15 research outputs found
Neutron-rich nuclei in cosmic rays and Wolf-Rayet stars
Wolf-Rayet stars figure prominently in astrophysical research. As a bonus, they seem to offer, in the recent past, an interesting connection between classical astronomy and high energy astrophysics due to their unusual composition and their huge mechanical power. The material flowing from WC stars (carbon-rich WR stars) contains gas which has been processed through core-helium burning, i.e., considerably enriched into 12C,16O, 22Ne, and 25,26Mg. This composition is reminiscent of the cosmic ray source anomalies. Encouraging agreement is obtained with observation in the mass range 12 A 26 assuming acceleration of wind particles at the shock that delineates the WR cavity, and adequate dilution with normal cosmic rays, but silicon poses
The s-process weak component: uncertainties due to convective overshooting
Using a new s-nucleosynthesis code, coupled with the stellar evolution code
Star2003, we performed simulations to study the impact of the convection
treatment on the s-process during core He-burning of a 25 Msun star (ZAMS mass)
with an initial metallicity of Z=0.02. Particular attention was devoted to the
impact of the extent of overshooting on the s-process efficiency. The results
show enhancements of about a factor 2-3 in s-process efficiency (measured as
the average overproduction factor of the 6 s-only nuclear species with
) with overshooting parameter values in the range
0.01-0.035, compared to results obtained with the same model but without
overshooting. The impact of these results on the p-process model based on type
II supernovae is discussed.Comment: 7 pages, 4 figures, accepted for publication in Astronomy &
Astrophysic
Convective overshooting and production of s-nuclei in massive stars during their core He-burning phase
With the "post-processing" technique we explore the role of the convective
overshooting on the production of s-nuclei in stellar models of different
initial mass and metallicity (; ), considering a range of values for the parameter , which
determines the overall efficiency of convective overshooting.We find
enhancements in the production of s-nuclei until a factor (measured as
the average overproduction factor of the 6 s-only nuclear species with
) in all our models of different initial mass and
metallicity with in the range (i.e. models with
overshooting) compared to the production obtained with "no-overshooting" models
(i.e. models with the same initial mass and metallicity, but ).
Moreover the results indicate that the link between the overshooting parameter
and the s-process efficiency is essentially monotonic in all our models of
different initial mass and metallicity. Also evident is the higher s-process
efficiency when we progressively increase for a given f value both the mass of
the models from 15 M to 25 M and the Z value from 10 to
0.02. We also briefly discuss the possible consequences of these results for
some open questions linked to the s-process weak component efficiency, as well
as a "rule of thumb" to evaluate the impact of the convective overshooting on
the yields of a generation of stars.Comment: 12 pages, 6 figures, A&A accepted (corrected typos plus minor changes
in order to fulfill the guidelines for A&A manuscripts
Constraints on the Formation of the Galactic Bulge from Na, Al, and Heavy Element Abundances in Plaut's Field
We report chemical abundances of Na, Al, Zr, La, Nd, and Eu for 39 red giant
branch (RGB) stars and 23 potential inner disk red clump stars located in
Plaut-s low extinction window. We also measure lithium for a super Li-rich RGB
star. The abundances were determined by spectrum synthesis of high resolution
(R~25,000), high signal-to-noise (S/N~50-100 pixel-1) spectra obtained with the
Blanco 4m telescope and Hydra multifiber spectrograph. For the bulge RGB stars,
we find a general increase in the [Na/Fe] and [Na/Al] ratios with increasing
metallicity, and a similar decrease in [La/Fe] and [Nd/Fe]. Additionally, the
[Al/Fe] and [Eu/Fe] abundance trends almost identically follow those of the
{\alpha}-elements, and the [Zr/Fe] ratios exhibit relatively little change with
[Fe/H]. The consistently low [La/Eu] ratios of the RGB stars indicate that at
least a majority of bulge stars formed rapidly (<1 Gyr) and before the main
s-process could become a significant pollution source. In contrast, we find
that the potential inner disk clump stars exhibit abundance patterns more
similar to those of the thin and thick disks. Comparisons between the abundance
trends at different bulge locations suggest that the inner and outer bulge
formed on similar timescales. However, we find evidence of some abundance
differences between the most metal-poor and metal-rich stars in various bulge
fields. The data also indicate that the halo may have had a more significant
impact on the outer bulge initial composition than the inner bulge composition.
