19,643 research outputs found
Galactic Nonlinear Dynamic Model
We develop a model for spiral galaxies based on a nonlinear realization of
the Newtonian dynamics starting from the momentum and mass conservations in the
phase space. The radial solution exhibits a rotation curve in qualitative
accordance with the observational data.Comment: 6 pages, 1 figure. Talk given in the 7th Alexander Friedmann
International Seminar, June 29 to July 5, 2008, Joao Pessoa, PB, Brazi
Evidence from stellar rotation of enhanced disc dispersal: (I) The case of the triple visual system BD-21 1074 in the Pictoris association
The early stage of stellar evolution is characterized by a star-disc locking
mechanism. The disc-locking prevents the star to spin its rotation up, and its
timescale depends on the disc lifetime. Some mechanisms can significantly
shorten this lifetime, allowing a few stars to start spinning up much earlier
than other stars. In the present study, we aim to investigate how the
properties of the circumstellar environment can shorten the disc lifetime. We
have identified a few multiple stellar systems, composed of stars with similar
masses, which belong to associations with a known age. Since all parameters
that are responsible for the rotational evolution, with the exception of
environment properties and initial stellar rotation, are similar for all
components, we expect that significant differences among the rotation periods
can only arise from differences in the disc lifetimes. A photometric timeseries
allowed us to measure the rotation periods of each component, while
high-resolution spectra provided us with the fundamental parameters,
and chromospheric line fluxes. The rotation periods of the components differ
significantly, and the component B, which has a closer companion C, rotates
faster than the more distant and isolated component A. We can ascribe the
rotation period difference to either different initial rotation periods or
different disc-locking phases arising from the presence of the close companion
C. In the specific case of BD21 1074, the second scenario seems to be more
favored. In our hypothesis of different disc-locking phase, any planet orbiting
this star is likely formed very rapidly owing to a gravitational instability
mechanism, rather than core accretion. Only a large difference of initial
rotation periods alone could account for the observed period difference,
leaving comparable disc lifetimes.Comment: Accepted by Astronomy & Astrophysics on July 31, 2014; Pages 12,
Figs.
Estimativas de parâmetros genéticos e fenotípicos em nível de indivíduo em populações segregantes de feijoeiro-comum.
O objetivo desse trabalho foi estimar os parâmetros genéticos e fenotípicos, em nível de indivíduo, de populações segregantes de feijoeiro-comum, grupo carioca, para caracteres de escurecimento e produção de grãos.CONAF
Vortex-Antivortex Lattice in Ultra-Cold Fermi Gases
We discuss ultra-cold Fermi gases in two dimensions, which could be realized
in a strongly confining one-dimensional optical lattice. We obtain the
temperature versus effective interaction phase diagram for an s-wave superfluid
and show that, below a certain critical temperature T_c, spontaneous
vortex-antivortex pairs appear for all coupling strengths. In addition, we show
that the evolution from weak to strong coupling is smooth, and that the system
forms a square vortex-antivortex lattice at a lower critical temperature T_M.Comment: Submitted to Physical Review Letter
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