1,061 research outputs found
Fixed boundary conditions analysis of the 3d Gonihedric Ising model with
The Gonihedric Ising model is a particular case of the class of models
defined by Savvidy and Wegner intended as discrete versions of string theories
on cubic lattices. In this paper we perform a high statistics analysis of the
phase transition exhibited by the 3d Gonihedric Ising model with in the
light of a set of recently stated scaling laws applicable to first order phase
transitions with fixed boundary conditions. Even though qualitative evidence
was presented in a previous paper to support the existence of a first order
phase transition at , only now are we capable of pinpointing the
transition inverse temperature at and of checking the
scaling of standard observables.Comment: 14 pages, 5 tables, 2 figures, uses elsart.cls packag
The cosmological Lithium problem outside the Galaxy: the Sagittarius globular cluster M54
The cosmological Li problem is the observed discrepancy between Li abundance,
A(Li), measured in Galactic dwarf, old and metal-poor stars (traditionally
assumed to be equal to the initial value A(Li)_0), and that predicted by
standard Big Bang Nucleosynthesis calculations (A(Li)_{BBN}). Here we attack
the Li problem by considering an alternative diagnostic, namely the surface Li
abundance of red giant branch stars that in a colour magnitude diagram populate
the region between the completion of the first dredge-up and the red giant
branch bump. We obtained high-resolution spectra with the FLAMES facility at
the Very Large Telescope for a sample of red giants in the globular cluster
M54, belonging to the Sagittarius dwarf galaxy. We obtain A(Li)=+0.93+-0.11
dex, translating -- after taking into account the dilution due to the dredge
up-- to initial abundances (A(Li)_0) in the range 2.35--2.29 dex, depending on
whether or not atomic diffusion is considered. This is the first measurement of
Li in the Sagittarius galaxy and the more distant estimate of A(Li)_0 in old
stars obtained so far. The A(Li)_0 estimated in M54 is lower by ~0.35 dex than
A(Li)_{BBN}, hence incompatible at a level of ~3sigma. Our result shows that
this discrepancy is a universal problem concerning both the Milky Way and
extra-galactic systems. Either modifications of BBN calculations, or a
combination of atomic diffusion plus a suitably tuned additional mixing during
the main sequence, need to be invoked to solve the discrepancy.Comment: Accepted by MNRAS, 10 pages, 5 figures, 1 tabl
A double stellar generation in the Globular Cluster NGC6656 (M 22). Two stellar groups with different iron and s-process element abundance
AIMS. In this paper we present the chemical abundance analysis from high
resolution UVES spectra of seventeen bright giant stars of the Globular Cluster
M~22. RESULTS. We obtained an average iron abundance of [Fe/H]=-1.76\pm0.02
(internal errors only) and an \alpha enhancement of 0.36\pm0.04 (internal
errors only). Na and O, and Al and O follow the well known anti-correlation
found in many other GCs. We identified two groups of stars with significantly
different abundances of the s-process elements Y, Zr and Ba. The relative
numbers of the two group members are very similar to the ratio of the stars in
the two SGBs of M22 recently found by Piotto (2009). Y and Ba abundances do not
correlate with Na, O and Al. The s-element rich stars are also richer in iron
and have higher Ca abundances. The results from high resolution spectra have
been further confirmed by lower resolution GIRAFFE spectra of fourteen
additional M22 stars. GIRAFFE spectra show also that the Eu -- a pure r-process
element -- abundance is not related to the iron content. We discuss the
chemical abundance pattern of M22 stars in the context of the multiple stellar
populations in GC scenario.Comment: 17 Pages, 21 figures, Accepted for publication in A&
Production and decays of supersymmetric Higgs bosons in spontaneously broken R-parity
We study the mass spectra, production and decay properties of the lightest
supersymmetric CP-even and CP-odd Higgs bosons in models with spontaneously
broken R-parity (SBRP). We compare the resulting mass spectra with expectations
of the Minimal Supersymmetric Standard Model (MSSM), stressing that the model
obeys the upper bound on the lightest CP-even Higgs boson mass. We discuss how
the presence of the additional scalar singlet states affects the Higgs
production cross sections, both for the Bjorken process and the "associated
production". The main phenomenological novelty with respect to the MSSM comes
from the fact that the spontaneous breaking of lepton number leads to the
existence of the majoron, denoted J, which opens new decay channels for
supersymmetric Higgs bosons. We find that the invisible decays of CP-even
Higgses can be dominant, while those of the CP-odd bosons may also be sizeable.Comment: 21 pages, 8 figures; minor changes, final version for publicatio
Ca II Triplet Spectroscopy of Small Magellanic Cloud Red Giants. III. Abundances and Velocities for a Sample of 14 Clusters
We obtained spectra of red giants in 15 Small Magellanic Cloud (SMC) clusters
in the region of the CaII lines with FORS2 on the Very Large Telescope (VLT).
