19 research outputs found
The effect of heavy element opacity on pre-main sequence Li depletion
Recent 3-D analysis of the solar spectrum data suggests a significant change
of the solar chemical composition. This may affect the temporal evolution of
the surface abundance of light elements since the extension of the convective
envelope is largely affected by the internal opacity value. We analyse the
influence of the adopted solar mixture on the opacity in the convective
envelope of pre-main sequence (PMS) stars and thus on PMS lithium depletion.
The surface Li abundance depends on the relative efficiency of several
processes, some of them still not known with the required precision; this paper
thus analyses one of the aspects of this ``puzzle''. Focusing on PMS evolution,
where the largest amount of Li burning occurs, we computed stellar models for
three selected masses (0.8, 1.0 and 1.2 Msun, with Z=0.013, Y=0.27, alpha=1.9)
by varying the chemical mixture, that is the internal element distribution in
Z. We analysed the contribution of the single elements to the opacity at the
temperatures and densities of interest for Li depletion. Several mixtures were
obtained by varying the abundance of the most important elements one at a time;
we then calculated the corresponding PMS Li abundance evolution. We found that
a mixture variation does change the Li abundance: at fixed total metallicity,
the Li depletion increases when increasing the fraction of elements heavier
than O.Comment: A&A accepted, 11 pages, 18 eps figure
Two distinct sequences of blue straggler stars in the globular cluster M30
Stars in globular clusters are generally believed to have all formed at the
same time, early in the Galaxy's history. 'Blue stragglers' are stars massive
enough that they should have evolved into white dwarfs long ago. Two possible
mechanisms have been proposed for their formation: mass transfer between binary
companions and stellar mergers resulting from direct collisions between two
stars. Recently, the binary explanation was claimed to be dominant. Here we
report that there are two distinct parallel sequences of blue stragglers in
M30. This globular cluster is thought to have undergone 'core collapse', during
which both the collision rate and the mass transfer activity in binary systems
would have been enhanced. We suggest that the two observed sequences arise from
the cluster core collapse, with the bluer population arising from direct
stellar collisions and the redder one arising from the evolution of close
binaries that are probably still experiencing an active phase of mass transfer.Comment: Published on the 24th December 2009 issue of Natur
The photometric evolution of dissolving star clusters: II. Realistic models. Colours and M/L ratios
Evolutionary synthesis models are the prime method to construct models of
stellar populations, and to derive physical parameters from observations. One
of the assumptions for such models so far has been the time-independence of the
stellar mass function. However, dynamical simulations of star clusters in tidal
fields have shown the mass function to change due to the preferential removal
of low-mass stars from clusters. Here we combine the results from dynamical
simulations of star clusters in tidal fields with our evolutionary synthesis
code GALEV to extend the models by a new dimension: the total cluster
disruption time. We reanalyse the mass function evolution found in N-body
simulations of star clusters in tidal fields, parametrise it as a function of
age and total cluster disruption time and use this parametrisation to compute
GALEV models as a function of age, metallicity and the total cluster disruption
time. We study the impact of cluster dissolution on the colour (generally, they
become redder) and magnitude (they become fainter) evolution of star clusters,
their mass-to-light ratios (off by a factor of ~2 -- 4 from standard
predictions), and quantify the effect on the cluster age determination from
integrated photometry (in most cases, clusters appear to be older than they
are, between 20 and 200%). By comparing our model results with observed M/L
ratios for old compact objects in the mass range 10^4.5 -- 10^8 Msun, we find a
strong discrepancy for objects more massive than 10^7 Msun (higher M/L). This
could be either caused by differences in the underlying stellar mass function
or be an indication for the presence of dark matter in these objects. Less
massive objects are well represented by the models. The models for a range of
total cluster disruption times are available online. (shortened)Comment: MNRAS, in press, data are available at
http://www.phys.uu.nl/~anders/data/SSP_varMF/, http://data.galev.org and soon
also from CDS ... sorry for the sometimes strange layout, that's LaTe
Calibrated stellar models for metal-poor population
We extend to lower metallicities recent evolutionary computations devoted to
Magellanic Clouds stars, presenting and discussing new stellar models with
Z=0.0002, 0.0004, 0.0006, 0.001 and suitable assumptions about the original He
content. As in the previous paper, evolutionary results are compared with
observational data to properly calibrate the assumptions about the efficiency
of the surface convection. On this basis, we follow the evolution of stellar
models in the mass range 0.6 to 11Mo from the Main Sequence (MS) to the C
ignition or the onset of thermal pulses in the advanced Asymptotic Giant Branch
(AGB) phase, presenting cluster isochrones covering the range of ages from 20
Myr to 20 Gyr. Selected predictions constraining the cluster ages are
discussed, presenting a calibration of the difference in magnitude between the
luminous MS termination and the He burning giants in terms of the cluster age.
Both evolutionary tracks and isochrones are available at the URL
http://astro.df.unipi.it/SAA/PEL/Z0.html. Data files are also available at the
CDS.Comment: In publication on Astronomy and Astrophysic
Blue Straggler Stars in Globular Clusters: a powerful tool to probe the internal dynamical evolution of stellar systems
This chapter presents an overview of the main observational results obtained
to date about Blue Straggler Stars (BSSs) in Galactic Globular Clusters (GCs).
