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Amanda I Karakas - One of the best experts on this subject based on the ideXlab platform.
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evolution and nucleosynthesis of extremely metal poor and metal free low and intermediate mass stars ii s process nucleosynthesis during the core he flash
Astronomy and Astrophysics, 2010Co-Authors: Simon Campbell, Maria Lugaro, Amanda I KarakasAbstract:Context. Models of primordial and hyper-metal-poor stars that have masses similar to the Sun are known to experience an ingestion of protons into the hot core during the core helium flash phase at the end of their red giant branch evolution. This produces a concurrent secondary flash powered by hydrogen burning that gives rise to further nucleosynthesis in the core. Aims. We aim to model the nucleosynthesis occurring during the proton ingestion event to ascertain if any significant neutron-capture nucleosynthesis occurs. Methods. We perform post-process nucleosynthesis calculations on a one-dimensional stellar evolution calculation of a star with mass 1 M ⊙ and a metallicity of [Fe/H] = -6.5 that suffers a proton ingestion episode. Our network includes 320 nuclear species and 2366 reactions and treats mixing and burning simultaneously. Results. We find that the mixing and burning of protons into the hot convective core leads to the production of 13 C, which then burns via the 13 C(α, n) 16 O reaction, releasing a large number of free neutrons. During the first two years of neutron production the neutron poison 14 N abundance is low, allowing the prodigious production of heavy elements such as strontium, barium, and lead via slow neutron captures (the s process). These nucleosynthetic products are later carried to the stellar surface and ejected via stellar winds. We compare our results with observations of the hyper-metal-poor halo star HE 1327-2326, which shows a strong Sr Overabundance. Conclusions. Our model provides the possibility of self-consistently explaining the Sr Overabundance in HE 1327-2326 together with its C, N, and O Overabundances (all within a factor of ~4) if the material were heavily diluted, for example, via mass transfer in a wide binary system. The model produces at least 18 times too much Ba than observed, but this may be within the large modelling uncertainties. In this scenario, binary systems of low mass must have formed in the early Universe. If this is true, it puts constraints on the primordial initial mass function.
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evolution and nucleosynthesis of extremely metal poor and metal free low and intermediate mass stars ii s process nucleosynthesis during the core he flash
arXiv: Solar and Stellar Astrophysics, 2010Co-Authors: Simon Campbell, Maria Lugaro, Amanda I KarakasAbstract:Models of primordial and hyper-metal-poor stars with masses similar to the Sun experience an ingestion of protons into the hot core during the core helium flash phase at the end of their red giant branch evolution. This produces a concurrent secondary flash powered by hydrogen burning that gives rise to further nucleosynthesis in the core. We perform post-process nucleosynthesis calculations on a one-dimensional stellar evolution calculation of a star of 1 solar mass and metallicity [Fe/H] = -6.5 that suffers a proton ingestion episode. Our network includes 320 nuclear species and 2,366 reactions and treats mixing and burning simultaneously. The mixing and burning of protons into the hot convective core leads to the production of 13C, which then burns via the 13C(alpha,n)16O reaction releasing a large number of free neutrons. During the first two years of neutron production the neutron poison 14N abundance is low, allowing the prodigious production of heavy elements such as strontium, barium, and lead via slow neutron captures (the s process). These nucleosynthetic products are later mixed to the stellar surface and ejected via stellar winds. We compare our results with observations of the hyper-metal-poor halo star HE 1327-2326, which shows a strong Sr Overabundance. Our model provides the possibility of self-consistently explaining the Sr Overabundance in HE 1327-2326 together with its C, N, and O Overabundances (all within a factor of ~4) if the material were heavily diluted, for example, via mass transfer in a wide binary system. The model produces at least 18 times too much Ba than observed, but this may be within the large modelling uncertainties. In this scenario, binary systems of low mass must have formed in the early Universe. If true then this puts constraints on the primordial initial mass function.
Simon Campbell - One of the best experts on this subject based on the ideXlab platform.
