6,242 research outputs found
Thermodynamical properties of metric fluctuations during inflation
I study a thermodynamical approach to scalar metric perturbations during the
inflationary stage. In the power-law expanding universe here studied, I find a
negative heat capacity as a manifestation of superexponential growing for the
number of states in super Hubble scales. The power spectrum depends on the
Gibbons-Hawking and Hagedorn temperatures.Comment: 7 pages, no figures (accepted to publication in General Relativity
and Gravitation
Fresh inflation: a warm inflationary model from a zero temperature initial state
A two-components mixture fluid which complies with the gamma law is
considered in the framework of inflation with finite temperature. The model is
developed for a quartic scalar potential without symmetry breaking. The
radiation energy density is assumed to be zero when inflation starts and
remains below the GUT temperature during the inflationary stage. Furthermore,
provides the necessary number of e-folds and sufficient radiation energy
density to GUT baryogenesis can take place near the minimum energetic
configuration.Comment: 11 pages, no figures, to be published in Phys. Rev.
Warm inflation and scalar perturbations of the metric
A second-order expansion for the quantum fluctuations of the matter field was
considered in the framework of the warm inflation scenario. The friction and
Hubble parameters were expended by means of a semiclassical approach. The
fluctuations of the Hubble parameter generates fluctuations of the metric.
These metric fluctuations produce an effective term of curvature. The power
spectrum for the metric fluctuations can be calculated on the infrared sector.Comment: 10 pages, no figures, to be published in General Rel. and Gravitatio
Effects of electronic correlation on X-Ray absorption and dichroic spectra at L edge
We present a new theoretical approach to describe X-Ray absorption and
Magnetic Circular Dichroism spectra in the presence of e-e correlation. Our
approach provides an unified picture to include correlations in both charged
and neutral excitations, namely in direct / inversion photoemission where
electrons are removed/added, and photo absorption where electrons are promoted
from core levels to empty states. We apply this approach to the prototypical
case of L edge of 3 transition metals and we show that the inclusion
of many body effects in the core level excitations is essential to reproduce,
together with satellite structures in core level photoemission, the observed
asymmetric line shapes in X-ray absorption and dichroic spectra.Comment: 3 figures, 5 pages, submitted to Phys. Rev.
Multi-wavelength Hubble Space Telescope photometry of stellar populations in NGC288
We present new UV observations for NGC288, taken with the WFC3 detector on
board the Hubble Space Telescope, and combine them with existing optical data
from the archive to explore the multiple-population phenomenon in this globular
cluster (GC). The WFC3's UV filters have demonstrated an uncanny ability to
distinguish multiple populations along all photometric sequences in GCs, thanks
to their exquisite sensitivity to the atmospheric changes that are tell-tale
signs of second-generation enrichment. Optical filters, on the other hand, are
more sensitive to stellar-structure changes related to helium enhancement. By
combining both UV and optical data we can measure helium variation. We quantify
this enhancement for NGC288 and find that its variation is typical of what we
have come to expect in other clusters.Comment: 15 pages, 5 figures, accepted for publication in Ap
Mean proper motions, space orbits and velocity dispersion profiles of Galactic globular clusters derived from Gaia DR2 data
We have derived the mean proper motions and space velocities of 154 Galactic globular clusters and the velocity dispersion profiles of 141 globular clusters based on a combination of Gaia DR2 proper motions with ground-based line-of-sight velocities. Combining the velocity dispersion profiles derived here with new measurements of the internal mass functions allows us to model the internal kinematics of 144 clusters, more than 90 per cent of the currently known Galactic globular cluster population. We also derive the initial cluster masses by calculating the cluster orbits backwards in time applying suitable recipes to account for mass-loss and dynamical friction. We find a correlation between the stellar mass function of a globular cluster and the amount of mass lost from the cluster, pointing to dynamical evolution as one of the mechanisms shaping the mass function of stars in clusters. The mass functions also show strong evidence that globular clusters started with a bottom-light initial mass function. Our simulations show that the currently surviving globular cluster population has lost about 80 per cent of its mass since the time of formation. If globular clusters started from a lognormal mass function, we estimate that the Milky Way contained about 500 globular clusters initially, with a combined mass of about 2.5 Ă— 10 M. For a power-law initial mass function, the initial mass in globular clusters could have been a factor of three higher
Spontaneous formation of optically induced surface relief gratings
A model based on Fick's law of diffusion as a phenomenological description of
the molecular motion, and on the coupled mode theory, is developped to describe
single-beam surface relief grating formation in azopolymers thin films. It
allows to explain the mechanism of spontaneous patterning, and
self-organization. It allows also to compute the surface relief profile and its
evolution in time with good agreement with experiments
Multiple stellar populations in Magellanic Cloud clusters. III. The first evidence of an extended main sequence turn-off in a young cluster: NGC1856
Recent studies have shown that the extended main-sequence turn off (eMSTO) is
a common feature of intermediate-age star clusters in the Magellanic Clouds
(MCs). The most simple explanation is that these stellar systems harbor
multiple generations of stars with an age difference of a few hundred Myrs.
However, while an eMSTO has been detected in a large number of clusters with
ages between ~1-2 Gyrs, several studies of young clusters in both MCs and in
nearby galaxies do not find any evidence for a prolonged star-formation
history, i.e. for multiple stellar generations. These results have suggested
alternative interpretation of the eMSTOs observed in intermediate-age star
clusters. The eMSTO could be due to stellar rotation mimicking an age spread or
to interacting binaries. In these scenarios, intermediate-age MC clusters would
be simple stellar populations, in close analogy with younger clusters.
Here we provide the first evidence for an eMSTO in a young stellar cluster.
We exploit multi-band Hubble Space Telescope photometry to study the ~300-Myr
old star cluster NGC1856 in the Large Magellanic Cloud and detected a broadened
MSTO that is consistent with a prolonged star-formation which had a duration of
about 150 Myrs. Below the turn-off, the MS of NGC1856 is split into a red and
blue component, hosting 33+/-5% and 67+/-5% of the total number of MS stars,
respectively. We discuss these findings in the context of
multiple-stellar-generation, stellar-rotation, and interacting-binary
hypotheses.Comment: 14 pages, 10 figures, accepted for publication in MNRA
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