67,921 research outputs found
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
Nebular abundances of southern symbiotic stars
We have calculated relative elemental abundances for a sample of 43 symbiotic
stars. Helium abundances and the relative elemental abundances N/O, Ne/O, Ar/O
were derived from new spectra collected in the optical range through low
dispersion spectroscopy. The He ionic abundances were derived taking into
account self-absorption effects in Balmer lines. We found that the symbiotic
stars in the galactic bulge have heavy element abundances showing the same wide
distribution as other bulge objects. In the galactic disk, the symbiotic stars
follow the abundance gradient as derived from different kinds of objects.Comment: 12 pages, 6 figures, A&A - accepte
Carbon-enhanced metal-poor stars: the most pristine objects?
Carbon-enhanced metal poor stars (CEMP) form a significant proportion of the
metal-poor stars, their origin is not well understood. Three very metal-poor
C-rich turnoff stars were selected from the SDSS survey, observed with the ESO
VLT (UVES) to precisely determine the element abundances. In turnoff stars
(unlike giants) the carbon abundance has not been affected by mixing with deep
layers and is therefore easier to interpret. The analysis was performed with 1D
LTE static model atmospheres. When available, non-LTE corrections were applied
to the classical LTE abundances. The 3D effects on the CH and CN molecular
bands were computed using hydrodynamical simulations of the stellar atmosphere
(CO5BOLD) and are found to be very important. To facilitate a comparison with
previous results, only 1D abundances are used in the discussion. The abundances
(or upper limits) of the elements enable us to place these stars in different
CEMP classes. The carbon abundances confirm the existence of a plateau at A(C)=
8.25 for [Fe/H] \geq -3.4. The most metal-poor stars ([Fe/H] < -3.4) have
significantly lower carbon abundances, suggesting a lower plateau at A(C)
\approx 6.5. Detailed analyses of a larger sample of very low metallicity
carbon-rich stars are required to confirm (or refute) this possible second
plateau and specify the behavior of the CEMP stars at very low metallicity
Chemical composition of A and F dwarfs members of the Pleiades open cluster
Abundances of 18 chemical elements have been derived for 16 A (normal and
chemically peculiar CP) and 5 F dwarfs members of the Pleiades open cluster in
order to set constraints on evolutionary models. The abundances, rotational
velocities and microturbulent velocities were derived by iteratively adjusting
synthetic spectra to observations at high resolution (R~42000 and R~75000) and
high signal-to-noise (S/N) ratios. The abundances obtained do not exhibit any
clear correlation with the effective temperature nor the projected rotational
velocity. Interestingly, A stars exhibit larger star-to-star variations in C,
Sc, Ti, V, Cr, Mn, Sr, Y, Zr and Ba than F stars. F stars exhibit solar
abundances for almost all the elements. In A stars, the abundances of Si, Ti
and Cr are found to be correlated with that of Fe, the [X/Fe] ratios being
solar for these three elements. The derived abundances have been compared to
the predictions of published evolutionary models at the age of Pleiades (100
Myr). For the F stars, the predicted slight underabundances of light elements
and overabundances of Cr, Fe and Ni are indeed confirmed by our findings. For A
stars, the predicted overabundances in iron peak elements are confirmed in a
few stars only. The large scatter of the abundances in A stars, already found
in the Hyades, Coma Berenices and the UMa group and in field stars appears to
be a characteristic property of dwarf A stars. The occurence of hydrodynamical
processes competing with radiative diffusion in the radiative zones of the A
dwarfs might account for the found scatter in abundances.Comment: 7 pages, 3 figures, accepted in A&
A holistic approach to carbon-enhanced metal-poor stars
By considering the various CEMP subclasses separately, we try to derive, from
the specific signatures imprinted on the abundances, parameters (such as
metallicity, mass, temperature, and neutron source) characterizing AGB
nucleosynthesis from the specific signatures imprinted on the abundances, and
separate them from the impact of thermohaline mixing, first dredge-up, and
dilution associated with the mass transfer from the companion.To put CEMP stars
in a broad context, we collect abundances for about 180 stars of various
metallicities, luminosity classes, and abundance patterns, from our own sample
and from literature. First, we show that there are CEMP stars which share the
properties of CEMP-s stars and CEMP-no stars (which we call CEMP-low-s stars).
