused by the organisms are illustrated in color. (CIAT)Desde 1967, muchas enfermedades se han asociado con los microorganismos semejantes a micoplasmas (MSM) especialmente cuando los sintomas se han caracterizado por una clorosis general de la planta, raquitismo, proliferacion excesiva de ramas (superbrotamiento) y desordenes en los organos florales (filodia y virescencia). Los saltahojas transmiten en forma natural numerosos agentes causales a diversos hospedantes. Se conocen varios MSM que infectan el frijol como la hoja pequena de las leguminosas transmitida por Orosius argentatus, el superbrotamiento y filodia y virescencia. En Colombia se registro un MSM conocido como machismo o amachamiento transmitido por Scaphytopius fuliginosus. Los sintomas de infeccion generalmente se manifiestan durante el periodo de floracion y formacion de vainas. Las medidas de control incluyen una adecuada rotacion de cultivos y eliminacion de las malezas hospedantes, y el uso de insecticidas. Se ilustran a color los sintomas y danos causados por los organismos. (RA

Mycoplasma-like diseases

Since 1967, many diseases have been associated with mycoplasma- like microorganisms (MLM), especially when symptoms have been characterized by general plant chlorosis, stunting, excessive proliferation of branches (witches' broom), bud proliferation and disorders of floral organs (phyllody and virescence). Many causal agents are transmitted naturally by leafhoppers to various hosts. Various MLM are known to infect beans, such as legume little-leaf transmitted by Orosius argentatus, witches'-broom, phyllody and virescence. In Colombia, a MLM was detected and is known as machismo or amachamiento, transmitted by Scaphytopius fuliginosus. The symptoms of infection generaly become apparent during flowering and pod development. Control measures include adequate crop rotation and elimination of weed hosts, and the use of insecticides. The symptoms and damage caused by the organisms are illustrated in color. (CIAT)Desde 1967, muchas enfermedades se han asociado con los microorganismos semejantes a micoplasmas (MSM) especialmente cuando los sintomas se han caracterizado por una clorosis general de la planta, raquitismo, proliferacion excesiva de ramas (superbrotamiento) y desordenes en los organos florales (filodia y virescencia). Los saltahojas transmiten en forma natural numerosos agentes causales a diversos hospedantes. Se conocen varios MSM que infectan el frijol como la hoja pequena de las leguminosas transmitida por Orosius argentatus, el superbrotamiento y filodia y virescencia. En Colombia se registro un MSM conocido como machismo o amachamiento transmitido por Scaphytopius fuliginosus. Los sintomas de infeccion generalmente se manifiestan durante el periodo de floracion y formacion de vainas. Las medidas de control incluyen una adecuada rotacion de cultivos y eliminacion de las malezas hospedantes, y el uso de insecticidas. Se ilustran a color los sintomas y danos causados por los organismos. (RA

Direct probing of band-structure Berry phase in diluted magnetic semiconductors

We report on experimental evidence of the Berry phase accumulated by the charge carrier wave function in single-domain nanowires made from a (Ga,Mn)(As,P) diluted ferromagnetic semiconductor layer. Its signature on the mesoscopic transport measurements is revealed as unusual patterns in the magnetoconductance, that are clearly distinguished from the universal conductance fluctuations. We show that these patterns appear in a magnetic field region where the magnetization rotates coherently and are related to a change in the band-structure Berry phase as the magnetization direction changes. They should be thus considered as a band structure Berry phase fingerprint of the effective magnetic monopoles in the momentum space. We argue that this is an efficient method to vary the band structure in a controlled way and to probe it directly. Hence, (Ga,Mn)As appears to be a very interesting test bench for new concepts based on this geometrical phase.Comment: 7 pages, 6 figure

Populations of rotating stars II. Rapid rotators and their link to Be-type stars

Even though it is broadly accepted that single Be stars are rapidly rotating stars surrounded by a flat rotating circumstellar disk, there is still a debate about how fast these stars rotate and also about the mechanisms involved in the angular-momentum and mass input in the disk. We study the properties of stars that rotate near their critical-rotation rate and investigate the properties of the disks formed by equatorial mass ejections. We used the most recent Geneva stellar evolutionary tracks for rapidly rotating stars that reach the critical limit and used a simple model for the disk structure. We obtain that for a 9 Msun star at solar metallicity, the minimum average velocity during the Main Sequence phase to reach the critical velocity is around 330 km/s, whereas it would be 390 km/s at the metallicity of the Small Magellanic Cloud (SMC). Red giants or supergiants originating from very rapid rotators rotate six times faster and show N/C ratios three times higher than those originating from slowly rotating stars. This difference becomes stronger at lower metallicity. It might therefore be very interesting to study the red giants in clusters that show a large number of Be stars on the MS band. On the basis of our single-star models, we show that the observed Be-star fraction with cluster age is compatible with the existence of a temperature-dependent lower limit in the velocity rate required for a star to become a Be star. The mass, extension, and diffusion time of the disks produced when the star is losing mass at the critical velocity, obtained from simple parametrized expressions, are not too far from those estimated for disks around Be-type stars. At a given metallicity, the mass and the extension of the disk increase with the initial mass and with age on the MS phase. Denser disks are expected in low-metallicity regions.Comment: Accepted for publication in A&A, language edite

