14,632 research outputs found
Toward in vitro fertilization in Brachiaria spp.
Brachiaria are forage grasses widely cultivated in tropical areas. In vitro pollination was applied to accessions of Brachiaria spp. by placing pollen of non-dehiscent anthers on a solid medium near isolated ovaries. Viability and in vitro germination were tested in order to establish good conditions for pollen development. Comparing sexual to apomictic plants, apomictic pollen has more abortion after meiosis during the microspore stage and a lower viability and, of both types, only some plants have sufficient germination in a high sugar concentration. Using in vitro pollination with the sexual plant, the pollen tube penetrates into the nucellus and micropyle, but the embryo sac degenerates and collapses. In the apomictic B. decumbens, in vitro pollination leads to the transfer of the sperm nuclei into the egg cell and the central cell. The results are discussed according to normal fertilization and barriers in sexual and apomictic plants
The Early Palomar Program (1950-1955) for the Discovery of Classical Novae in M81: Analysis of the Spatial Distribution, Magnitude Distribution, and Distance Suggestion
Data obtained in the 1950-1955 Palomar campaign for the discovery of
classical novae in M81 are set out in detail. Positions and apparent B
magnitudes are listed for the 23 novae that were found. There is modest
evidence that the spatial distribution of the novae does not track the B
brightness distribution of either the total light or the light beyond an
isophotal radius that is 70\arcsec from the center of M81. The nova
distribution is more extended than the aforementioned light, with a significant
fraction of the sample appearing in the outer disk/spiral arm region. We
suggest that many (perhaps a majority) of the M81 novae that are observed at
any given epoch (compared with say years ago) are daughters of
Population I interacting binaries. The conclusion that the present day novae
are drawn from two population groups, one from low mass white dwarf secondaries
of close binaries identified with the bulge/thick disk population, and the
other from massive white dwarf secondaries identified with the outer thin
disk/spiral arm population, is discussed. We conclude that the M81 data are
consistent with the two population division as argued previously from (1) the
observational studies on other grounds by Della Valle et al. (1992, 1994),
Della Valle & Livio (1998), and Shafter et al. (1996) of nearby galaxies, (2)
the Hatano et al. (1997a,b) Monte Carlo simulations of novae in M31 and in the
Galaxy, and (3) the Yungelson et al. (1997) population synthesis modeling of
nova binaries. Two different methods of using M81 novae as distance indicators
give a nova distance modulus for M81 as , consistent with the
Cepheid modulus that is the same value.Comment: 24 pages, 7 figures, accepted to PAS
Superconducting/magnetic three state nanodevice for memory and reading applications
We present a simple nanodevice that can operate in two modes: i) three-state
memory and ii) reading device. The nanodevice is fabricated with an array of
ordered triangular-shaped nanomagnets embedded in a superconducting thin film.
The input signal is ac current and the output signal is dc voltage. Vortex
ratchet effect in combination with out of plane magnetic anisotropy of the
nanomagnets is the background physics which governs the nanodevice performance.Comment: 10 pages, 4 figure
Scaling of the conductance in gold nanotubes
A new form of gold nanobridges has been recently observed in ultrahigh-vacuum
experiments, where the gold atoms rearrange to build helical nanotubes, akin in
some respects to carbon nanotubes. The good reproducibility of these wires and
their unexpected stability will allow for conductance measurements and make
them promising candidates for future applications . We present here a study of
the transport properties of these nanotubes in order to understand the role of
chirality and of the different orbitals in quantum transport observables. The
conductance per atomic row shows a light decreasing trend as the diameter
grows, which is also shown through an analytical formula based on a one-orbital
model.Comment: 5 pages, 6 figure
A new web-based genomics resource for bioinformatics analysis of Rhipicephalus (Boophilus) microplus: CattleTickBase
No abstract availabl
Dynamics of formation and decay of coherence in a polariton condensate
We study the dynamics of formation and decay of a condensate of microcavity
polaritons. We investigate the relationship between the number of particles,
the emission's linewidth and its degree of linear polarization which serves as
the order parameter. Tracking the condensate's formation, we show that, even
when interactions are negligible, coherence is not determined only by
occupation of the ground state. As a result of the competition between the
coherent and thermal fractions of the condensate, the highest coherence is
obtained some time after the particle number has reached its maximum
The strong-CP question in SU(3)_c X SU(3)_L X U(1)_N models
We analyze two recent models based on the gauge group
SU(3)SU(3)U(1) where each generation is not
anomaly-free, but anomaly cancels when three generations are taken into
account. We show that the most general Yukawa couplings of these models admit
of a Peccei-Quinn symmetry. This symmetry can be extended to the entire
Lagrangian by using extra fields in a very elegant way so that the resulting
axion can be made invisible.Comment: Latex, 8 pages, no figure
A NICMOS search for obscured Supernovae in starburst galaxies
The detection of obscured supernovae (SNe) in near-infrared monitoring campaigns of starburst galaxies has shown that a significant fraction of SNe is missed by optical surveys. However, the number of SNe detected in ground-based near-IR observations is still significantly lower than the number of SNe extrapolated from the FIR luminosity of the hosts. A possibility is that most SNe occur within the nuclear regions, where the limited angular resolution of ground-based observations prevents their detection. This issue prompted us to exploit the superior angular resolution of NICMOS-HST to search for obscured SNe within the first kpc from the nucleus of strong starbursting galaxies. A total of 17 galaxies were observed in SNAPSHOT mode. Based on their FIR luminosity, we expected to detect not less than ~12 SNe. However, no confirmed SN event was found. From our data we derive an observed nuclear SN rate 11; ii) most SNe occur within the first 0.5" (which corresponds in our sample to about 500pc) where even NICMOS is unable to detect SN events
Parity Violation in Aharonov-Bohm Systems: The Spontaneous Hall Effect
We show how macroscopic manifestations of (and ) symmetry breaking can
arise in a simple system subject to Aharonov-Bohm interactions. Specifically,
we study the conductivity of a gas of charged particles moving through a dilute
array of flux tubes. The interaction of the electrons with the flux tubes is
taken to be of a purely Aharonov-Bohm type. We find that the system exhibits a
non-zero transverse conductivity, i.e., a spontaneous Hall effect. This is in
contrast with the fact that the cross sections for both scattering and
bremsstrahlung (soft photon emission) of a single electron from a flux tube are
invariant under reflections. We argue that the asymmetry in the conductivity
coefficients arises from many-body effects. On the other hand, the transverse
conductivity has the same dependence on universal constants that appears in the
Quantum Hall Effect, a result that we relate to the validity of the Mean Field
approximation.Comment: 12 pages (4 figures available upon request), RevTex, EHU-FT-93/1
Z0 Boson Measurement with the ALICE Central Barrel in pp collisions at 14 TeV
The possibility to detect the Z0 in the ALICE central barrel is studied via
the electronic decay channel Z0->e+e-. The signal and the background are
simulated with the leading order event generator PYTHIA 6. The total
cross-sections are taken from NLO calculations. Based on test beam data, the
electron identification performance of the Transition Radiation Detector is
extrapolated to high momenta. The expected yields for minimum-bias pp
collisions at 14 TeV are presented. An isolation cut on the single electron,
together with a minimum transverse momentum cut, allows to obtain a clear
signal. The expected background is of the order of 1 % with the main
contribution coming from misidentified pions from jets.Comment: 4 pages, 3 figures, QM2008 proceeding
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