The BSS specific frequency, radial distribution, chemical composition and
rotational properties are presented and discussed in the framework of using
this stellar population as probe of GC internal dynamics. In particular, the
shape of the BSS radial distribution has been found to be a powerful tracer of
the dynamical age of stellar systems, thus allowing the definition of the first
empirical "dynamical clock".Comment: Chapter 5, in Ecology of Blue Straggler Stars, H.M.J. Boffin, G.
Carraro & G. Beccari (Eds), Astrophysics and Space Science Library, Springe
Phosphorylation of huntingtin at residue T3 is decreased in Huntington’s disease and modulates mutant huntingtin protein conformation
Posttranslational modifications can have profound effects on the biological and biophysical properties of proteins associated with misfolding and aggregation. However, their detection and quantification in clinical samples and an understanding of the mechanisms underlying the pathological properties of misfolding- and aggregation-prone proteins remain a challenge for diagnostics and therapeutics development. We have applied an ultrasensitive immunoassay platform to develop and validate a quantitative assay for detecting a posttranslational modification (phosphorylation at residue T3) of a protein associated with polyglutamine repeat expansion, namely Huntingtin, and characterized its presence in a variety of preclinical and clinical samples. We find that T3 phosphorylation is greatly reduced in samples from Huntington\u2019s disease models and in Huntington\u2019s disease patients, and we provide evidence that bona-fide T3 phosphorylation alters Huntingtin exon 1 protein conformation and aggregation properties. These findings have significant implications for both mechanisms of disease pathogenesis and the development of therapeutics and diagnostics for Huntington\u2019s disease
The Pisa Evolutionary Library
We supplement recent evolutionary computations of canonical stellar models (i.e. with inefficient core overshooting) with Z=0.0002, 0.0004, 0.0006, 0.001, 0.004, 0.008 and suitable assumptions about the original He content. Evolutionary results have been compared with observational data in order to properly calibrate the efficiency of the surface convection. On this basis, we follow the evolution of stellar models in the mass range 0.6 to 11 Msun from the Main Sequence (MS) until C ignition or the onset of thermal pulses in the advanced Asymptotic Giant Branch (AGB) phase, presenting cluster isochrones covering the range of ages from 20 Myr to 20 Gyr. To allow a comparison with evolutionary investigations appeared in the recent literature, we computed additional sets of models which take into account moderate core overshooting during the H burning phase (for the metallicities suitable for stars in the Magellanic Clouds: Z=0.004, Z=0.008). Selected predictions constraining the cluster ages are discussed, presenting a calibration of the difference in magnitude between the luminous MS termination and the He burning giants in terms of the cluster age. Both evolutionary tracks and isochrones have been made available at the URL http://gipsy.cjb.net in the ``Pisa Evolutionary Library'' directory
The Pisa Evolutionary Library
We supplement recent evolutionary computations of canonical stellar models (i.e. with inefficient core overshooting) with Z=0.0002, 0.0004, 0.0006, 0.001, 0.004, 0.008 and suitable assumptions about the original He content. Evolutionary results have been compared with observational data in order to properly calibrate the efficiency of the surface convection. On this basis, we follow the evolution of stellar models in the mass range 0.6 to 11 M from the Main Sequence (MS) until C ignition or the onset of thermal pulses, presenting cluster isochrones covering the range of ages from 20 Myr to 20 Gyr. To allow a comparison with evolutionary investigations appeared in the recent literature, we computed additional sets of models which take into account moderate core overshooting during the H burning phase. Selected predictions constraining the cluster ages are discussed, presenting a calibration of the difference in magnitude between the luminous MS termination and the He burning giants in terms of the cluster age. Both evolutionary tracks and isochrones have been made available at the URL http://gipsy.cjb.net in the ``Pisa Evolutionary Library'' directory
The Pisa Evolutionary Library
We supplement recent evolutionary computations of canonical stellar models (i.e. with inefficient core overshooting) with Z=0.0002, 0.0004, 0.0006, 0.001, 0.004, 0.008 and suitable assumptions about the original He content. Evolutionary results have been compared with observational data in order to properly calibrate the efficiency of the surface convection. On this basis, we follow the evolution of stellar models in the mass range 0.6 to 11 Msun from the Main Sequence (MS) until C ignition or the onset of thermal pulses in the advanced Asymptotic Giant Branch (AGB) phase, presenting cluster isochrones covering the range of ages from 20 Myr to 20 Gyr. To allow a comparison with evolutionary investigations appeared in the recent literature, we computed additional sets of models which take into account moderate core overshooting during the H burning phase (for the metallicities suitable for stars in the Magellanic Clouds: Z=0.004, Z=0.008). Selected predictions constraining the cluster ages are discussed, presenting a calibration of the difference in magnitude between the luminous MS termination and the He burning giants in terms of the cluster age. Both evolutionary tracks and isochrones have been made available at the URL http://gipsy.cjb.net in the ``Pisa Evolutionary Library'' directory