Presolar SiC grains of the mainstream, Y, and Z type are believed to come from carbon stars. We compared their C and Si isotopic ratios with theoretical models for the envelope compositions of AGB ...stars. Two sets of models (FRANEC and Monash) use a range of stellar masses (1.5-5 M sub( )) and metallicities, different prescriptions for mass loss, and two sets of neutron-capture cross sections for the Si isotopes. They predict that the shifts in Si isotopic ratios and the increase of super(12)C/ super(13)C in the envelope during third dredge-up are higher for higher stellar mass, lower metallicity, and lower mass-loss rate. Because the super(22)Ne neutron source dominates Si nucleosynthesis, the effect of the super(13)C source is negligible. Comparison of the model predictions with grain data confirms an AGB origin for these grains, with Y and Z grains having originated in stars with lower than solar metallicity. The Si isotopic ratios of the Z grains favor the Si cross sections by Guber et al. over those by Bao et al. The super(12)C/ super(13)C ratios of low-metallicity models are much higher than those found in Z grains, and cool bottom processing must be invoked to explain the grains' C isotopic ratios. By combining Z grain Si data with the models, we determined the evolution of the super(29)Si/ super(28)Si ratio in the Galaxy as function of metallicity Z. At Z< 0.01 this ratio rises much faster than current Galactic evolution models predict and suggests an early source of the heavy Si isotopes not considered in these models.
ABSTRACT
The abundance patterns observed in the Sun and in metal-poor stars show a clear odd-even effect. An important question is whether the odd-even effect in solar-metallicity stars is similar to ...the Sun, or if there are variations that can tell us about different chemical enrichment histories. In this work, we report for the first time observational evidence of a differential odd-even effect in the solar twin HIP 11915, relative to the solar odd-even abundance pattern. The spectra of this star were obtained with high-resolving power (140 000) and signal-to-noise ratio (∼420) using the ESPRESSO spectrograph and the VLT telescope. Thanks to the high spectral quality, we obtained extremely precise stellar parameters (σ(Teff) = 2 K, $\sigma (\rm {Fe/H})$ = 0.003 dex, and σ(log g) = 0.008 dex). We determine the chemical abundance of 20 elements (Z ≤ 39) with high precision (∼0.01 dex), which shows a strong pattern of the odd-even effect even after performing galactic chemical evolution corrections. The odd-even effect is reasonably well-reproduced by a core-collapse supernova of 13 $\rm {M_{\odot }}$ and metallicity Z = 0.001 diluted into a metal-poor gas of 1 $\rm {M_{\odot }}$. Our results indicate that HIP 11915 has an odd-even effect slightly different than the Sun, thus confirming a different supernova enrichment history.
We study with unprecedented detail the chemical composition and stellar parameters of the solar twin 18 Sco in a strictly differential sense relative to the Sun. Our study is mainly based on ...high-resolution (R ~ 110,000), high signal-to-noise ratio (800-1,000) Very Large Telescope UVES spectra, which allow us to achieve a precision of about 0.005 dex in differential abundances. The effective temperature and surface gravity of 18 Sco are T sub(eff) = 5823 + or - 6 K and log g = 4.45 + or - 0.02 dex, i.e., 18 Sco is 46 + or - 6 K hotter than the Sun and log g is 0.01 + or - 0.02 dex higher. Its metallicity is Fe/H = 0.054 + or - 0.005 dex, and its microturbulence velocity is +0.02 + or - 0.01 km s super(-1) higher than solar. Our precise stellar parameters and differential isochrone analysis show that 18 Sco has a mass of 1.04 + or - 0.02 M sub(odot) and that it is ~1.6 Gyr younger than the Sun. We use precise High Accuracy Radial velocity Planet Searcher (HARPS) radial velocities to search for planets, but none are detected. The chemical abundance pattern of 18 Sco displays a clear trend with condensation temperature, thus showing higher abundances of refractories in 18 Sco than in the Sun. Intriguingly, there are enhancements in the neutron-capture elements relative to the Sun. Despite the small element-to-element abundance differences among nearby n-capture elements (~0.02 dex), we successfully reproduce the r-process pattern in the Solar System. This is independent evidence for the universality of the r process. Our results have important implications for chemical tagging in our Galaxy and nucleosynthesis in general.
A new synthetic model for asymptotic giant branch stars Izzard, Robert G.; Tout, Christopher A.; Karakas, Amanda I. ...
Monthly notices of the Royal Astronomical Society,
11 May 2004, Letnik:
350, Številka:
2
Journal Article
Recenzirano
Odprti dostop
ABSTRACT
We present a synthetic model for thermally pulsing asymptotic giant branch (TPAGB) evolution constructed by fitting expressions to full evolutionary models in the metallicity range 0.0001 ...≤Z≤ 0.02. Our model includes parametrizations of third dredge‐up and hot‐bottom burning with mass and metallicity. The Large Magellanic Cloud and Small Magellanic Cloud carbon star luminosity functions are used to calibrate third dredge‐up. We calculate yields appropriate for galactic chemical evolution models for 1H, 4He, 12C, 13C, 14N, 15N, 16O and 17O. The initial–final mass relation is examined for our stars and found to fit to within 0.1 M⊙ of the observations. We also reproduce well the white dwarf mass function for masses above about 0.58 M⊙. The new model is to be implemented in a rapid binary star evolution code.
