378 research outputs found
On the primordial scenario for abundance variations within globular clusters. The isochrone test
Self-enrichment processes occurring in the early stages of a globular cluster
lifetime are generally invoked to explain the observed CNONaMgAl abundance
anticorrelations within individual Galactic globulars.We have tested, with
fully consistent stellar evolution calculations, if theoretical isochrones for
stars born with the observed abundance anticorrelations satisfy the
observational evidence that objects with different degrees of these anomalies
lie on essentially identical sequences in the Color-Magnitude-Diagram (CMD). To
this purpose, we have computed for the first time low-mass stellar models and
isochrones with an initial metal mixture that includes the extreme values of
the observed abundance anticorrelations, and varying initial He mass fractions.
Comparisons with 'normal' alpha-enhanced isochrones and suitable Monte Carlo
simulations that include photometric errors show that a significant broadening
of the CMD sequences occurs only if the helium enhancement is extremely large
(in this study, when Y=0.35) in the stars showing anomalous abundances. Stellar
luminosity functions up to the Red Giant Branch tip are also very weakly
affected, apart from - depending on the He content of the polluting material -
the Red Giant Branch bump region. We also study the distribution of stars along
the Zero Age Horizontal Branch, and derive general constraints on the relative
location of objects with and without abundance anomalies along the observed
horizontal branches of globular clusters.Comment: Accepted for publication in Ap
The Role of Dust in Models of Population Synthesis
We have employed state-of-the-art evolutionary models of low and
intermediate-mass AGB stars, and included the effect of circumstellar dust
shells on the spectral energy distribution (SED) of AGB stars, to revise the
Padua library of isochrones (Bertelli et al. 1994). The major revision involves
the thermally pulsing AGB phase, that is now taken from fully evolutionary
calculations by Weiss & Ferguson (2009). Two libraries of about 600 AGB
dust-enshrouded SEDs each have also been calculated, one for oxygen-rich
M-stars and one for carbon-rich C-stars. Each library accounts for different
values of input parameters like the optical depth {\tau}, dust composition, and
temperature of the inner boundary of the dust shell. These libraries of dusty
AGB spectra have been implemented into a large composite library of theoretical
stellar spectra, to cover all regions of the Hertzsprung-Russell Diagram (HRD)
crossed by the isochrones. With the aid of the above isochrones and libraries
of stellar SEDs, we have calculated the spectro-photometric properties (SEDs,
magnitudes, and colours) of single-generation stellar populations (SSPs) for
six metallicities, more than fifty ages (from 3 Myr to 15 Gyr), and nine
choices of the Initial Mass Function. The new isochrones and SSPs have been
compared to the colour-magnitude diagrams (CMDs) of field populations in the
LMC and SMC, with particular emphasis on AGB stars, and the integrated colours
of star clusters in the same galaxies, using data from the SAGE (Surveying the
Agents of Galaxy Evolution) catalogues. We have also examined the integrated
colours of a small sample of star clusters located in the outskirts of M31. The
agreement between theory and observations is generally good. In particular, the
new SSPs reproduce the red tails of the AGB star distribution in the CMDs of
field stars in the Magellanic Clouds.Comment: Accepted for publication in MNRA
A large stellar evolution database for population synthesis studies. II. Stellar models and isochrones for an alpha-enhanced metal distribution
[Abridged] We present a large, new set of stellar evolution models and
isochrones for an alpha-enhanced metal distribution typical of Galactic halo
and bulge stars; it represents a homogeneous extension of our stellar model
library for a distribution already presented in Pietrinferni et al.(2004). The
effect of the alpha-element enhancement has been properly taken into account in
the nuclear network, opacity, equation of state and, for the first time, the
bolometric corrections, and color transformations. This allows us to avoid the
inconsistent use - common to all alpha-enhanced model libraries currently
available - of scaled-solar bolometric corrections and color transformations
