103 research outputs found
Asymptotic Giant Brach Stars as Astroparticle Laboratories
We show that the inclusion of axion emission during stellar evolution
introduces important changes into the evolutionary behaviour of AGB stars. The
mass of the resulting C/O white dwarf is much lower than the equivalent
obtained from standard evolution. This implies a deficit in luminous AGB stars
and in massive WDs. Moreover the total mass processed in the nuclear burning
shells that is dredged-up to the surface () increases when axion
emission is included, modifying the chemical composition of the photosphere. We
conclude that the AGB is a promising phase to put constraints on particle
physicsComment: 8 pages, 3 tables, 8 figures, accepted for publication in MNRA
Heavy elements in Globular Clusters: the role of AGB stars
Recent observations of heavy elements in Globular Clusters reveal intriguing
deviations from the standard paradigm of the early galactic nucleosynthesis. If
the r-process contamination is a common feature of halo stars, s-process
enhancements are found in a few Globular Clusters only. We show that the
combined pollution of AGB stars with mass ranging between 3 to 6 M may
account for most of the features of the s-process overabundance in M4 and M22.
In these stars, the s process is a mixture of two different neutron-capture
nucleosynthesis episodes. The first is due to the 13C(a,n)16O reaction and
takes place during the interpulse periods. The second is due to the
22Ne(a,n)25Mg reaction and takes place in the convective zones generated by
thermal pulses. The production of the heaviest s elements (from Ba to Pb)
requires the first neutron burst, while the second produces large
overabundances of light s (Sr, Y, Zr). The first mainly operates in the
less-massive AGB stars, while the second dominates in the more-massive. From
the heavy-s/light-s ratio, we derive that the pollution phase should last for
Myr, a period short enough compared to the formation timescale of
the Globular Cluster system, but long enough to explain why the s-process
pollution is observed in a few cases only. With few exceptions, our theoretical
prediction provides a reasonable reproduction of the observed s-process
abundances, from Sr to Hf. However, Ce is probably underproduced by our models,
while Rb and Pb are overproduced. Possible solutions are discussed.Comment: Accepted by the Ap
Type Ia supernovae: differences due to progenitors within delayed detonation explosions
At this moment, the use of SNIa for cosmology lies on the assumption that the
SNe at high redshifts are equal to the local ones. However, some observations
indicate a correlation between the light curve (LC) properties and the
morphological type of the host galaxy. This could indicate a dependence with
the age (mass/composition) of the underlying population. In this work we have
chosen the delayed detonation explosion model in CO Chandrasekhar mass WDs to
explore the dependence of the SNIa LC and nucleosynthesis with the initial mass
and composition of the WD progenitor. The progenitor influences the final SNIa
via the mass of the CO core formed and the C/O ratio within it (1D explosion
models). We have followed the evolution of stars with masses between 1.5 and 8
Mo and metallicity, Z=0, 1.E-05, 0.001 and 0.02, from the pre-main sequence to
the TP-AGB phase. The differences obtained in the final C/O ratio within the
explosive WD are smaller than 22%. This results in a difference at maximum of
0.03 mag and of 0.1 mag when the brightness-decline relation is applied.Comment: 4 pages, 1 figure, needs espcrc1.sty; conference "Nuclei in the
Cosmos 2000", held in Arhus, Denmark, June 27-July 1, 2000; submitted to
Nucl. Phys.
On the need of the Light Elements Primary Process (LEPP)
Extant chemical evolution models underestimate the Galactic production of Sr,
Y and Zr as well as the Solar System abundances of s-only isotopes with
90<A<130. To solve this problem, an additional (unknown) process has been
invoked, the so-called LEPP (Light Element Primary Process). In this paper we
investigate possible alternative solutions. Basing on Full Network Stellar
evolutionary calculations, we investigate the effects on the Solar System
s-only distribution induced by the inclusion of some commonly ignored physical
processes (e.g. rotation) or by the variation of the treatment of convective
overshoot, mass-loss and the efficiency of nuclear processes. Our main findings
are: 1) at the epoch of the formation of the Solar System, our reference model
produces super-solar abundances for the whole s-only distribution, even in the
range 90<A<130; 2) within errors, the s-only distribution relative to 150Sm is
flat; 3) the s-process contribution of the less massive AGB stars (M<1.5 M_SUN)
as well as of the more massive ones (M>4.0 M_SUN) are negligible; 4) the
inclusion of rotation implies a downward shift of the whole distribution with
an higher efficiency for the heavy s-only isotopes, leading to a flatter s-only
distribution; 5) different prescriptions on convection or mass-loss produce
nearly rigid shifts of the whole distribution. In summary, a variation of the
standard paradigm of AGB nucleosynthesis would allow to reconcile models
predictions with Solar System s-only abundances. Nonetheless, the LEPP cannot
be definitely ruled out, because of the uncertainties still affecting stellar
and Galactic chemical evolution models.Comment: Accepted for publication on Ap
Impact of Axions on the Minimum Mass of Core Collapse Supernova Progenitors
In this study we include axions in stellar evolution models adopting the
current stringest constraints for their coupling to photons and electrons. We
obtain that the minimum stellar mass of Core Collapse Supernova (CCSN)
