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 $1500 \leq cz \leq 3500$. 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 ($\geq$ 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