177 research outputs found
Observational Properties of SNe Ia Progenitors Close to the Explosion
We determine the expected signal in various observational bands of Supernovae
Ia progenitors just before the explosion by assuming the rotating Double
Degenerate scenario. Our results are valid also for all the evolutionary
scenarios invoking rotation as the driving mechanism of the accretion process
as well as the evolution up to the explosion. We find that the observational
properties depend mainly on the mass of the exploding object, even if the
angular momentum evolution after the end of the mass accretion phase and before
the onset of C-burning plays a non-negligible role. Just before the explosion
the magnitude M_V ranges between 9 and 11 mag, while the colour (F225W-F555W)
is about -1.64 mag. The photometric properties remain constant for a few
decades before the explosion. During the last few months the luminosity
decreases very rapidly. The corresponding decline in the optical bands varies
from few hundredths up to one magnitude, the exact value depending on both the
WD total mass and the braking efficiency at the end of the mass transfer. This
feature is related to the exponentially increasing energy production which
drives the formation of a convective core rapidly extending over a large part
of the exploding object. Also a drop in the angular velocity occurs. We find
that observations in the soft X band (0.5 -2 keV) may be used to check if the
SNe Ia progenitors evolution up to explosion is driven by rotation and, hence,
to discriminate among different progenitor scenarios.Comment: 8 pages, 6 figures, 2 tables. Accepted for the publication on MNRA
Disentangling age and metallicity in distant unresolved stellar systems
We present some results of an observational and theoretical study on
unresolved stellar systems based on the Surface Brightness Fluctuations (SBF)
technique. It is shown that SBF magnitudes are a valuable tracer of stellar
population properties, and a reliable distance indicator. SBF magnitudes,
SBF-colors, and SBF-gradients can help to constrain within relatively narrow
limits the metallicity and age of the dominant stellar component in distant
stellar systems, especially if coupled with other spectro-photometric
indicators.Comment: A contributed paper to the Cefalu' (Italy) "Probing Stellar
Populations out to the Distant Universe", 4 pages. To appear as an AIP
Conference Proceedin
Core collapse and horizontal-branch morphology in galactic globular clusters
Context. Stellar collision rates in globular clusters (GCs) do not appear to
correlate with horizontal branch (HB) morphology, sug- gesting that dynamics
does not play a role in the second-parameter problem. However, core densities
and collision rates derived from surface-brightness may be significantly
underestimated as the surface-brightness profile of GCs is not necessarily a
good indicator of the dynamical state of GC cores. Core-collapse may go
unnoticed if high central densities of dark remnants are present. Aims. We test
whether GC HB morphology data supports a dynamical contribution to the
so-called second-parameter effect. Methods. To remove first-parameter
dependence we fitted the maximum effective temperature along the HB as a
function of metal- licity in a sample of 54 Milky Way GCs. We plotted the
residuals to the fit as a function of second-parameter candidates, namely
dynamical age and total luminosity. We considered dynamical age (i.e. the ratio
between age and half-light relaxation time) among possible second-parameters.
We used a set of direct N-body simulations, including ones with dark remnants
to illustrate how core density peaks, due to core collapse, in a dynamical-age
range similar to that in which blue HBs are overabundant with respect to the
metallicity expectation, especially for low-concentration initial conditions.
Results. GC total luminosity shows nonlinear behavior compatible with the
self-enrichment picture. However, the data are amenable to a different
interpretation based on a dynamical origin of the second-parameter effect.
Enhanced mass-stripping in the late red-giant- branch phase due to stellar
interactions in collapsing cores is a viable candidate mechanism. In this
picture, GCs with HBs bluer than expected based on metallicity are those
undergoing core-collapse.Comment: 7 pages, 7 figures, A&A accepte
Detection of Surface Brightness Fluctuations in Elliptical Galaxies imaged with the Advanced Camera for Surveys. B- and I-band measurements
Taking advantage of the exceptional capabilities of ACS on board of HST, we
derive Surface Brightness Fluctuation (SBF) measurements in the B and I bands
from images of six elliptical galaxies with . Given the
low S/N ratio of the SBF signal in the blue band images, the reliability of the
measurements is verified both with numerical simulations and experimental data
tests.
This paper presents the first published B- and I-band SBF measurements for
distant ( 20 Mpc) galaxies, essential for the comparisons of the models
to observations of normal ellipticals. By comparing I-band data with our new
Simple Stellar Population (SSP) models we find an excellent agreement and we
confirm that I-band SBF magnitudes are mainly sensitive to the metallicity of
the dominant stellar component in the galaxy, and are not strongly affected by
the contribution of possible secondary stellar components. As a consequence
I-band fluctuations magnitudes are ideal for distance studies. On the other
hand, we show that standard SSP models do not reproduce the B-band SBF
magnitudes of red ((B-I)_0 \gsim 2.1) galaxies in our sample. We explore the
capability of two non--canonical models in properly reproducing the high
sensitivity of B SBF to the presence of even small fractions of bright, hot
stars (metal poor stars, hot evolved stars, etc.). The disagreement is solved
both by taking into account hot (Post--AGB) stars in SSP models and/or by
adopting Composite Stellar Population models. Finally, we suggest a limit value
of the S/N for the B-band SBF signal required to carry out a detailed study of
stellar population properties based on this technique.Comment: ApJ accepte
Advances on GRB as cosmological tools
Several interesting correlations among Gamma Ray Bursts (GRB) prompt and
afterglow properties have been found in the recent years. Some of these
correlations have been proposed also to standardize GRB energetics to use them
as standard candles in constraining the expansion history of the universe up to
z>6. However, given the still unexplained nature of most of these correlations,
only the less scattered correlations can be used for constraining the
cosmological parameters. The updated E_peak-E_gamma correlation is presented.
Caveats of alternative methods of standardizing GRB energetics are discussed.Comment: 8 parges, AIP conf. proc. "Probing stellar populations out to the
distant universe, Cefalu' 2008" Vol. 1111, pp. 579-58
Unsolved Problems about Supernovae
A number of unsolved problems and open questions about the nature and the
properties of supernovae are identified and briefly discussed. Some suggestions
and directions toward possible solutions are also considered.Comment: 7 pages, Proceedings of 'Probing Stellar Populations out to the
Distant Universe', Cefalu, Italy, Sep 7-19, 2008, AIP Conf. Proc. Serie
The formation and evolution of early-type galaxies : solid results and open questions
The most recent results and some of the open key questions on the evolution
of early-type galaxies are reviewed in the general cosmological context of
massive galaxy formation.Comment: 8 pages, invited review at the workshop "Probing Stellar Populations
out to the Distant Universe", Cefalu` (Italy), September 7 - 19, 200
Spectral libraries and their uncertainties
Libraries of stellar spectra are fundamental tools in the study of stellar
populations and in automatic determination of atmospheric parameters for large
samples of observed stars. In the context of the present volume, here I give an
overview of the current status of stellar spectral libraries from the
perspective of stellar population modeling: what we have currently available,
how good they are, and where we need further improvement
Early metal enrichment in high-redshift quasars
Quasars are powerful systems whose spectrum is rich of metal features that
allow us to investigate the chemical evolution of galaxies at very high
redshift, even close to the reionization epoch. I review the main observational
constraints on the metallicity of quasars host galaxies at high redshift and
discuss the implications and issues for models of galaxy evolution in the early
universe.Comment: 8 pages, invited review at the workshop "Probing Stellar Populations
out to the Distant Universe
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