for alpha-enhanced models and isochrones. We show how bolometric corrections to
magnitudes obtained for the U,B portion of stellar spectra for T_{eff}<=6500K,
are significantly affected by the metal mixture, especially at the higher
metallicities. We also provide complete sets of evolutionary models for
low-mass, He-burning stellar structures covering the whole metallicity range,
to enable synthetic horizontal branch simulations. We compare our database with
several widely used stellar model libraries from different authors, as well as
with various observed color magnitude and color-color diagrams (Johnson-Cousins
BVI and near infrared magnitudes, Stromgren colors) of Galactic field stars and
globular clusters. We also test our isochrones comparing integrated optical
colors and Surface Brightness Fluctuation magnitudes with selected globular
cluster data. We find a general satisfactory agreement with the empirical
constraints.Comment: 46 pages, 20 figures, ApJ in press, the whole database presented in
this paper can be foud at http://www.te.astro.it/BASTI/index.ph
A Rapid, Widely Applicable Screen for Drugs that Suppress Free Radical Formation in Ischemia/Reperfusion
Substantial injury can occur during reoxygenation of previously ischemic tissue in many experimental models, as the result of the generation of oxygen-derived free radicals. To test the antiradical activity of potentially protective compounds in this setting, we developed a simple screening system, applicable to fresh biopsy specimens, in which warm ischemia and reoxygenation of excised tissue are performed in vitro. Tissue production of malondialdehyde (MDA) equivalents is used as a nonspecific-but-sensitive marker of oxygen radical damage. Test compounds with putative antiradical activity are added prior to the reoxygenation phase, and their ability to suppress MDA production is an index of activity in preventing reoxygenation injury. Comparison with ischemic but not reoxygenated controls confirms the oxygen-dependent nature of the effect. Standard positive controls of known effective agents, such as butylated hydroxytoluene or deferoxamine, provide a reference for the activity of the test compound. The method is applicable to surgical biopsy specimens in veterinary and human medicine
Optical Identification of He White Dwarfs Orbiting Four Millisecond Pulsars in the Globular Cluster 47 Tucanae
We used ultra-deep UV observations obtained with the Hubble Space Telescope
to search for optical companions to binary millisecond pulsars (MSPs) in the
globular cluster 47 Tucanae. We identified four new counterparts (to MSPs
47TucQ, 47TucS, 47TucT and 47TucY) and confirmed those already known (to MSPs
47TucU and 47TucW). In the color magnitude diagram, the detected companions are
located in a region between the main sequence and the CO white dwarf cooling
sequences, consistent with the cooling tracks of He white dwarfs of mass
between 0.15 Msun and 0.20 Msun. For each identified companion, mass, cooling
age, temperature and pulsar mass (as a function of the inclination angle) have
been derived and discussed. For 47TucU we also found that the past accretion
history likely proceeded in a sub-Eddington rate. The companion to the redback
47TucW is confirmed to be a non degenerate star, with properties particularly
similar to those observed for black widow systems. Two stars have been
identified within the 2-sigma astrometric uncertainty from the radio positions
of 47TucH and 47TucI, but the available data prevent us from firmly assessing
whether they are the true companions of these two MSPs.Comment: 27 pages, 7 figures, Accepted for publication by Ap
The optical companion to the intermediate mass millisecond pulsar J1439-5501 in the Galactic field
We present the identification of the companion star to the intermediate mass
binary pulsar J1439-5501 obtained by means of ground-based deep images in the
B, V and I bands, acquired with FORS2 mounted at the ESO-VLT. The companion is
a massive white dwarf (WD) with B=23.57+-0.02, V=23.21+-0.01 and I=22.96+-0.01,
located at only ~0.05" from the pulsar radio position. Comparing the WD
location in the (B, B-V) and (V, V-I) Color-Magnitude diagrams with theoretical
cooling sequences we derived a range of plausible combinations of companion
masses (1<~Mcom<~1.3 Msun), distances (d<~1200 pc), radii (<~7.8 10^3 Rsun) and
temperatures (T=31350^{+21500}_{-7400}). From the PSR mass function and the
