52,072 research outputs found
Topology and Strings: Topics in
A review on topological strings and the geometry of the space of two
dimensional theories. (Lectures given by C. Gomez at the Enrico Fermi Summer
School, Varenna, July 1994)Comment: 61 pages, late
A Note on the String Analog of Super-Symmetric Yang-Mills
A connection between the conifold locus of the type II string on the
Calabi-Yau manifold and the geometry of the quantum moduli of
super Yang-Mills is presented. This relation is obtained from
the anomalous behaviour of the super Yang-Mills special coordinates
under -duality transformation in .Comment: 7 pages, Late
Holography and the C-Theorem
We review the geometric definition of C-function in the context of field
theories that admit a holographic gravity dual.Comment: Contribution to the proceedings of the TMR 2000 Paris Conferenc
On the Ages of Exoplanet Host Stars
We obtained spectra, covering the CaII H and K region, for 49 exoplanet host
(EH) stars, observable from the southern hemisphere. We measured the
chromospheric activity index, Rhk. We compiled previously published values of
this index for the observed objects as well as the remaining EH stars in an
effort to better smooth temporal variations and derive a more representative
value of the average chromospheric activity for each object. We used the
average index to obtain ages for the group of EH stars. In addition we applied
other methods, such as: Isochrone, lithium abundance, metallicity and
transverse velocity dispersions, to compare with the chromospheric results. The
kinematic method is a less reliable age estimator because EH stars lie red-ward
of Parenago's discontinuity in the transverse velocity dispersion vs dereddened
B-V diagram. The chromospheric and isochrone techniques give median ages of 5.2
and 7.4 Gyr, respectively, with a dispersion of 4 Gyr. The median age of F and
G EH stars derived by the isochrone technique is 1--2 Gyr older than that of
identical spectral type nearby stars not known to be associated with planets.
However, the dispersion in both cases is large, about 2--4 Gyr. We searched for
correlations between the chromospheric and isochrone ages and Lir/L* (the
excess over the stellar luminosity) and the metallicity of the EH stars. No
clear tendency is found in the first case, whereas the metallicy dispersion
seems to slightly increase with age.Comment: 22 pages, 25 figures, A&A accepte
The formation of spiral arms and rings in barred galaxies
In this and in a previous paper (Romero-Gomez et al. 2006) we propose a
theory to explain the formation of both spirals and rings in barred galaxies
using a common dynamical framework. It is based on the orbital motion driven by
the unstable equilibrium points of the rotating bar potential. Thus, spirals,
rings and pseudo-rings are related to the invariant manifolds associated to the
periodic orbits around these equilibrium points. We examine the parameter space
of three barred galaxy models and discuss the formation of the different
morphological structures according to the properties of the bar model. We also
study the influence of the shape of the rotation curve in the outer parts, by
making families of models with rising, flat, or falling rotation curves in the
outer parts. The differences between spiral and ringed structures arise from
differences in the dynamical parameters of the host galaxies. The results
presented here will be discussed and compared with observations in a
forthcoming paper.Comment: 16 pages, 13 figures, accepted in A&A. High resolution version
available at http://www.oamp.fr/dynamique/pap/merce.htm
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