Close binary evolution I. The tidally induced shear mixing in rotating binaries

We study how tides in a binary system induce some specific internal shear mixing, able to substantially modify the evolution of close binaries prior to mass transfer. We construct numerical models accounting for tidal interactions, meridional circulation, transport of angular momentum, shears and horizontal turbulence and consider a variety of orbital periods and initial rotation velocities. Depending on orbital periods and rotation velocities, tidal effects may spin down (spin down Case) or spin up (spin up Case) the axial rotation. In both cases, tides may induce a large internal differential rotation. The resulting tidally induced shear mixing (TISM) is so efficient that the internal distributions of angular velocity and chemical elements are greatly influenced. The evolutionary tracks are modified, and in both cases of spin down and spin up, large amounts of nitrogen can be transported to the stellar surfaces before any binary mass transfer. Meridional circulation, when properly treated as an advection, always tends to counteract the tidal interaction, tending to spin up the surface when it is braked down and vice versa. As a consequence, the times needed for the axial angular velocity to become equal to the orbital angular velocity may be larger than given by typical synchronization timescales. Also, due to meridional circulation some differential rotation remains in tidally locked binary systems.Comment: 10 pages, 18 figures, Accepted for publication in Astronomy and Astrophysic

Localized magnetoplasmon modes arising from broken translational symmetry in semiconductor superlattices

The electromagnetic propagator associated with the localized collective magnetoplasmon excitations in a semiconductor superlattice with broken translational symmetry, is calculated analytically within linear response theory. We discuss the properties of these collective excitations in both radiative and non-radiative regimes of the electromagnetic spectra. We find that low frequency retarded modes arise when the surface density of carriers at the symmetry breaking layer is lower than the density at the remaining layers. Otherwise a doublet of localized, high-frequency magnetoplasmon-like modes occurs.Comment: Revtex file + separate pdf figure

Multi-epoch <i>L</i>-band Spectroscopy of the Be Star μ Centauri Prior to Outburst

With the aim of contributing to the understanding of the disk formation process in Be stars, we pursued a one-year spectroscopic observing campaign of the Be star μ Centauri in the L-band, using VLT/ISAAC. We present the nine near-IR spectra we obtained in an epoch of relative photometric quiescence prior to an outburst of ΔV = 0.4 magnitude. Visual estimates during the epoch of our L-band spectroscopy are also presented for the first time, together with the unpublished complete visual light curve between the years 1998 and 2014. We observe significant and monotonic changes in emission line strength of Bracket-α and Pfund-γ lines relative to Humphreys lines, and also in the continuum slope. We interpret these observed changes in terms of important changes to the column density of the line emitting regions, moving from an optically thin to an optically thick stage just prior to a major outburst. For each observing date, we provide estimates for the column density and relative extension of the line emitting region. If the changes observed toward the end of our observing campaign were related to mass-loss changes from the central star, they would correspond to an increase in a factor of two in the mass of the disk in the innermost region. If related to the visual outburst observed one month later, the variability observed in our spectra would be the first detection of the early disk formation process in the L-band.Instituto de Astrofísica de La PlataFacultad de Ciencias Astronómicas y Geofísica

The impact of mass-loss on the evolution and pre-supernova properties of red supergiants

The post main-sequence evolution of massive stars is very sensitive to many parameters of the stellar models. Key parameters are the mixing processes, the metallicity, the mass-loss rate and the effect of a close companion. We study how the red supergiant lifetimes, the tracks in the Hertzsprung-Russel diagram (HRD), the positions in this diagram of the pre-supernova progenitor as well as the structure of the stars at that time change for various mass-loss rates during the red supergiant phase (RSG), and for two different initial rotation velocities. The surface abundances of RSGs are much more sensitive to rotation than to the mass-loss rates during that phase. A change of the RSG mass-loss rate has a strong impact on the RSG lifetimes and therefore on the luminosity function of RSGs. At solar metallicity, the enhanced mass-loss rate models do produce significant changes on the populations of blue, yellow and red supergiants. When extended blue loops or blue ward excursions are produced by enhanced mass-loss, the models predict that a majority of blue (yellow) supergiants are post RSG objects. These post RSG stars are predicted to show much smaller surface rotational velocities than similar blue supergiants on their first crossing of the HR gap. The position in the HRD of the end point of the evolution depends on the mass of the hydrogen envelope. More precisely, whenever, at the pre-supernova stage, the H-rich envelope contains more than about 5\% of the initial mass, the star is a red supergiant, and whenever the H-rich envelope contains less than 1\% of the total mass the star is a blue supergiant. For intermediate situations, intermediate colors/effective temperatures are obtained. Yellow progenitors for core collapse supernovae can be explained by the enhanced mass-loss rate models, while the red progenitors are better fitted by the standard mass-loss rate models.Comment: 19 pages, 11 figures, 6 tables, accepted for publication in Astronomy and Astrophysic