progenitors is shifted up by nearly 2 Mo. This result seems to be in tension
with the observationaly derived minimum mass of CCSN progenitors.Comment: Contributed to the 13th Patras Workshop on Axions, WIMPs and WISPs,
Thessaloniki, May 15 to 19, 201
Two barium stars in the Galactic bulge
Barium stars conserve important information on the s-process and the third
dredge-up in intermediate mass stars. Their discovery in various environments
is therefore of great help to test nucleosynthesis and mixing models. Our aim
is to analyse two stars with a very strong barium line detected in a large
survey of red giants in the Galactic bulge. Abundance analysis was done
comparing synthetic model spectra based on the COMARCS code with our medium
resolution spectra. Abundances of Ba, La, Y, and Fe were determined. Beside the
two main targets, the analysis was also applied to two comparison stars. We
confirm that both stars are barium stars. They are the first ones of this kind
identified in the Galactic bulge. Their barium excesses are among the largest
values found up to now. The elemental abundances are compared with current
nucleosynthesis and mixing models. Furthermore, we estimate a frequency of
barium stars in the Galactic bulge of about 1%, which is identical to the value
for disc stars.Comment: 4 pages, accepted for publication in A&
Axion-electron coupling from the RGB tip of Globular Clusters
We present a preliminary study of the Globular Cluster RGB devoted to improve
the available constraint for the axion-electron coupling. By means of
multi-band IR photometry of the cluster M3 we obtain g_{ae}/10^{-13} < 2.57
(95\% C.L.).Comment: paper submitted to Proceedings of the 13th Patras Workshop on Axions,
WIMPs and WISPs (Patras2017)
The puzzle of the CNO isotope ratios in AGB carbon stars
Previous determinations of the oxygen isotopic ratios in AGB carbon stars
were at odds with the existing theoretical predictions. We aim to redetermine
the oxygen ratios in these stars using new spectral analysis tools and further
develop discussions on the carbon and nitrogen isotopic ratios in order to
elucidate this problem. Oxygen isotopic ratios were derived from spectra in the
K-band in a sample of galactic AGB carbon stars of different spectral types and
near solar metallicity. Synthetic spectra calculated in LTE with spherical
carbon-rich atmosphere models and updated molecular line lists were used. The
CNO isotope ratios derived in a homogeneous way, were compared with theoretical
predictions for low-mass (1.5-3 M_o) AGB stars computed with the FUNS code
assuming extra mixing both during the RGB and AGB phases. For most of the stars
the 16O/17O/18O ratios derived are in good agreement with theoretical
predictions confirming that, for AGB stars, are established using the values
reached after the FDU according to the initial stellar mass. This fact, as far
as the oxygen isotopic ratios are concerned, leaves little space for the
operation of any extra mixing mechanism during the AGB phase. Nevertheless, for
a few stars with large 16O/17O/18O, the operation of such a mechanism might be
required, although their observed 12C/13C and 14N/15N ratios would be difficult
to reconcile within this scenario. Furthermore, J-type stars tend to have lower
16O/17O ratios than the normal carbon stars, as already indicated in previous
studies. Excluding these peculiar stars, AGB carbon stars occupy the same
region as pre-solar type I oxide grains in a 17O/16O vs. 18O/16O diagram,
showing little spread. This reinforces the idea that these grains were probably
formed in low-mass stars during the previous O-rich phases.Comment: Accepted fo publication in A&
Oxygen isotopic ratios in intermediate-mass red giants
Context. The abundances of the three main isotopes of oxygen are altered in
the course of the CNO-cycle. When the first dredge-up mixes the burning
products to the surface, the nucleosynthesis processes can be probed by
measuring oxygen isotopic ratios. Aims. By measuring 16O/17O and 16O/18O in red
giants of known mass we compare the isotope ratios with predictions from
stellar and galactic evolution modelling. Methods. Oxygen isotopic ratios were
derived from the K-band spectra of six red giants. The sample red giants are
open cluster members with known masses of between 1.8 and 4.5 Msun . The
abundance determination employs synthetic spectra calculated with the COMARCS
code. The effect of uncertainties in the nuclear reaction rates, the mixing
length, and of a change in the initial abundance of the oxygen isotopes was
determined by a set of nucleosynthesis and mixing models using the FUNS code.
Results. The observed 16O/17O ratios are in good agreement with the model
results, even if the measured values do not present clear evidence of a
variation with the stellar mass. The observed 16O/18O ratios are clearly lower
than the predictions from our reference model. Variations in nuclear reaction
rates and mixing length parameter both have only a very weak effect on the
predicted values. The 12C/13C ratios of the K giants studied implies the
absence of extra-mixing in these objects. Conclusions. A comparison with
galactic chemical evolution models indicates that the 16O/18O abundance ratio
underwent a faster decrease than predicted. To explain the observed ratios, the
most likely scenario is a higher initial 18O abundance combined with a lower
initial 16 O abundance. Comparing the measured 18 O/17 O ratio with the
corresponding value for the ISM points towards an initial enhancement of 17O as
well. Limitations imposed by the observations prevent this from being a
conclusive result.Comment: 9 pages, accepted for publication in Astronomy & Astrophysic
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