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evolution and nucleosynthesis of extremely metal poor and metal free low and intermediate mass stars ii s process nucleosynthesis during the core he flash
Astronomy and Astrophysics, 2010Co-Authors: Simon Campbell, Maria Lugaro, Amanda I KarakasAbstract:Context. Models of primordial and hyper-metal-poor stars that have masses similar to the Sun are known to experience an ingestion of protons into the hot core during the core helium flash phase at the end of their red giant branch evolution. This produces a concurrent secondary flash powered by hydrogen burning that gives rise to further nucleosynthesis in the core. Aims. We aim to model the nucleosynthesis occurring during the proton ingestion event to ascertain if any significant neutron-capture nucleosynthesis occurs. Methods. We perform post-process nucleosynthesis calculations on a one-dimensional stellar evolution calculation of a star with mass 1 M ⊙ and a metallicity of [Fe/H] = -6.5 that suffers a proton ingestion episode. Our network includes 320 nuclear species and 2366 reactions and treats mixing and burning simultaneously. Results. We find that the mixing and burning of protons into the hot convective core leads to the production of 13 C, which then burns via the 13 C(α, n) 16 O reaction, releasing a large number of free neutrons. During the first two years of neutron production the neutron poison 14 N abundance is low, allowing the prodigious production of heavy elements such as strontium, barium, and lead via slow neutron captures (the s process). These nucleosynthetic products are later carried to the stellar surface and ejected via stellar winds. We compare our results with observations of the hyper-metal-poor halo star HE 1327-2326, which shows a strong Sr Overabundance. Conclusions. Our model provides the possibility of self-consistently explaining the Sr Overabundance in HE 1327-2326 together with its C, N, and O Overabundances (all within a factor of ~4) if the material were heavily diluted, for example, via mass transfer in a wide binary system. The model produces at least 18 times too much Ba than observed, but this may be within the large modelling uncertainties. In this scenario, binary systems of low mass must have formed in the early Universe. If this is true, it puts constraints on the primordial initial mass function.
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evolution and nucleosynthesis of extremely metal poor and metal free low and intermediate mass stars ii s process nucleosynthesis during the core he flash
arXiv: Solar and Stellar Astrophysics, 2010Co-Authors: Simon Campbell, Maria Lugaro, Amanda I KarakasAbstract:Models of primordial and hyper-metal-poor stars with masses similar to the Sun experience an ingestion of protons into the hot core during the core helium flash phase at the end of their red giant branch evolution. This produces a concurrent secondary flash powered by hydrogen burning that gives rise to further nucleosynthesis in the core. We perform post-process nucleosynthesis calculations on a one-dimensional stellar evolution calculation of a star of 1 solar mass and metallicity [Fe/H] = -6.5 that suffers a proton ingestion episode. Our network includes 320 nuclear species and 2,366 reactions and treats mixing and burning simultaneously. The mixing and burning of protons into the hot convective core leads to the production of 13C, which then burns via the 13C(alpha,n)16O reaction releasing a large number of free neutrons. During the first two years of neutron production the neutron poison 14N abundance is low, allowing the prodigious production of heavy elements such as strontium, barium, and lead via slow neutron captures (the s process). These nucleosynthetic products are later mixed to the stellar surface and ejected via stellar winds. We compare our results with observations of the hyper-metal-poor halo star HE 1327-2326, which shows a strong Sr Overabundance. Our model provides the possibility of self-consistently explaining the Sr Overabundance in HE 1327-2326 together with its C, N, and O Overabundances (all within a factor of ~4) if the material were heavily diluted, for example, via mass transfer in a wide binary system. The model produces at least 18 times too much Ba than observed, but this may be within the large modelling uncertainties. In this scenario, binary systems of low mass must have formed in the early Universe. If true then this puts constraints on the primordial initial mass function.
Karin Lind - One of the best experts on this subject based on the ideXlab platform.
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the gaia eso survey sodium and aluminium abundances in giants and dwarfs implications for stellar and galactic chemical evolution
Astronomy and Astrophysics, 2016Co-Authors: R Smiljanic, P Ventura, D Romano, A Bragaglia, P Donati, L Magrini, Eileen D Friel, Heather R Jacobson, S Randich, Karin LindAbstract:Context. Stellar evolution models predict that internal mixing should cause some sodium Overabundance at the surface of red giants more massive than similar to 1.5-2.0 M-circle dot. The surface alu ...
