7,763 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 and decoherence of super Hubble fluctuations
I study a stochastic approach to the recently introduced fresh inflation
model for super Hubble scales. I find that the state loses its coherence at the
end of the fresh inflationary period as a consequence of the damping of the
interference function in the reduced density matrix. This fact should be a
consequence of a) the relative evolutions of both the scale factor and the
horizon and b) the additional thermal and dissipative effects. This implies a
relevant difference with respect to supercooled inflationary scenarios which
require a very rapid expansion of the scale factor to give the decoherence of
super Hubble fluctuations.Comment: version with minor changes. To appear 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
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.
Observing multiple stellar populations with FORS2@VLT - Main sequence photometry in outer regions of NGC 6752, NGC 6397, and NGC 6121 (M 4)
We present the photometric analysis of the external regions of three Galactic
Globular Clusters: NGC 6121, NGC 6397 and NGC 6752. The main goal is the
characterization of the multiple stellar populations along the main sequence
(MS) and the study of the radial trend of the different populations hosted by
the target clusters. The data have been collected using FORS2 mounted at the
ESO/VLT@UT1 telescope in UBVI filters. From these data sets we extracted
high-accuracy photometry and constructed color-magnitude diagrams. We exploit
appropriate combination of colors and magnitudes which are powerful tools to
identify multiple stellar populations, like B versus U-B and V versus
c_{U,B,I}=(U-B)-(B-I) CMDs. We confirm previous findings of a split MS in NGC
6752 and NGC 6121. Apart from the extreme case of omega Centauri, this is the
first detection of multiple MS from ground-based photometry. For NGC 6752 and
NGC 6121 we compare the number ratio of the blue MS to the red MS in the
cluster outskirts with the fraction of first and second generation stars
measured in the central regions. There is no evidence for significant radial
trend. The MS of NGC 6397 is consistent with a simple stellar population. We
propose that the lack of multiple sequences is due both to observational errors
and to the limited sensitivity of U,B,V,I photometry to multiple stellar
populations in metal-poor GCs. Finally, we compute the helium abundance for the
stellar populations hosted by NGC 6121 and NGC 6752, finding a mild (Delta Y ~
0.02) difference between stars in the two sequences.Comment: 16 pages, 5 tables, 17 figures, accepted for pubblication in A&
The "UV-route" to search for Blue Straggler Stars in Globular Clusters: first results from the HST UV Legacy Survey
We used data from the HST UV Legacy Survey of Galactic Globular Clusters to
select the Blue Straggler Star (BSS) population in four intermediate/high
density systems (namely NGC 2808, NGC 6388, NGC 6541 and NGC 7078) through a
"UV-guided search". This procedure consists in using the F275W images in each
cluster to construct the master list of detected sources, and then force it to
the images acquired in the other filters. Such an approach optimizes the
detection of relatively hot stars and allows the detection of complete sample
of BSSs even in the central region of high-density clusters, because the light
from the bright cool giants, which dominates the optical emission in old
stellar systems, is sensibly reduced at UV wavelengths. Our UV-guided
selections of BSSs have been compared to the samples obtained in previous,
optical-driven surveys, clearly demonstrating the efficiency of the UV
approach. In each cluster we also measured the parameter A+, defined as the
area enclosed between the cumulative radial distribution of BSSs and that of a
reference population, which traces the level of BSS central segregation and the
level of dynamical evolution suffered by the system. The values measured for
the four clusters studied in this paper nicely fall along the dynamical
sequence recently presented for a sample of 25 clusters.Comment: 30 pages, 9 figures, 4 tables; accepted for publication in Ap
Magnetic resonance tumor regression grade (MR-TRG) to assess pathological complete response following neoadjuvant radiochemotherapy in locally advanced rectal cancer
