77 research outputs found
Distances and Stellar Population properties using the SBF method
We present some results on the study of stellar population properties and
distances of galaxies using the SBF technique. The applications summarized here
show that the Surface Brightness Fluctuations (SBF) method is able to i)
provide accurate distances of resolved and unresolved stellar systems from ~10
Kpc to ~150 Mpc, and ii) to reliably constrain the physical properties (e.g.
age and metallicity) of unresolved stellar systems.Comment: 4 page
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
Pulsational Evidence for Mass Loss in NGC 1866 Cepheids
Available observational data for the 20 known Cepheids in the LMC cluster NGC 1866 have been compared with Hubble Space Telescope observations, discovering in the cluster central region five additional variables, one of which appears to be a Cepheid candidate. We also reach the conclusion that only the photometric data for the seven variables in the cluster periphery appear accurate enough to allow a meaningful comparison with the results of pulsational theories. Out of these seven well-observed Cepheids, we find that the six probable cluster members are located in the color-magnitude diagram at the hot tip of the blue nose experienced by He-burning giants. Since evolutionary theory predicts for these giants an original mass on the order of 4 M⊙, we extend down to ~3 M⊙ the theoretical pulsational scenario already presented for M ≥ 5.0 M⊙. On this basis we discuss the four member Cepheids with VI magnitudes accurate enough to produce robust constraints on the pulsating structures. Among these variables, one finds evidence for a spread of masses by about 7%, with the structures following a tight mass-luminosity relation. Moreover, we show that periods and colors of the Cepheids give a robust indication of pulsator masses smaller than predicted by stellar evolution theory without mass loss, independently of the occurrence of core overshooting
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
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
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