We present a detailed abundance analysis of 23 elements for a newly discovered carbon-enhanced metal-poor (CEMP) star, HE 0414-0343, from the Chemical Abundances of Stars in the Halo Project. Its ...spectroscopic stellar parameters are T sub(eff) = 4863 K, log g = 1.25, xi = 2.20 km s super(-1), and Fe/H = -2.24. Radial velocity measurements covering seven years indicate HE 0414-0343 to be a binary. HE 0414-0343 has C/Fe = 1.44 and is strongly enhanced in neutron-capture elements but its abundances cannot be reproduced by a solar-type s-process pattern alone. Traditionally, it could be classified as a "CEMP-r/s" star. Based on abundance comparisons with asymptotic giant branch (AGB) star nucleosynthesis models, we suggest a new physically motivated origin and classification scheme for CEMP-s stars and the still poorly understood CEMP-r/s. The new scheme describes a continuous transition between these two so-far distinctly treated subgroups: CEMP-sA, CEMP-sB, and CEMP-sC. Possible causes for a continuous transition include the number of thermal pulses the AGB companion underwent, the effect of different AGB star masses on their nucleosynthetic yields, and physics that is not well approximated in 1D stellar models such as proton ingestion episodes and rotation. Based on a set of detailed AGB models, we suggest the abundance signature of HE 0414-0343 to have arisen from a >1.3 M sub(middot in circle) mass AGB star and a late-time mass transfer that transformed HE 0414-0343 into a CEMP-sC star. We also find that the Y/Ba ratio well parametrizes the classification and can thus be used to easily classify any future such stars.
Extremely metal-poor (EMP) stars are an integral piece in the puzzle that is the early universe, and although anomolous subclasses of EMP stars such as carbon-enhanced metal-poor (CEMP) stars are ...well studied, they make up less than half of all EMP stars with Fe/H ∼ −3.0. The amount of carbon depletion occurring on the red giant branch (carbon offset) is used to determine the evolutionary status of EMP stars, and this offset will differ between CEMP and normal EMP stars. The depletion mechanism employed in stellar models (from which carbon offsets are derived) is very important; however, the only widely available carbon offsets in the literature are derived from stellar models using a thermohaline mixing mechanism that cannot simultaneously match carbon and lithium abundances to observations for a single diffusion coefficient. Our stellar evolution models utilize a modified thermohaline mixing model that can match carbon and lithium in the metal-poor globular cluster NGC 6397. We compare our models to the bulk of the EMP star sample at Fe/H = −3 and show that our modified models follow the trend of the observations and deplete less carbon compared to the standard thermohaline mixing theory. We conclude that stellar models that employ the standard thermohaline mixing formalism overestimate carbon offsets and hence CEMP star frequencies, particularly at metallicities where carbon-normal stars dominate the EMP star population.
Abstract
We present a high-resolution (
R
∼ 35,000), high signal-to-noise ratio (S/N > 200) Magellan/MIKE spectrum of the star RAVE J094921.8−161722, a bright (
V
= 11.3) metal-poor red giant star ...with Fe/H = −2.2, identified as a carbon-enhanced metal-poor (CEMP) star from the RAVE survey. We report its detailed chemical abundance signature of light fusion elements and heavy neutron-capture elements. We find J0949−1617 to be a CEMP star with
s
-process enhancement that must have formed from gas enriched by a prior
r
-process event. Light neutron-capture elements follow a low-metallicity
s
-process pattern, while the heavier neutron-capture elements above Eu follow an
r
-process pattern. The Pb abundance is high, in line with an
s
-process origin. Thorium is also detected, as expected from an
r
-process origin, as Th is not produced in the
s
-process. We employ nucleosynthesis model predictions that take an initial
r
-process enhancement into account, and then determine the mass transfer of carbon and
s
-process material from a putative more massive companion onto the observed star. The resulting abundances agree well with the observed pattern. We conclude that J0949−1617 is the first bonafide CEMP-
r
+
s
star identified. This class of objects has previously been suggested to explain stars with neutron-capture element patterns that originate from neither the
r
- nor the
s
-process alone. We speculate that J0949−1617 formed in an environment similar to those of ultra-faint dwarf galaxies like Tucana III and Reticulum II, which were enriched in
r
-process elements by one or multiple neutron star mergers at the earliest times.
We present a comprehensive abundance analysis of two newly discovered carbon-enhanced metal-poor (CEMP) stars. HE 2138-3336 is a s-process-rich star with Fe/H = -2.79, and has the highest Pb/Fe ...abundance ratio measured thus far, if non-local thermodynamic equilibrium corrections are included (Pb/Fe = +3.84). HE 2258-6358, with Fe/H = -2.67, exhibits enrichments in both s- and r-process elements. These stars were selected from a sample of candidate metal-poor stars from the Hamburg/ESO objective-prism survey, and followed up with medium-resolution (R ~ 2000) spectroscopy with GEMINI/GMOS. We report here on derived abundances (or limits) for a total of 34 elements in each star, based on high-resolution (R ~ 30,000) spectroscopy obtained with Magellan-Clay/MIKE. Our results are compared to predictions from new theoretical asymptotic giant branch (AGB) nucleosynthesis models of 1.3 M sub(middot in circle) with Fe/H = -2.5 and -2.8, as well as to a set of AGB models of 1.0 to 6.0 M sub(middot in circle) at Fe/H = -2.3. The agreement with the model predictions suggests that the neutron-capture material in HE 2138-3336 originated from mass transfer from a binary companion star that previously went through the AGB phase, whereas for HE 2258-6358, an additional process has to be taken into account to explain its abundance pattern. We find that a narrow range of progenitor masses (1.0 < or =, slant M(M sub(middot in circle)) < or =, slant 1.3) and metallicities (-2.8 < or =, slant Fe/H < or =, slant -2.5) yield the best agreement with our observed elemental abundance patterns.
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