650 research outputs found
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
Radiation-hydrodynamical modelling of Core-Collapse Supernovae: light curves and the evolution of photospheric velocity and temperature
We have developed a relativistic, radiation-hydrodynamics Lagrangian code,
specifically tailored to simulate the evolution of the main observables (light
curve, evolution of photospheric velocity and temperature) in core-collapse
supernova (CC-SN) events. The distinctive features of the code are an accurate
treatment of radiative transfer coupled to relativistic hydrodynamics, a
self-consistent treatment of the evolution of the innermost ejecta taking into
account the gravitational effects of the central compact remnant, and a fully
implicit Lagrangian approach to the solution of the coupled non-linear finite
difference system of equations. Our aim is to use it as numerical tool to
perform calculations of grid of models to be compared with observation of
CC-SNe. In this paper we present some testcase simulations and a comparison
with observations of SN 1987A, as well as with the results obtained with other
numerical codes. We also briefly discuss the influence of the main physical
parameters (ejected mass, progenitor radius, explosion energy, amount of
\chem{56}{Ni}) on the evolution of the ejecta, and the implications of our
results in connection with the possibility to "standardize" hydrogen-rich
CC-SNe for using them as candles to measure cosmological distances.Comment: Accepted for publication in ApJ (16 pages, 22 figures
Supernova dust yields: the role of metallicity, rotation, and fallback
Supernovae (SNe) are considered to have a major role in dust enrichment of
high redshift galaxies and, due to the short lifetimes of interstellar grains,
in dust replenishment of local galaxies. Here we explore how SN dust yields
depend on the mass, metallicity, and rotation rate of the progenitor stars, and
on the properties of the explosion. To this aim, assuming uniform mixing inside
the ejecta, we quantify the dust mass produced by a sample of SN models with
progenitor masses , metallicity , rotation rate and ~km/s, that
explode with a fixed energy of ~erg (FE models) or with
explosion properties calibrated to reproduce the - relation
inferred from SN observations (CE models). We find that rotation favours more
efficient dust production, particularly for more massive, low metallicity
stars, but that metallicity and explosion properties have the largest effects
on the dust mass and its composition. In FE models, SNe with are more efficient at forming dust: between 0.1 and 1 is
formed in a single explosion, with a composition dominated by silicates, carbon
and magnetite grains when , and by carbon and magnetite grains
when . In CE models, the ejecta are massive and metal-rich and
dust production is more efficient. The dust mass increases with and it is
dominated by silicates, at all [Fe/H].Comment: MNRAS, in pres
Hydrodynamic simulations of shell convection in stellar cores
Shell convection driven by nuclear burning in a stellar core is a common
hydrodynamic event in the evolution of many types of stars. We encounter and
simulate this convection (i) in the helium core of a low-mass red giant during
core helium flash leading to a dredge-down of protons across an entropy
barrier, (ii) in a carbon-oxygen core of an intermediate-mass star during core
carbon flash, and (iii) in the oxygen and carbon burning shell above the
silicon-sulfur rich core of a massive star prior to supernova explosion. Our
results, which were obtained with the hydrodynamics code HERAKLES, suggest that
both entropy gradients and entropy barriers are less important for stellar
structure than commonly assumed. Our simulations further reveal a new dynamic
mixing process operating below the base of shell convection zones.Comment: 8 pages, 3 figures .. submitted to a proceedings of conference about
"Red Giants as Probes of the Structure and Evolution of the Milky Way" which
has taken place between 15-17 November 2010 in Rom
Isotopic Titanium Abundances in Local M Dwarfs
Relative abundances of the five stable isotopes of titanium (^46Ti to ^50Ti)
are measured for 11 M dwarfs belonging to the thin disk (four stars), thick
disk (three stars), the halo (one star), and either the thick or the thin disk
(three stars). Over the metallicity range of the sample (-1<[Fe/H]<0), the
isotopic ratios are approximately constant to the solar system ratios. There is
no discernible difference between the isotopic ratios for thin and thick disk
stars. Isotopic ratios are in fair accord with recent calculations of Galactic
chemical evolution despite the fact that such calculations underpredict [Ti/Fe]
by about 0.4 dex at all metallicities.Comment: 33 pages, 8 figures, accepted for publication in Ap
Heavy element abundances in giant stars of the globular clusters M4 and M5
We present a comprehensive abundance analysis of 27 heavy elements in bright
giant stars of the globular clusters M4 and M5 based on high resolution, high
signal-to-noise ratio spectra obtained with the Magellan Clay Telescope. We
confirm and expand upon previous results for these clusters by showing that (1)
all elements heavier than, and including, Si have constant abundances within
each cluster, (2) the elements from Ca to Ni have indistinguishable
compositions in M4 and M5, (3) Si, Cu, Zn, and all s-process elements are
approximately 0.3 dex overabundant in M4 relative to M5, and (4) the r-process
elements Sm, Eu, Gd, and Th are slightly overabundant in M5 relative to M4. The
cluster-to-cluster abundance differences for Cu and Zn are intriguing,
especially in light of their uncertain nucleosynthetic origins. We confirm that
stars other than Type Ia supernovae must produce significant amounts of Cu and
Zn at or below the clusters' metallicities. If intermediate-mass AGB stars or
massive stars are responsible for the Cu and Zn enhancements in M4, the similar
[Rb/Zr] ratios and (preliminary) Mg isotope ratios in both clusters may be
problematic for either scenario. For the elements from Ba to Hf, we assume that
the s- and r-process contributions are scaled versions of the solar s- and
r-process abundances. We quantify the relative fractions of s- and r-process
material for each cluster and show that they provide an excellent fit to the
observed abundances.Comment: Accepted for publication in Ap
TOPoS: II. On the bimodality of carbon abundance in CEMP stars. Implications on the early chemical evolution of galaxies
In the course of the TOPoS (Turn Off Primordial Stars) survey, aimed at
discovering the lowest metallicity stars, we have found several carbon-enhanced
metal-poor (CEMP) stars. We here present our analysis of six CEMP stars.
