44 research outputs found
Stars of extragalactic origin in the solar neighborhood
We computed the spatial velocities and the galactic orbital elements using
Hipparcos data for 77 nearest main-sequence F-G-stars with published the iron,
magnesium, and europium abundances determined from high dispersion spectra and
with the ages estimated from theoretical isochrones. A comparison with the
orbital elements of the globular clusters that are known was accreted by our
Galaxy in the past reveals stars of extragalactic origin. We show that the
relative elemental abundance ratios of r- and \alpha- elements in all the
accreted stars differ sharply from those in the stars that are genetically
associated with the Galaxy. According to current theoretical models, europium
is produced mainly in low mass Type II supernovae (SNe II), while magnesium is
synthesized in larger amounts in high mass SN II progenitors. Since all the old
accreted stars of our sample exhibit a significant Eu overabundance relative to
Mg, we conclude that the maximum masses of the SNII progenitors outside the
Galaxy were much lower than those inside it are. On the other hand, only a
small number of young accreted stars exhibit low negative ratios .
The delay of primordial star formation burst and the explosions of high mass
SNe II in a relatively small part of extragalactic space can explain this
situation. We provide evidence that the interstellar medium was weakly mixed at
the early evolutionary stages of the Galaxy formed from a single proto-galactic
cloud and that the maximum mass of the SN II progenitors increased in it with
time simultaneously with the increase in mean metallicity.Comment: Accepted for 2004, Astronomy Letters, Vol. 30, No. 3, P.148-158 15
pages, 3 figure
Noncommutative resolutions of ADE fibered Calabi-Yau threefolds
In this paper we construct noncommutative resolutions of a certain class of Calabi-Yau threefolds studied by F. Cachazo, S. Katz and C. Vafa. The threefolds under consideration are fibered over a complex plane with the fibers being deformed Kleinian singularities. The construction is in terms of a noncommutative algebra introduced by V. Ginzburg, which we call the "N=1 ADE quiver algebra"
Dark Energy from structure: a status report
The effective evolution of an inhomogeneous universe model in any theory of
gravitation may be described in terms of spatially averaged variables. In
Einstein's theory, restricting attention to scalar variables, this evolution
can be modeled by solutions of a set of Friedmann equations for an effective
volume scale factor, with matter and backreaction source terms. The latter can
be represented by an effective scalar field (`morphon field') modeling Dark
Energy.
The present work provides an overview over the Dark Energy debate in
connection with the impact of inhomogeneities, and formulates strategies for a
comprehensive quantitative evaluation of backreaction effects both in
theoretical and observational cosmology. We recall the basic steps of a
description of backreaction effects in relativistic cosmology that lead to
refurnishing the standard cosmological equations, but also lay down a number of
challenges and unresolved issues in connection with their observational
interpretation.
The present status of this subject is intermediate: we have a good
qualitative understanding of backreaction effects pointing to a global
instability of the standard model of cosmology; exact solutions and
perturbative results modeling this instability lie in the right sector to
explain Dark Energy from inhomogeneities. It is fair to say that, even if
backreaction effects turn out to be less important than anticipated by some
researchers, the concordance high-precision cosmology, the architecture of
current N-body simulations, as well as standard perturbative approaches may all
fall short in correctly describing the Late Universe.Comment: Invited Review for a special Gen. Rel. Grav. issue on Dark Energy, 59
pages, 2 figures; matches published versio