We also show that there is a strong correlation between Ba and C abundances in
the s-only CEMP stars. This strongly points at the operation of the 13C neutron
source in low-mass AGB stars. For the CEMP-rs stars (seemingly enriched with
elements from both the s- and r-processes), the correlation of the N abundances
with abundances of heavy elements from the 2nd and 3rd s-process peaks bears
instead the signature of the 22Ne neutron source. Adding the fact that CEMP-rs
stars exhibit O and Mg enhancements, we conclude that extremely hot conditions
prevailed during the thermal pulses of the contaminating AGB stars. Finally, we
argue that most CEMP-no stars (with no overabundances for the neutron-capture
elements) are likely the extremely metal-poor counterparts of CEMP
neutron-capture-rich stars. We also show that the C enhancement in CEMP-no
stars declines with metallicity at extremely low metallicity ([Fe/H]~< -3.2).
This trend is not predicted by any of the current AGB models.Comment: 27 pages, 24 figures, accepted for publication in A&
The abundance of HCN in circumstellar envelopes of AGB stars of different chemical types
A multi-transition survey of HCN (sub-) millimeter line emission from a large
sample of AGB stars of different chemical type is presented. The data are
analysed and circumstellar HCN abundances are estimated. The sample stars span
a large range of properties such as mass-loss rate and photospheric C/O-ratio.
The analysis of the new data allows for more accurate estimates of the
circumstellar HCN abundances and puts new constraints on chemical models. In
order to constrain the circumstellar HCN abundance distribution a detailed
non-LTE excitation analysis, based on the Monte Carlo method, is performed.
Effects of line overlaps and radiative excitation from dust grains are
included. The median values for the derived abundances of HCN (with respect to
H2) are 3x10-5, 7x10-7 and 10-7 for carbon stars (25 stars), S-type AGB stars
(19 stars) and M-type AGB stars (25 stars), respectively. The estimated sizes
of the HCN envelopes are similar to those obtained in the case of SiO for the
same sample of sources and agree well with previous results from
interferometric observations, when these are available. We find that there is a
clear dependence of the derived circumstellar HCN abundance on the C/O-ratio of
the star, in that carbon stars have about two orders of magnitude higher
abundances than M-type AGB stars, on average. The derived HCN abundances of the
S-type AGB stars have a larger spread and typically fall in between those of
the two other types, however, slightly closer to the values for the M-type AGB
stars. For the M-type stars, the estimated abundances are much higher than what
would be expected if HCN is formed in thermal equilibrium. However, the results
are also in contrast to predictions from recent non-LTE chemical models, where
very little difference is expected in the HCN abundances between the various
types of AGB stars.Comment: Accepted for publication in A&
Detailed Analysis of Nearby Bulgelike Dwarf Stars II. Lithium Abundances
Li abundances are derived for a sample of bulgelike stars with isochronal
ages of 10-11 Gyr. These stars have orbits with pericentric distances, Rp, as
small as 2-3 kpc and Zmax < 1 kpc. The sample comprises G and K dwarf stars in
the metallicity range -0.80<[Fe/H]< +0.40. Few data of Li abundances in old
turn-off stars (> 4.5 Gyr) within the present metallicity range are available.
M67 (4.7 Gyr) and NGC 188 (6 Gyr) are the oldest studied metal-rich open
clusters with late-type stars. Li abundances have also been studied for few
samples of old metal-rich field stars. In the present work a high dispersion in
Li abundances is found for bulgelike stars for all the metallicity range,
comparable with values in M67. The role of metallicity and age on a Li
depletion pattern is discussed. The possible connection between Li depletion
and oxygen abundance due to atmospheric opacity effects is investigated.Comment: 9 pages, 7 figure
Carbon and Oxygen abundances across the Hertzsprung gap
We derived atmospheric parameters and spectroscopic abundances for C and O
for a large sample of stars located in the Hertzsprung gap in the
Hertzsprung-Russell Diagram in order to detect chemical peculiarities and get a
comprehensive overview of the population of stars in this evolutionary state.
We have observed and analyzed high resolution spectra (R = 60 000) of 188 stars
in the mass range 2 - 5 Msun with the 2.7 m Harlan J. Smith Telescope at the
McDonald Observatory including 28 stars previously identified as Am/Ap stars.
We find that the C and O abundances of the majority of stars in the Hertzsprung
gap are in accordance with abundances derived for local lower mass dwarfs but
detect expected peculiarities for the Am/Ap stars. The C and O abundances of
stars with Teff < 6500 K are slightly lower than for the hotter objects but the
C/O ratio is constant in the analyzed temperature domain. No indication of an
alteration of the C and O abundances of the stars by mixing during the
evolution across the Hertzsprung gap could be found before the homogenization
of their atmospheres by the first dredge-up.Comment: 13 pages, 12 figures, accepted for publication in the Astrophysical
Journa
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