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the gaia eso survey sodium and aluminium abundances in giants and dwarfs implications for stellar and galactic chemical evolution
arXiv: Solar and Stellar Astrophysics, 2016Co-Authors: R Smiljanic, P Ventura, D Romano, A Bragaglia, P Donati, L Magrini, Eileen D Friel, Heather R Jacobson, S Randich, Karin LindAbstract:Stellar evolution models predict that internal mixing should cause some sodium Overabundance at the surface of red giants more massive than ~ 1.5--2.0 Msun. The surface aluminium abundance should not be affected. Nevertheless, observational results disagree about the presence and/or the degree of the Na and Al Overabundances. In addition, Galactic chemical evolution models adopting different stellar yields lead to quite different predictions for the behavior of [Na/Fe] and [Al/Fe] versus [Fe/H]. Overall, the observed trends of these abundances with metallicity are not well reproduced. We readdress both issues, using new Na and Al abundances determined within the Gaia-ESO Survey, using two samples: i) more than 600 dwarfs of the solar neighborhood and of open clusters and ii) low- and intermediate-mass clump giants in six open clusters. Abundances of Na in giants with mass below ~2.0 Msun, and of Al in giants below ~3.0 Msun, seem to be unaffected by internal mixing processes. For more massive giants, the Na Overabundance increases with stellar mass. This trend agrees well with predictions of stellar evolutionary models. Chemical evolution models that are able to fit well the observed [Na/Fe] vs. [Fe/H] trend in solar neighborhood dwarfs can not simultaneously explain the run of [Al/Fe] with [Fe/H], and viceversa. The comparison with stellar ages is hampered by severe uncertainties. Indeed, reliable age estimates are available for only a half of the stars of the sample. We conclude that Al is underproduced by the models, except for stellar ages younger than about 7 Gyr. In addition, some significant source of late Na production seems to be missing in the models. Either current Na and Al yields are affected by large uncertainties, and/or some important Galactic source(s) of these elements has not been taken into account up to now. [abridged]
M Laine - One of the best experts on this subject based on the ideXlab platform.
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heavy quark chemical equilibration rate as a transport coefficient
Journal of High Energy Physics, 2012Co-Authors: Dietrich Bodeker, M LaineAbstract:Motivated by indications that heavy (charm and bottom) quarks interact strongly at temperatures generated in heavy ion collision experiments, we suggest a non- perturbative definition of a heavy quark chemical equilibration rate as a transport coefficient. Within leading-order perturbation theory (corresponding to 3-loop level), the definition is argued to reduce to an expression obtained from the Boltzmann equation. Around T ~ 400 MeV, an order-of-magnitude estimate for charm yields a rate $\Gamma_{\text{chem}}^{ - 1} \gtrsim {6}0{{\text{fm}} \left/ {\text{c}} \right.}$ which remains too slow to play a practical role in current experiments. However, the rate increases rapidly with T and, due to non-linear effects, also if the initial state contains an Overabundance of heavy quarks.
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heavy quark chemical equilibration rate as a transport coefficient
arXiv: High Energy Physics - Phenomenology, 2012Co-Authors: Dietrich Bodeker, M LaineAbstract:Motivated by indications that heavy (charm and bottom) quarks interact strongly at temperatures generated in heavy ion collision experiments, we suggest a non-perturbative definition of a heavy quark chemical equilibration rate as a transport coefficient. Within leading-order perturbation theory (corresponding to 3-loop level), the definition is argued to reduce to an expression obtained from the Boltzmann equation. Around T ~ 400 MeV, an order-of-magnitude estimate for charm yields a rate Gamma^{-1}_{chem} > 60 fm/c which remains too slow to play a practical role in current experiments. However, the rate increases rapidly with T and, due to non-linear effects, also if the initial state contains an Overabundance of heavy quarks.
Timothy C Beers - One of the best experts on this subject based on the ideXlab platform.
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chemical cartography i a carbonicity map of the galactic halo
The Astrophysical Journal, 2017Co-Authors: Young Sun Lee, Timothy C Beers, Young Kwang Kim, Vinicius M Placco, Jinmi Yoon, D Carollo, T Masseron, Jaehun JungAbstract:We present the first map of carbonicity, [C/Fe], for the halo system of the Milky Way, based on a sample of over 100,000 main-sequence turnoff stars with available spectroscopy from the Sloan Digital Sky Survey. This map, which explores distances up to 15 kpc from the Sun, reveals clear evidence for the dual nature of the Galactic halo, based on the spatial distribution of stellar carbonicity. The metallicity distribution functions of stars in the inner- and outer-halo regions of the carbonicity map reproduce those previously argued to arise from contributions of the inner- and outer-halo populations, with peaks at [Fe/H] = -1.5 and -2.2, respectively. From consideration of the absolute carbon abundances for our sample, A(C), we also confirm that the carbon-enhanced metal-poor (CEMP) stars in the outer-halo region exhibit a higher frequency of CEMP-no stars (those with no Overabundances of heavy neutron-capture elements) than of CEMP-s stars (those with strong Overabundances of elements associated with the s-process), whereas the stars in the inner-halo region exhibit a higher frequency of CEMP-s stars. We argue that the contrast in the behavior of the CEMP-no and CEMP-s fractions in these regions arises from differences in the mass distributions of the mini-halos from which the stars of the inner- and outer-halo populations formed, which gives rise in turn to the observed dichotomy of the Galactic halo.
