217 research outputs found
Study of the Helium Enrichment in Globular Clusters
Globular clusters (GCs) are spheroidal concentrations typically containing of
the order of 10^5 to 10^6, predominantly old, stars. Historically, they have
been considered as the closest counterparts of the idealized concept of "simple
stellar populations." However, some recent observations suggest than, at least
in some GCs, some stars are present that have been formed with material
processed by a previous generation of stars. In this sense, it has also been
suggested that such material might be enriched in helium, and that blue
horizontal branch stars in some GCs should accordingly be the natural progeny
of such helium-enhanced stars. In this contribution we show that, at least in
the case of M3 (NGC 5272), the suggested level of helium enrichment is not
supported by the available, high-precision observations.Comment: 4 pages, 2 figures. To appear in the proceedings of IAU Symp. 262
(ed. G. Bruzual & S. Charlot
Quark Cluster Model Study of Isospin-Two Dibaryons
Based on a quark cluster model for the non-strange sector that reproduces
reasonably well the nucleon-nucleon system and the excitation of the
isobar, we generate a nucleon- interaction and present the predictions
for the several isospin two channels. The only attractive channels are
and , but not attractive enough to generate a resonance. If a resonance is
artificially generated and is required to have the observed experimental mass,
then our model predicts a width that agrees with the experimental result.Comment: 12 pages, 5 poscript figures available under request. To appear in
Phys. Rev.
Pulsation Period Changes as a Tool to Identify Pre-Zero Age Horizontal Branch Stars
One of the most dramatic events in the life of a low-mass star is the He
flash, which takes place at the tip of the red giant branch (RGB) and is
followed by a series of secondary flashes before the star settles into the
zero-age horizontal branch (ZAHB). Yet, no stars have been positively
identified in this key evolutionary phase, mainly for two reasons: first, this
pre-ZAHB phase is very short compared to other major evolutionary phases in the
life of a star; and second, these pre-ZAHB stars are expected to overlap the
loci occupied by asymptotic giant branch (AGB), HB and RGB stars observed in
the color-magnitude diagram (CMD). We investigate the possibility of detecting
these stars through stellar pulsations, since some of them are expected to
rapidly cross the Cepheid/RR Lyrae instability strip in their route from the
RGB tip to the ZAHB, thus becoming pulsating stars along the way. As a
consequence of their very high evolutionary speed, some of these stars may
present anomalously large period change rates. We constructed an extensive grid
of stellar models and produced pre-ZAHB Monte Carlo simulations appropriate for
the case of the Galactic globular cluster M3 (NGC 5272), where a number of RR
Lyrae stars with high period change rates are found. Our results suggest that
some -- but certainly not all -- of the RR Lyrae stars in M3 with large period
change rates are in fact pre-ZAHB pulsators.Comment: Conference Proceedings HELAS Workshop on 'Synergies between solar and
stellar modelling', Rome, June 2009, Astrophys. Space Sci., in the pres
Nonlocal calculation for nonstrange dibaryons and tribaryons
We study the possible existence of nonstrange dibaryons and tribaryons by
solving the bound-state problem of the two- and three-body systems composed of
nucleons and deltas. The two-body systems are , , and
, while the three-body systems are , ,
, and . We use as input the nonlocal ,
, and potentials derived from the chiral quark cluster
model by means of the resonating group method. We compare with previous results
obtained from the local version based on the Born-Oppenheimer approximation.Comment: 19 pages. To be published in Physical Review
Are there compact heavy four-quark bound states?
We present an exact method to study four-quark systems based on the
hyperspherical harmonics formalism. We apply it to several physical systems of
interest containing two heavy and two light quarks using different quark-quark
potentials. Our conclusions mark the boundaries for the possible existence of
compact, non-molecular, four-quark bound states. While states
may be stable in nature, the stability of states would imply
the existence of quark correlations not taken into account by simple quark
dynamical modelsComment: 10 pages, 1 figure. Accepted for publication in Phys. Rev.
Does the quark cluster model predict any isospin two dibaryon resonance?
We analyze the possible existence of a resonance in the channel
with isospin two by means of nucleon- interactions based on the
constituent quark model. We solve the bound state and the scattering problem
using two different potentials, a local and a non-local one. The non-local
potential results to be the more attractive, although not enough to generate
the experimentally predicted resonance.Comment: 9 pages in Latex (revtex), 2 eps figures available under reques
Long- and medium-range components of the nuclear force in quark-model based calculations
Quark-model descriptions of the nucleon-nucleon interaction contain two main
ingredients, a quark-exchange mechanism for the short-range repulsion and
meson-exchanges for the medium- and long-range parts of the interaction. We
point out the special role played by higher partial waves, and in particular
the 1F3, as a very sensitive probe for the meson-exchange part employed in
these interaction models. In particular, we show that the presently available
models fail to provide a reasonable description of higher partial waves and
indicate the reasons for this shortcoming.Comment: 19 pages, 7 figure
Mass and width of the resonance in nuclei
We calculated the mass and width of the resonance inside nuclei within a
nucleon- model by including the self-energy of the in the
propagator. We found that in the nuclear medium the width of the
is increased by one order of magnitude while its mass changes only by a few
MeV. This broadening of the width of the resonance embedded in nuclei is
consistent with the experimental observations so that the can be
understood as a resonance. Thus, given the freedom between either
isospin 0 or isospin 2 for the , our results give weigth to the isospin-2
assignment.Comment: 14 pages, RevteX type, 2 eps figures. To be published in Phys. Rev. C
(September
- …