Calcium and carbon are the only elements that can be measured in all six stars.
The range is -5.0<=[Ca/H]< -2.1 and 7.12<=A(C)<=8.65. For star SDSS J1742+2531
we were able to detect three FeI lines from which we deduced [Fe/H]=-4.80, from
four CaII lines we derived [Ca/H]=-4.56, and from synthesis of the G-band we
derived A(C)=7.26. For SDSS J1035+0641 we were not able to detect any iron
lines, yet we could place a robust (3sigma) upper limit of [Fe/H]< -5.0 and
measure the Ca abundance, with [Ca/H]=-5.0, and carbon, A(C)=6.90. No lithium
is detected in the spectrum of SDSS J1742+2531 or SDSS J1035+0641, which
implies a robust upper limit of A(Li)<1.8 for both stars. Our measured carbon
abundances confirm the bimodal distribution of carbon in CEMP stars,
identifying a high-carbon band and a low-carbon band. We propose an
interpretation of this bimodality according to which the stars on the
high-carbon band are the result of mass transfer from an AGB companion, while
the stars on the low-carbon band are genuine fossil records of a gas cloud that
has also been enriched by a faint supernova (SN) providing carbon and the
lighter elements. (Abridged)Comment: to be published on A&
The effect of 12C + 12C rate uncertainties on s-process yields
The slow neutron capture process in massive stars (the weak s-process)
produces most of the s-only isotopes in the mass region 60 < A < 90. The
nuclear reaction rates used in simulations of this process have a profound
effect on the final s-process yields. We generated 1D stellar models of a 25
solar mass star varying the 12C + 12C rate by a factor of 10 and calculated
full nucleosynthesis using the post-processing code PPN. Increasing or
decreasing the rate by a factor of 10 affects the convective history and
nucleosynthesis, and consequently the final yields.Comment: Conference proceedings for the Nuclear Physics in Astrophysics IV
conference, 8-12 June 2009. 4 pages, 3 figures. Accepted for publication to
the Journal of Physics: Conference Serie
Intermediate mass stars: updated models
A new set of stellar models in the mass range 1.2 to 9 is
presented. The adopted chemical compositions cover the typical galactic values,
namely and . A comparison among
the most recent compilations of similar stellar models is also discussed. The
main conclusion is that the differencies among the various evolutionary results
are still rather large. For example, we found that the H-burning evolutionary
time may differ up to 20 %. An even larger disagreement is found for the
He-burning phase (up to 40-50 %). Since the connection between the various
input physics and the numerical algorithms could amplify or counterbalance the
effect of a single ingredient on the resulting stellar model, the origin of
this discrepancies is not evident. However most of these discrepancies, which
are clearly found in the evolutionary tracks, are reduced on the isochrones. By
means of our updated models we show that the ages inferred by the theory of
stellar evolution is in excellent agreement with those obtained by using other
independent methods applied to the nearby Open Clusters. Finally, the
theoretical initial/final mass relation is revised.Comment: 35 pages, 24 figures, 4 tables, accepted for publication in the
Astrophisycal Journa
Transcranial magnetic stimulation (TMS) application in sport medicine: A brief review
Since 1985, transcranial magnetic stimulation (TMS) has been used for non-invasive exploration of motor control in humans and for a wide range of applications in all ages of life. This brief review examined briefly the potential interest in sport medicine
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