The [Na/Fe] and to a lesser extent [La/Fe] abundances further indicate that the
metal-poor bulge, at least at ~1 kpc from the Galactic center, and thick disk
may not share an identical chemistry.Comment: Accepted for publication in ApJ; 66 pages, 17 figures, 3 tables;
prior to publication, data tables in electronic form will be made available
upon reques
Protostellar collapse induced by compression. II: rotation and fragmentation
We investigate numerically and semi-analytically the collapse of low-mass,
rotating prestellar cores. Initially, the cores are in approximate equilibrium
with low rotation (the initial ratio of thermal to gravitational energy is
, and the initial ratio of rotational to gravitational
energy is ). They are then subjected to a steady
increase in external pressure. Fragmentation is promoted -- in the sense that
more protostars are formed -- both by more rapid compression, and by higher
rotation (larger ). In general, the large-scale collapse is
non-homologous, and follows the pattern described in Paper I for non-rotating
clouds, viz. a compression wave is driven into the cloud, thereby increasing
the density and the inflow velocity. The effects of rotation become important
at the centre, where the material with low angular momentum forms a central
primary protostar (CPP), whilst the material with higher angular momentum forms
an accretion disc around the CPP. More rapid compression drives a stronger
compression wave and delivers material more rapidly into the outer parts of the
disc.Comment: 17 pages, accepted for publication in MNRA
The stellar mass spectrum from non-isothermal gravoturbulent fragmentation
Identifying the processes that determine the initial mass function of stars
(IMF) is a fundamental problem in star formation theory. One of the major
uncertainties is the exact chemical state of the star forming gas and its
influence on the dynamical evolution. Most simulations of star forming clusters
use an isothermal equation of state (EOS). However, theoretical predictions and
observations suggest that the effective polytropic exponent gamma in the EOS
varies with density.
We address these issues and study the effect of a piecewise polytropic EOS on
the formation of stellar clusters in turbulent, self-gravitating molecular
clouds using three-dimensional, smoothed particle hydrodynamics simulations. To
approximate the results of published predictions of the thermal behavior of
collapsing clouds, we increase the polytropic exponent gamma from 0.7 to 1.1 at
some chosen density n_c, which we vary. The change of thermodynamic state at
n_c selects a characteristic mass scale for fragmentation M_ch, which we relate
to the peak of the observed IMF. Our investigation generally supports the idea
that the distribution of stellar masses depends mainly on the thermodynamic
state of the star-forming gas. The thermodynamic state of interstellar gas is a
result of the balance between heating and cooling processes, which in turn are
determined by fundamental atomic and molecular physics and by chemical
abundances. Given the abundances, the derivation of a characteristic stellar
mass can thus be based on universal quantities and constants.Comment: 13 pages, 7 figures, accepted by A&
The effect of 12C + 12C rate uncertainties on the evolution and nucleosynthesis of massive stars
[Shortened] The 12C + 12C fusion reaction has been the subject of
considerable experimental efforts to constrain uncertainties at temperatures
relevant for stellar nucleosynthesis. In order to investigate the effect of an
enhanced carbon burning rate on massive star structure and nucleosynthesis, new
stellar evolution models and their yields are presented exploring the impact of
three different 12C + 12C reaction rates. Non-rotating stellar models were
generated using the Geneva Stellar Evolution Code and were later post-processed
with the NuGrid Multi-zone Post-Processing Network tool. The enhanced rate
causes core carbon burning to be ignited more promptly and at lower
temperature. This reduces the neutrino losses, which increases the core carbon
burning lifetime. An increased carbon burning rate also increases the upper
initial mass limit for which a star exhibits a convective carbon core. Carbon
shell burning is also affected, with fewer convective-shell episodes and
convection zones that tend to be larger in mass. Consequently, the chance of an
overlap between the ashes of carbon core burning and the following carbon shell
convection zones is increased, which can cause a portion of the ashes of carbon
core burning to be included in the carbon shell. Therefore, during the
supernova explosion, the ejecta will be enriched by s-process nuclides
synthesized from the carbon core s process. The yields were used to estimate
the weak s-process component in order to compare with the solar system
abundance distribution. The enhanced rate models were found to produce a
significant proportion of Kr, Sr, Y, Zr, Mo, Ru, Pd and Cd in the weak
component, which is primarily the signature of the carbon-core s process.
Consequently, it is shown that the production of isotopes in the Kr-Sr region
can be used to constrain the 12C + 12C rate using the current branching ratio
for a- and p-exit channels.Comment: The paper contains 17 figures and 7 tables. Table 7 will be published
in full online onl
The Effects of Element Diffusion on the Pulsational Properties of Variable DA White Dwarf Stars
We explore the effects of element diffusion due to gravitational settling and
thermal and chemical diffusion on the pulsational properties of DA white
dwarfs. To this end, we employ an updated evolutionary code coupled with a
pulsational, finite difference code for computing the linear, non-radial
g-modes in the adiabatic approximation. We follow the evolution of a 0.55 \msun
white dwarf model in a self-consistent way with the evolution of chemical
abundance distribution as given by time dependent diffusion processes. Results
are compared with the standard treatment of diffusive equilibrium in the trace
element approximation. Appreciable differences are found between the two
employed treatments. We conclude that time dependent element diffusion plays an
important role in determining the whole oscillation pattern and the temporal
derivative of the periods in DAV white dwarfs. In addition, we discuss the
plausibility of the standard description employed in accounting for diffusion
in most of white dwarf asteroseismological studies.Comment: 6 pages, 5 figures, accepted for publication in MNRA