25,241 research outputs found
Searching for chemical inhomogeneities in Open Clusters: Analysis of the CN and CH Molecular Band Strengths in NGC 2158, NGC 2420, NGC 2682, NGC 7789 and Berkeley 29
Context: The total mass of a cluster, being the main parameter determining
its ability to host more than one stellar generation, may constitute a
threshold below which the cluster is able to form only a single stellar
population. AIms: Our goal is to investigate the existence of star-to-star
variations of CN and CH band strengths, related to the N and C abundances,
respectively, among the stars in five open cluster (NGC 2158, NGC 2420, NGC
2682, NGC 7789 and Berkeley 29) similar to those observed in globular clusters
and linked with the existence of multiple populations therein. Since these
systems are less massive than globulars, our results may allow us to constrain
the lower mass necessary to form more than one stellar population. Methods: We
measured the strength of the CN and CH bands, which correlate with the C and N
abundances, using four molecular indices in low-resolution SDSS/SEGUE spectra.
Results: We found that for four of the open clusters (NGC 2158, NGC 2420, NGC
2682 and Berkeley 29) all the stars studied in each of them have similar CN and
CH band strengths within the uncertainties since neither anomalous spreads nor
bimodalities have been detected in their CN and CH distributions. In contrast,
for NGC 7789 we found an anomalous spread in the strength of the CN molecular
band at 3839 \AA which is larger than the uncertainties. However, the small
number of stars studied in this cluster implies that further analysis is needed
to confirm the existence of chemical inhomogeneities in this cluster.Comment: 11 pages, 9 figures, Accepted for publication in A&A, Tables 7, 8 and
9 will be publish onlin
BPS Skyrme neutron stars in generalized gravity
We study the coupling of nuclear matter described by the BPS Skyrme model to
generalized gravity. Concretely, we consider the Starobinsky model which
provides the leading-order correction to the Einstein-Hilbert action. Static
solutions describing neutron stars are found both for the full field theory and
for the mean-field approximation. We always consider the full Starobinsky model
in the nonperturbative approach, using appropriately generalized shooting
methods for the numerical neutron star calculations. Many of our results are
similar to previous investigations of neutron stars for the Starobinsky model
using other models of nuclear matter, but there are some surprizing
discrepancies. The "Newtonian mass" relevant for the surface redshift, e.g.,
results larger than the ADM mass in our model, in contrast to other
investigations. This difference is related to the particularly high stiffness
of nuclear matter described by the BPS Skyrme model and offers an interesting
possibility to distinguish different models of nuclear matter within
generalized gravity.Comment: LaTex, 28 pages, 13 figures; v2: minor change
The influence of persuasion in opinion formation and polarization
We present a model that explores the influence of persuasion in a population
of agents with positive and negative opinion orientations. The opinion of each
agent is represented by an integer number that expresses its level of
agreement on a given issue, from totally against to totally in favor
. Same-orientation agents persuade each other with probability ,
becoming more extreme, while opposite-orientation agents become more moderate
as they reach a compromise with probability . The population initially
evolves to (a) a polarized state for , where opinions' distribution is
peaked at the extreme values , or (b) a centralized state for ,
with most opinions around . When , polarization lasts for a
time that diverges as , where is the population's size. Finally,
an extremist consensus ( or ) is reached in a time that scales as
for
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