estimated mass range we also constrained the inclination angle i >~ 55 degrees
and the pulsar mass (Mpsr <~2.2 Msun). The comparison between the WD cooling
age and the spin down age suggests that the latter is overestimated by a factor
of about ten.Comment: Accepted for publication by ApJ; 19 pages, 5 figures, 1 tabl
A Bound on the Light Emitted During the TP-AGB Phase
The integrated luminosity of the TP-AGB phase is a major uncertainty in
stellar population synthesis models. We use the white dwarf initial final mass
relation and stellar interiors models to demonstrate that a significant
fraction of the core mass growth for intermediate (1.5 < Msun < 6) mass stars
takes place during the TP-AGB phase. We find evidence that the peak fractional
core mass contribution for TP-AGB stars is ~20% and occurs for stars between 2
Msun and 3.5 Msun. Using a simple fuel consumption argument we couple this core
mass increase to a lower limit on the TP-AGB phase energy output. Roughly half
of the energy released in models of TP-AGB stars can be directly accounted for
by this core growth; while the remainder is predominantly the stellar yield of
He. A robust measurement of the emitted light in this phase will therefore set
strong constraints on helium enrichment from TP-AGB stars, and we estimate the
yields predicted by current models as a function of initial mass. Implications
for stellar population studies and prospects for improvements are discussed.Comment: Submitted to the Astrophysical Journal. 25 pages, 2 figures
Methylene Blue as an Inhibitor of Superoxide Generation by Xanthene Oxidase: A Potential New Drug for the Attenuation of Ischemia/Reperfusion Injury
Tissue oxidases, especially xanthine oxidase, have been proposed as primary sources of toxic oxygen radicals in many experimental models of disease states. Among these, ischemia-reperfusion injury may be of the greatest clinical interest. In this paper we propose the use of methylene blue as a means of suppressing the production of superoxide radicals, O2, by acting as an alternative electron acceptor for xanthine oxidase. Previous work has indicated that methylene blue accepts electrons from xanthine oxidase at the iron-sulfur center. Initial experiments in our laboratory demonstrated that (1) pairs of electrons from each enzymatic oxidation are transferred to methylene blue, (2) the reduction of methylene blue can be achieved by model iron-sulfur centers, similar to the iron-sulfur center of xanthine oxidase, (3) reduced methylene blue auto-oxidizes to produce H2O2 directly, rather than O2, and (4) methylene blue is effective at non-toxic levels (2-5 mg/kg) in preventing free radical damage to liver and kidney tissues in an in vitro model of ischemia and reoxygenation. Accordingly, we propose that methylene blue may represent a new class of antioxidant drugs that competitively inhibit reduction of molecular oxygen to superoxide by acting as alternative electron acceptors for tissue oxidases. We have termed these agents “parasitic” electron acceptors
Evidence That Free Fatty Acid-Iron Complexes Directly Initiate Lipid Peroxidation in Vitro and in Vivo: A New Mechanism of Oxidative Stress
Through a series of biochemical and histochemical experiments we explored the novel hypothesis that iron and free fatty acids, liberated after tissue injury, combine to form liposoluble complexes that directly initiate lipid peroxidation. The addition of 100 M ferric iron to 30 mM linoleate suspensions at pH 7.4 produced time dependent lipid peroxidation, measured as conjugated diene formation. Complexes of 100 M ferric iron and 600 M pentanoate also initiated formation of conjugated dienes in linoleate suspensions and formation of malondialdehyde-like materials in rat liver slices. A histochemical stain for free fatty acids revealed positive reactions within cell membranes in traumatized regions of rat liver tissue that underwent compression injury followed by thirty minutes of blood perfusion, but not in nontraumatized control regions. The diaminobenzidine-H2O2 histochemical reaction for iron, revealed increased levels of redox cyclable iron in the membranes and the cytoplasm of traumatized hepatocytes. We propose that traumatic injury initiates cascades leading to liberation of iron from storage proteins and free fatty acids from membranes, which combine, distribute to the lipid domains of cell membranes, and directly initiate lipid peroxidation
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