40 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
The Influence of N14(e-,nu)C14(alpha,gamma)O18 reaction on the He-Ignition in Degenerate Physical Conditions
The importance of NCO chain on the onset of the He-flash in degenerate
physical conditions has been reevaluated. We find that low-mass, metal-rich (Z
0.001) structures climbing the Red Giant Branch do never attain the
physical conditions suitable for the onset of this chain, while at lower
metallicities the energy contribution provided by NCO reaction is too low to
affect the onset of the central He-flash. At the same time, our evolutionary
models suggest that for a Carbon-Oxygen White Dwarf of mass M_{WD}=0.6 M_sun
accreting He-rich matter, directly or as a by-product of an overlying H-burning
shell, at rates suitable for a dynamical He-flash, the NCO energy contribution
is not able to keep hot enough the He-shell and in turn to avoid the occurrence
of a strong electron degeneracy and the ensuing final explosion.Comment: 15 pages, 3 tables, 10 figure, to appear in Ap
Carbon-Oxygen White Dwarf Accreting CO-Rich Matter. II. Self-Regulating Accretion Process up to the Explosive Stage
We investigate the effect of rotation on the evolution of double-degenerate white dwarf systems, which are possible progenitors of Type Ia supernovae. We assume that prior to merging, the two white dwarfs rotate synchronously at the orbital frequency and that in the merger process, the lighter white dwarf is transformed into a thick disk from which the more massive white dwarf initially accretes at a very high rate (~10-5 M☉ yr-1). Because of the lifting effect of rotation, the accreting white dwarf expands until the gravitational acceleration and centripetal acceleration required for binding at the surface become equal, initiating a Roche instability. The white dwarf continues to accrete matter from the disk, but at a rate that is determined by the balance between two competing processes operating in outer layers: (1) heating, expansion, and spin-up due to accretion and (2) cooling and contraction due to thermal diffusion. The balance produces an accretion rate such that the angular velocity of the white dwarf ωWD and the break-up angular velocity ωcr remain equal. Because of the deposition of angular momentum by accreted matter and the contraction of the accreting star, ωWD increases continuously until the rotational energy reaches about 14% of the gravitational binding energy; then, another instability sets in: the structure is forced to adopt an elliptical shape and emit gravitational waves. Thereafter, a balance between the rate of deposition of angular momentum by accreted matter and the rate of loss of angular momentum by gravitational waves produces a nearly constant or "plateau" accretion rate of ~4 × 10-7 M☉ yr-1. The mass of the accreting white dwarf can increase up to and beyond the Chandresekhar mass limit for nonrotating white dwarfs before carbon ignition occurs. Independent of the initial value of the accretion rate, the physical conditions suitable for carbon ignition are achieved at the center of the accreting white dwarf and, because of the high electron degeneracy, the final outcome is an event of SN Ia proportions. Our results apply to merged binary white dwarf systems which, at the onset of explosive carbon ignition, have a total mass in the range 1.4-1.5 M☉
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
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
Hydrogen-Accreting Carbon-Oxygen White Dwarfs of Low Mass: Thermal and Chemical Behavior of Burning Shells
Numerical experiments have been performed to investigate the thermal behavior
of a cooled down white dwarf of initial mass M_{\rm WD} = 0.516 M_{\sun}
which accretes hydrogen-rich matter with Z = 0.02 at the rate
\msun \yrm1, typical for a recurrent hydrogen shell flash regime. The evolution
of the main physical quantities of a model during a pulse cycle is examined in
detail. From selected models in the mass range
\msunend, we derive the borders in the - plane of the
steady state accretion regime when hydrogen is burned at a constant rate as
rapidly as it is accreted. The physical properties during a hydrogen shell
flash in white dwarfs accreting hydrogen-rich matter with metallicities Z =
0.001 and Z = 0.0001 are also studied. For a fixed accretion rate, a decrease
in the metallicity of the accreted matter leads to an increase in the thickness
of the hydrogen-rich layer at outburst and a decrease in the hydrogen-burning
shell efficiency. In the - plane, the borders of the
steady state accretion band are critically dependent on the metallicity of the
accreted matter: on decreasing the metallicity, the band is shifted to lower
accretion rates and its width in is reduced.Comment: 31 pages and 10 Postscript figures; Accepted for publication on Ap