This study aims to evaluate the feasibility of a magnetic resonance (MR) automatic method for quantitative assessment of the percentage of fibrosis developed within locally advanced rectal cancers (LARC) after neoadjuvant radiochemotherapy (RCT). A total of 65 patients were enrolled in the study and MR studies were performed on 3.0 Tesla scanner; patients were followed-up for 30 months. The percentage of fibrosis was quantified on T2-weighted images, using automatic K-Means clustering algorithm. According to the percentage of fibrosis, an optimal cut-off point for separating patients into favorable and unfavorable pathologic response groups was identified by ROC analysis and tumor regression grade (MR-TRG) classes were determined and compared to histopathologic TRG. An optimal cut-off point of 81% of fibrosis was identified to differentiate between favorable and unfavorable pathologic response groups resulting in a sensitivity of 78.26% and a specificity of 97.62% for the identification of complete responders (CRs). Interobserver agreement was good (0.85). The agreement between P-TRG and MR-TRG was excellent (0.923). Significant differences in terms of overall survival (OS) and disease free survival (DFS) were found between favorable and unfavorable pathologic response groups. The automatic quantification of fibrosis determined by MR is feasible and reproducible
Proper motions and membership probabilities of stars in the region of globular cluster NGC 6809
NGC 6809 is a luminous metal-poor halo globular cluster that is relatively
easy to study due to its proximity and low concentration. Because of its high
Galactic latitude (b = -23deg), interstellar reddening and contamination is not
very high. We aim to determine the relative proper motion and membership
probability of the stars in the wide area of globular cluster NGC 6809. To
target cluster members reliably during spectroscopic surveys and both spatial
and radial distributions in the cluster outskirts without including field
stars, a good proper motion and membership probability catalogue of NGC 6809 is
required.The archival data of two epochs with a time-base line of 7.1 years
have been collected with Wide Field Imager (WFI) mounted on the 2.2m MPG/ESO
telescope. The CCD images of both epochs have been reduced using the
astrometric techniques as described in Anderson et al. (2006). The calibrated
UBVI magnitudes have been derived using Stetson's secondary standard stars. We
derived the relative proper motion and membership probabilities for \sim 12600
stars in the field of globular cluster NGC 6809. The measurement error in
proper motions for the stars of V \sim 17 mag is 2.0 mas/yr, gradually
increasing up to \sim 3 mas/yr at V = 20 mag. We also provide the membership
probability for the published different types of sources in NGC 6809. An
electronic catalogue with proper motion and membership probability for the
stars will be available to the astronomical community.Comment: Accepted for publication in The A&A. 11 pages. 11 figures,5 Table
Astrometry with MCAO: HST-GeMS proper motions in the globular cluster NGC 6681
Aims: for the first time the astrometric capabilities of the Multi-Conjugate
Adaptive Optics (MCAO) facility GeMS with the GSAOI camera on Gemini-South are
tested to quantify the accuracy in determining stellar proper motions in the
Galactic globular cluster NGC 6681. Methods: proper motions from HST/ACS for a
sample of its stars are already available, and this allows us to construct a
distortion-free reference at the epoch of GeMS observations that is used to
measure and correct the temporally changing distortions for each GeMS exposure.
In this way, we are able to compare the corrected GeMS images with a
first-epoch of HST/ACS images to recover the relative proper motion of the
Sagittarius dwarf spheroidal galaxy with respect to NGC 6681. Results: we find
this to be (\mu_{\alpha}cos\delta, \mu_{\delta}) = (4.09,-3.41) mas/yr, which
matches previous HST/ACS measurements with a very good accuracy of 0.03 mas/yr
and with a comparable precision (r.m.s of 0.43 mas/yr). Conclusions: this study
successfully demonstrates that high-quality proper motions can be measured for
quite large fields of view (85 arcsec X 85 arcsec) with MCAO-assisted,
ground-based cameras and provides a first, successful test of the performances
of GeMS on multi-epoch data.Comment: 5 pages, 4 figures. Accepted for publication by A&A Letter
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