207 research outputs found
Evolution, nucleosynthesis and yields of AGB stars at different metallicities (III): intermediate mass models, revised low mass models and the ph-FRUITY interface
We present a new set of models for intermediate mass AGB stars (4.0, 5.0 and,
6.0 Msun) at different metallicities (-2.15<=Fe/H]<=+0.15). This integrates the
existing set of models for low mass AGB stars (1.3<=M/M<=3.0) already included
in the FRUITY database. We describe the physical and chemical evolution of the
computed models from the Main Sequence up to the end of the AGB phase. Due to
less efficient third dredge up episodes, models with large core masses show
modest surface enhancements. The latter is due to the fact that the interpulse
phases are short and, then, Thermal Pulses are weak. Moreover, the high
temperature at the base of the convective envelope prevents it to deeply
penetrate the radiative underlying layers. Depending on the initial stellar
mass, the heavy elements nucleosynthesis is dominated by different neutron
sources. In particular, the s-process distributions of the more massive models
are dominated by the \nean~reaction, which is efficiently activated during
Thermal Pulses. At low metallicities, our models undergo hot bottom burning and
hot third dredge up. We compare our theoretical final core masses to available
white dwarf observations. Moreover, we quantify the weight that intermediate
mass models have on the carbon stars luminosity function. Finally, we present
the upgrade of the FRUITY web interface, now also including the physical
quantities of the TP-AGB phase of all the models included in the database
(ph-FRUITY).Comment: Accepted for publication on ApJ
On the mass of supernova progenitors: the role of the CC reaction
A precise knowledge of the masses of supernova progenitors is essential to
answer various questions of modern astrophysics, such as those related to the
dynamical and chemical evolution of Galaxies. In this paper we revise the upper
bound for the mass of the progenitors of CO white dwarfs (\mup) and the lower
bound for the mass of the progenitors of normal type II supernovae (\mups). In
particular, we present new stellar models with mass between 7 and 10 \msun,
discussing their final destiny and the impact of recent improvements in our
understanding of the low energy rate of the \c12c12 reaction.Comment: To be published on the proceedings of NIC 201
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
On the Origin of the Early Solar System Radioactivities. Problems with the AGB and Massive Star Scenarios
Recent improvements in stellar models for intermediate-mass and massive stars
are recalled, together with their expectations for the synthesis of radioactive
nuclei of lifetime Myr, in order to re-examine the origins
of now extinct radioactivities, which were alive in the solar nebula. The
Galactic inheritance broadly explains most of them, especially if -process
nuclei are produced by neutron star merging according to recent models.
Instead, Al, Ca, Cs and possibly Fe require
nucleosynthesis events close to the solar formation. We outline the persisting
difficulties to account for these nuclei by Intermediate Mass Stars (2
M/M). Models of their final stages now
predict the ubiquitous formation of a C reservoir as a neutron capture
source; hence, even in presence of Al production from Deep Mixing or Hot
Bottom Burning, the ratio Al/Pd remains incompatible with
measured data, with a large excess in Pd. This is shown for two recent
approaches to Deep Mixing. Even a late contamination by a Massive Star meets
problems. In fact, inhomogeneous addition of Supernova debris predicts
non-measured excesses on stable isotopes. Revisions invoking specific low-mass
supernovae and/or the sequential contamination of the pre-solar molecular cloud
might be affected by similar problems, although our conclusions here are
weakened by our schematic approach to the addition of SN ejecta. The limited
parameter space remaining to be explored for solving this puzzle is discussed.Comment: Accepted for publication on Ap
Carbon-Oxygen White Dwarfs Accreting CO-Rich Matter I: A Comparison Between Rotating and Non-Rotating Models
We investigate the lifting effect of rotation on the thermal evolution of CO
WDs accreting CO-rich matter. We find that rotation induces the cooling of the
accreting star so that the delivered gravitational energy causes a greater
expansion with respect to the standard non-rotating case. The increase in the
surface radius produces a decrease in the surface value of the critical angular
velocity and, therefore, the accreting WD becomes gravitationally unbound
(Roche instability). This occurrence is due to an increase in the total angular
momentum of the accreting WD and depends critically on the amount of specific
angular momentum deposited by the accreted matter. If the specific angular
momentum of the accreted matter is equal to that of the outer layers of the
accreting structure, the Roche instability occurs well before the accreting WD
can attain the physical conditions for C-burning. If the values of both initial
angular velocity and accretion rate are small, we find that the accreting WD
undergoes a secular instability when its total mass approaches 1.4 Msun. At
this stage, the ratio between the rotational and the gravitational binding
energy of the WD becomes of the order of 0.1, so that the star must deform by
adopting an elliptical shape. In this case, since the angular velocity of the
WD is as large as 1 rad/s, the anisotropic mass distribution induces the loss
of rotational energy and angular momentum via GWR. We find that, independent of
the braking efficiency, the WD contracts and achieves the physical conditions
suitable for explosive C-burning at the center so that a type Ia supernova
event is produced.Comment: 39 pages, 22 eps-figures; accepted for publication in Astrophysical
Journa
Asymptotic Giant Branch models at very low metallicity
In this paper we present the evolution of a low mass model (initial mass
M=1.5 Msun) with a very low metal content (Z=5x10^{-5}, equivalent to
[Fe/H]=-2.44). We find that, at the beginning of the AGB phase, protons are
ingested from the envelope in the underlying convective shell generated by the
first fully developed thermal pulse. This peculiar phase is followed by a deep
third dredge up episode, which carries to the surface the freshly synthesized
13C, 14N and 7Li. A standard TP-AGB evolution, then, follows. During the proton
ingestion phase, a very high neutron density is attained and the s-process is
efficiently activated. We therefore adopt a nuclear network of about 700
isotopes, linked by more than 1200 reactions, and we couple it with the
physical evolution of the model. We discuss in detail the evolution of the
surface chemical composition, starting from the proton ingestion up to the end
of the TP-AGB phase.Comment: Accepted for Publication on PAS
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