602 research outputs found
Carbon-enhanced metal-poor stars: a window on AGB nucleosynthesis and binary evolution. I. Detailed analysis of 15 binary stars with known orbital periods
AGB stars are responsible for producing a variety of elements, including
carbon, nitrogen, and the heavy elements produced in the slow neutron-capture
process (-elements). There are many uncertainties involved in modelling the
evolution and nucleosynthesis of AGB stars, and this is especially the case at
low metallicity, where most of the stars with high enough masses to enter the
AGB have evolved to become white dwarfs and can no longer be observed. The
stellar population in the Galactic halo is of low mass () and only a few observed stars have evolved beyond the first
giant branch. However, we have evidence that low-metallicity AGB stars in
binary systems have interacted with their low-mass secondary companions in the
past. The aim of this work is to investigate AGB nucleosynthesis at low
metallicity by studying the surface abundances of chemically peculiar very
metal-poor stars of the halo observed in binary systems. To this end we select
a sample of 15 carbon- and -element-enhanced metal-poor (CEMP-) halo
stars that are found in binary systems with measured orbital periods. With our
model of binary evolution and AGB nucleosynthesis, we determine the binary
configuration that best reproduces, at the same time, the observed orbital
period and surface abundances of each star of the sample. The observed periods
provide tight constraints on our model of wind mass transfer in binary stars,
while the comparison with the observed abundances tests our model of AGB
nucleosynthesis.Comment: 18 pages, 20 figures, accepted for publication on A&
Carbon-enhanced metal-poor stars: a window on AGB nucleosynthesis and binary evolution. II. Statistical analysis of a sample of 67 CEMP- stars
Many observed CEMP stars are found in binary systems and show enhanced
abundances of -elements. The origin of the chemical abundances of these
CEMP- stars is believed to be accretion in the past of enriched material
from a primary star in the AGB phase. We investigate the mechanism of mass
transfer and the process of nucleosynthesis in low-metallicity AGB stars by
modelling the binary systems in which the observed CEMP- stars were formed.
For this purpose we compare a sample of CEMP- stars with a grid of
binary stars generated by our binary evolution and nucleosynthesis model. We
classify our sample CEMP- stars in three groups based on the observed
abundance of europium. In CEMP stars the europium-to-iron ratio is more
than ten times higher than in the Sun, whereas it is lower than this threshold
in CEMP stars. No measurement of europium is currently available for
CEMP- stars. On average our models reproduce well the abundances observed
in CEMP- stars, whereas in CEMP- stars and CEMP- stars the
abundances of the light- elements are systematically overpredicted by our
models and in CEMP- stars the abundances of the heavy- elements are
underestimated. In all stars our modelled abundances of sodium overestimate the
observations. This discrepancy is reduced only in models that underestimate the
abundances of most of the -elements. Furthermore, the abundance of lead is
underpredicted in most of our model stars. These results point to the
limitations of our AGB nucleosynthesis model, particularly in the predictions
of the element-to-element ratios. Finally, in our models CEMP- stars are
typically formed in wide systems with periods above 10000 days, while most of
the observed CEMP- stars are found in relatively close orbits with periods
below 5000 days.Comment: 23 pages, 8 figures, accepted for publication on Astronomy &
Astrophysic
Heavy Element Nucleosynthesis in the Brightest Galactic Asymptotic Giant Branch stars
We present updated calculations of stellar evolutionary sequences and
detailed nucleosynthesis predictions for the brightest asymptotic giant branch
(AGB) stars in the Galaxy with masses between 5Msun to 9Msun, with an initial
metallicity of Z =0.02 ([Fe/H] = 0.14). In our previous studies we used the
Vassiliadis & Wood mass-loss rate, which stays low until the pulsation period
reaches 500 days after which point a superwind begins. Vassiliadis & Wood noted
that for stars over 2.5Msun the superwind should be delayed until P ~ 750 days
at 5Msun. We calculate evolutionary sequences where we delay the onset of the
superwind to pulsation periods of P ~ 700-800 days in models of M = 5, 6, and
7Msun. Post-processing nucleosynthesis calculations show that the 6 and 7Msun
models produce the most Rb, with [Rb/Fe] ~ 1 dex, close to the average of most
of the Galactic Rb-rich stars ([Rb/Fe] ~ 1.4 plus or minus 0.8 dex). Changing
the rate of the 22Ne + alpha reactions results in variations of [Rb/Fe] as
large as 0.5 dex in models with a delayed superwind. The largest enrichment in
heavy elements is found for models that adopt the NACRE rate of the
22Ne(a,n)25Mg reaction. Using this rate allows us to best match the composition
of most of the Rb-rich stars. A synthetic evolution algorithm is then used to
remove the remaining envelope resulting in final [Rb/Fe] of ~ 1.4 dex although
with C/O ratios > 1. We conclude that delaying the superwind may account for
the large Rb overabundances observed in the brightest metal-rich AGB stars.Comment: 37 pages, accepted for publication in the Astrophysical Journal,
minor modifications to text and Tables 2 and 3, reference adde
The Curious Conundrum Regarding Sulfur Abundances In Planetary Nebulae
Sulfur abundances derived from optical emission line measurements and
ionization correction factors in planetary nebulae are systematically lower
than expected for the objects' metallicities. We have carefully considered a
large range of explanations for this "sulfur anomaly", including: (1)
correlations between the size of the sulfur deficit and numerous nebular and
central star properties; (2) ionization correction factors which under-correct
for unobserved ions; (3) effects of dielectronic recombination on the sulfur
ionization balance; (4) sequestering of S into dust and/or molecules; and (5)
excessive destruction of S or production of O by AGB stars. It appears that all
but the second scenario can be ruled out. However, we find evidence that the
sulfur deficit is generally reduced but not eliminated when S^+3 abundances
determined directly from IR measurements are used in place of the customary
sulfur ionization correction factor. We tentatively conclude that the sulfur
anomaly is caused by the inability of commonly used ICFs to properly correct
for populations of ionization stages higher than S^+2.Comment: 40 pages, 14 figures, 3 tables. Accepted for publication in the
Astrophysical Journa
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