We determined the mean metallicity and radial velocity with mean errors of 0.05
dex and 2.6 km/s, respectively, from a mean of 6.5 members per cluster. One
cluster (B113) was too young for a reliable metallicity determination and was
excluded from the sample. We combined the sample studied here with 15 clusters
previously studied by us using the same technique, and with 7 clusters whose
metallicities determined by other authors are on a scale similar to ours. This
compilation of 36 clusters is the largest SMC cluster sample currently
available with accurate and homogeneously determined metallicities. We found a
high probability that the metallicity distribution is bimodal, with potential
peaks at -1.1 and -0.8 dex. Our data show no strong evidence of a metallicity
gradient in the SMC clusters, somewhat at odds with recent evidence from CaT
spectra of a large sample of field stars Dobbie et al. (2014). This may be
revealing possible differences in the chemical history of clusters and field
stars. Our clusters show a significant dispersion of metallicities, whatever
age is considered, which could be reflecting the lack of a unique AMR in this
galaxy. None of the chemical evolution models currently available in the
literature satisfactorily represents the global chemical enrichment processes
of SMC clusters.Comment: 49 pages, 15 figures. Accepted for publication in A
New HST WFC3/UVIS observations augment the stellar-population complexity of omega Centauri
We used archival multi-band Hubble Space Telescope observations obtained with
the Wide-Field Camera 3 in the UV-optical channel to present new important
observational findings on the color-magnitude diagram (CMD) of the Galactic
globular cluster omega Centauri. The ultraviolet WFC3 data have been coupled
with available WFC/ACS optical-band data. The new CMDs, obtained from the
combination of colors coming from eight different bands, disclose an even more
complex stellar population than previously identified. This paper discusses the
detailed morphology of the CMDs.Comment: 17 pages, 14 figures (11 in low res), 3 tables. Accepted for
publication in AJ on June 19, 201
Degenerate neutrinos from a supersymmetric A_4 model
We investigate the supersymmetric A_4 model recently proposed by Babu, Ma and
Valle. The model naturally gives quasi-degenerate neutrinos that are bi-largely
mixed, in agreement with observations. Furthermore, the mixings in the quark
sector are constrained to be small, making it a complete model of the flavor
structure. Moreover, it has the interesting property that CP-violation in the
leptonic sector is maximal (unless vanishing). The model exhibit a close
relation between the slepton and lepton sectors and we derive the slepton
spectra that are compatible with neutrino data and the present bounds on
flavor-violating charged lepton decays. The prediction for the branching ratio
of the decay tau -> mu gamma has a lower limit of 10^{-9}. In addition, the
overall neutrino mass scale is constrained to be larger than 0.3 eV. Thus, the
model will be tested in the very near future.Comment: 11 pages, 6 figures. Talk given at the International Workshop on
Astroparticle and High Energy Physics (AHEP), Valencia, Spain, 14-18 Oct.
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