3,780 research outputs found
Free Differential Algebras: Their Use in Field Theory and Dual Formulation
The gauging of free differential algebras (FDA's) produces gauge field
theories containing antisymmetric tensors. The FDA's extend the Cartan-Maurer
equations of ordinary Lie algebras by incorporating p-form potentials (). We study here the algebra of FDA transformations. To every p-form in the
FDA we associate an extended Lie derivative generating a corresponding
``gauge" transformation. The field theory based on the FDA is invariant under
these new transformations. This gives geometrical meaning to the antisymmetric
tensors. The algebra of Lie derivatives is shown to close and provides the dual
formulation of FDA's.Comment: 10 pages, latex, no figures. Talk presented at the 4-th Colloquium on
"Quantum Groups and Integrable Sysytems", Prague, June 199
Stellar evolution and large extra dimensions
We discuss in detail the information on large extra dimensions which can be
derived in the framework of stellar evolution theory and observation. The main
effect of large extra dimensions arises from the production of the Kaluza-Klein
(KK) excitations of the graviton. The KK-graviton and matter interactions are
of gravitational strength, so the KK states never become thermalized and always
freely escape. In this paper we first pay attention to the sun. Production of
KK gravitons is incompatible with helioseismic constraints unless the 4+n
dimensional Planck mass M_s exceeds 300 Gev/c^2. Next we show that stellar
structures in their advanced phase of H burning evolution put much more severe
constraints, M_s > 3-4 TeV/c^2, improving on current laboratory lower limits.Comment: 13 pages RevTeX file, 8 figures ps file
Which radius for the Sun?
The high accuracy reached by solar limb observations, by helioseismic
measurements and by Standard Solar Models (SSMs) calculations suggests that
general relativity corrections are included when discussing the solar radius.
The Allen value (R = 695.99 0.07 Mm) has to be reduced by 1.5
Km. This correction, which is small as compared with present accuracy, should
be kept in mind for future more precise measurements and/or calculations.Comment: Latex, 3 page
Stellar models for very low mass main sequence stars: the role of model atmospheres
We present Very Low Mass stellar models as computed including non-grey model
atmospheres for selected assumptions about the star metallicities. The role of
atmospheres is discussed and the models are compared with models based on the
Eddington approximation and with similar models appeared in the recent
literature. Theoretical predictions concerning both the HR diagram location and
the mass-luminosity relation are presented and discussed in terms of
expectations in selected photometric bands. Comparison with available
observational data concerning both galactic globular clusters and dwarfs in the
solar neighborhood reveals a satisfactory agreement together with the existence
of some residual mismatches.Comment: 10 pages including 13 figures, gzip postscript file, To be published
in MNRA
Theoretical Zero Age Main Sequences revisited
Zero Age Main Sequence (ZAMS) models with updated physical inputs are
presented for selected assumptions about the chemical composition, covering the
ranges 0.6 < M/Mo < 1.2, 0.0001 < Z < 0.04, 0.23 < Y < 0.34.The HR diagram
location of the ZAMS as a function of Y and Z is discussed both in the
theoretical and in the observational HR diagrams, showing that the V magnitude
presents an increased dependence on Z to be taken into account when discussing
observational evidences. Analytical relations quantifying both these
dependences are derived. Implications for the galactic helium to heavier
elements enrichment are finally discussed.Comment: 4 pages, 4 postscript figures, accepted for publication on Astronomy
& Astrophysic
Stellar Evolutionary Models for Magellanic Clouds
We supplement current evolutionary computations concerning Magellanic Cloud
stars by exploring the evolutionary behavior of canonical stellar models
(i.e.,with inefficient core overshooting) with metallicities suitable for stars
in the Clouds. After discussing the adequacy of the adopted evolutionary
scenario, we present evolutionary sequences as computed following a selected
sample of stellar models in the mass range 0.8-8 Mo from the Main Sequence till
the C ignition or the onset of thermal pulses in the advanced Asymptotic Giant
Branch phase. On this basis, cluster isochrones covering the range of ages from
100 Myr to 15 Gyr are presented and discussed. To allow a comparison with
evolutionary investigations appeared in the recent literature, we computed
additional sets of models which take into account moderate core overshooting
during the H burning phase, discussing the comparison in terms of current
uncertainties in the stellar evolutionary models. Selected predictions
constraining the cluster ages are finally discussed, presenting a calibration
of the difference in magnitude between the luminous MS termination and the He
burning giants in terms of cluster age. Both evolutionary tracks and isochrones
have been made available at the node http://gipsy.cjb.net as a first step of a
planned ``Pisa Evolutionary Library''.Comment: 11 pages, 9 eps figures, A&A accepted, evolutionary tracks and
isochrones available at http://gipsy.cjb.net at the link ``Pisa Evolutionary
Library'
Synthetic Stellar Clusters for Pop III
We present preliminary results of an incoming theoretical work concerning the
integrated properties of the Population III clusters of stars. On the basis of
synthetic Color-Magnitude Diagrams, we provide a grid of optical and near-IR
colors of Simple Stellar Populations with very low metallicity (Z=10
and Z=10) and age which spans from 10 Myr to 15 Gyr. A comparison with
higher metallicities up to 0.006 is also shown, disclosing sizable differences
in the CMD morphology, integrated colors and Spectral Energy Distribution
(SED).Comment: 2 pages, incl. 2 figures, "The First Stars", Proceedings of the
second MPA/ESO workshop, Eds.: Weiss, Abel, Hill, Springer, Heidelberg, 200
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