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first stars iii a detailed elemental abundance study of four extremely metal poor giant stars
Astronomy and Astrophysics, 2003Co-Authors: P Francois, E Depagne, V Hill, M Spite, F Spite, B Plez, Timothy C Beers, B Barbuy, R Cayrel, J AndersenAbstract:This paper reports detailed abundance analyses for four extremely metal-poor (XMP) giant stars with (Fe=H)< 3:8, based on high-resolution, high-S=N spectra from the ESO VLT (Kueyen/UVES) and LTE model atmosphere calculations. The derived (/Fe) ratios in our sample exhibit a small dispersion, confirming previous findings in the literature, i.e. a constant Overabundance of the-elements with a very small (if any) dependence on (Fe/H). In particular, the very small scatter we determine for (Si/Fe) suggests that this element shows a constant Overabundance at very low metallicity, a conclusion which could not have been derived from the widely scattered (Si/Fe) values reported in the literature for less metal-poor stars. For the iron-peak elements, our precise abundances for the four XMP stars in our sample confirm the decreasing trend of Cr and Mn with decreasing (Fe/H), as well as the increasing trend for Co and the absence of any trend for Sc and Ni. In contrast to the significant spread of the ratios (Sr/Fe) and (Ba/Fe), we find (Sr/Ba) in our sample to be roughly solar, with a much lower dispersion than previously found for stars in the range 3:5< (Fe=H)< 2:5.
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the chemical composition of carbon rich very metal poor stars a new class of mildly carbon rich objects without excess of neutron capture elements
The Astrophysical Journal, 2002Co-Authors: Wako Aoki, Timothy C Beers, John E Norris, Sean G Ryan, Hiroyasu AndoAbstract:We report on an analysis of the chemical composition of —ve carbon-rich, very metal poor stars based on high-resolution spectra. One star, CS 22948-027, exhibits very large Overabundances of carbon, nitrogen, and the neutron-capture elements, as found in the previous study of Hill et al. This result can be interpreted as a consequence of mass transfer from a binary companion that previously evolved through the asymptotic giant branch stage. By way of contrast, the other four stars we investigate exhibit no Overabundances of barium ([Ba/Fe] \ 0), while three of them have mildly enhanced carbon and/or nitrogen ([C ] N] D ]1). We have been unable to determine accurate carbon and nitrogen abundances for the remaining star (CS 30312-100). These stars are rather similar to the carbon-rich, neutron-capture
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the chemical composition of carbon rich very metal poor stars a new class of mildly carbon rich objects without excess of neutron capture elements
arXiv: Astrophysics, 2001Co-Authors: Wako Aoki, Timothy C Beers, John E Norris, Sean G Ryan, Hiroyasu AndoAbstract:We report on an analysis of the chemical composition of five carbon-rich, very metal-poor stars based on high-resolution spectra. One star, CS22948-027, exhibits very large Overabundances of carbon, nitrogen, and the neutron-capture elements, as found in the previous study of Hill et al.. This result may be interpreted as a consequence of mass transfer from a binary companion that previously evolved through the asymptotic giant branch stage. By way of contrast, the other four stars we investigate exhibit no Overabundances of barium ([Ba/Fe]<0), while three of them have mildly enhanced carbon and/or nitrogen ([C+N]+1). We have been unable to determine accurate carbon and nitrogen abundances for the remaining star (CS30312-100). These stars are rather similar to the carbon-rich, neutron-capture-element-poor star CS22957-027 discussed previously by Norris et al., though the carbon Overabundance in this object is significantly larger ([C/Fe]=+2.2). Our results imply that these carbon-rich objects with ``normal'' neutron-capture element abundances are not rare among very metal-deficient stars. One possible process to explain this phenomenon is as a result of helium shell flashes near the base of the AGB in very low-metallicity, low-mass (M~< 1M_sun) stars, as recently proposed by Fujimoto et al.. The moderate carbon enhancements reported herein ([C/Fe]+1) are similar to those reported in the famous r-process-enhanced star CS22892-052. We discuss the possibility that the same process might be responsible for this similarity, as well as the implication that a completely independent phenomenon was responsible for the large r-process enhancement in CS22892-052.