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Anna Frebel - One of the best experts on this subject based on the ideXlab platform.
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neutron star mergers might not be the only source of r Process elements in the milky way
The Astrophysical Journal, 2019Co-Authors: Anna Frebel, Benoit Cote, Marius Eichler, Almudena Arcones, C J Hansen, Paolo Simonetti, Chris L FryerAbstract:Probing the origin of r-Process elements in the universe represents a multidisciplinary challenge. We review the observational evidence that probes the properties of r-Process sites, and address them using galactic chemical evolution simulations, binary population synthesis models, and nucleosynthesis calculations. Our motivation is to define which astrophysical sites have significantly contributed to the total mass of r-Process elements present in our Galaxy. We found discrepancies with the neutron star (NS–NS) merger scenario. When we assume that they are the only site, the decreasing trend of [Eu/Fe] at [Fe/H] > −1 in the disk of the Milky Way cannot be reproduced while accounting for the delay-time distribution (DTD) of coalescence times (∝t −1) derived from short gamma-ray bursts (GRBs) and population synthesis models. Steeper DTD functions (∝t −1.5) or power laws combined with a strong burst of mergers before the onset of supernovae (SNe) Ia can reproduce the [Eu/Fe] trend, but this scenario is inconsistent with the similar fraction of short GRBs and SNe Ia occurring in early-type galaxies, and it reduces the probability of detecting GW170817 in an early-type galaxy. One solution is to assume an additional production site of Eu that would be active in the early universe, but would fade away with increasing metallicity. If this is correct, this additional site could be responsible for roughly 50% of the Eu production in the early universe before the onset of SNe Ia. Rare classes of supernovae could be this additional r-Process source, but hydrodynamic simulations still need to ensure the conditions for a robust r-Process Pattern.
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neutron star mergers might not be the only source of r Process elements in the milky way
arXiv: High Energy Astrophysical Phenomena, 2018Co-Authors: Anna Frebel, Benoit Cote, Marius Eichler, Almudena Arcones, C J Hansen, Paolo Simonetti, Chris L FryerAbstract:Probing the origin of r-Process elements in the universe represents a multi-disciplinary challenge. We review the observational evidence that probe the properties of r-Process sites, and address them using galactic chemical evolution simulations, binary population synthesis models, and nucleosynthesis calculations. Our motivation is to define which astrophysical sites have significantly contributed to the total mass of r-Process elements present in our Galaxy. We found discrepancies with the neutron star (NS-NS) merger scenario. Assuming they are the only site, the decreasing trend of [Eu/Fe] at [Fe/H]\,$>-1$ in the disk of the Milky Way cannot be reproduced while accounting for the delay-time distribution (DTD) of coalescence times ($\propto~t^{-1}$) derived from short gamma-ray bursts and population synthesis models. Steeper DTD functions ($\propto~t^{-1.5}$) or power laws combined with a strong burst of mergers before the onset of Type~Ia supernovae can reproduce the [Eu/Fe] trend, but this scenario is inconsistent with the similar fraction of short gamma-ray bursts and Type~Ia supernovae occurring in early-type galaxies, and reduces the probability of detecting GW170817 in an early-type galaxy. One solution is to assume an extra production site of Eu that would be active in the early universe, but would fade away with increasing metallicity. If this is correct, this extra site could be responsible for roughly 50% of the Eu production in the early universe, before the onset of Type~Ia supernovae. Rare classes of supernovae could be this additional r-Process source, but hydrodynamic simulations still need to ensure the conditions for a robust r-Process Pattern.
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the r Process alliance chemical abundances for a trio of r Process enhanced stars one strong one moderate and one mild
The Astrophysical Journal, 2018Co-Authors: Madelyn Cain, Anna Frebel, Timothy C Beers, Maude Gull, Alexander P Ji, Vinicius M Placco, Jorge MelendezAbstract:We present detailed chemical abundances of three new bright (V ~ 11), extremely metal-poor ([Fe/H] ~ -3.0), r-Process-enhanced halo red giants based on high-resolution, high-S/N Magellan/MIKE spectra. We measured abundances for 20-25 neutron-capture elements in each of our stars. J1432-4125 is among the most r-Process rich r-II stars, with [Eu/Fe]= +1.44+-0.11. J2005-3057 is an r-I star with [Eu/Fe] = +0.94+-0.07. J0858-0809 has [Eu/Fe] = +0.23+-0.05 and exhibits a carbon abundance corrected for evolutionary status of [C/Fe]_corr = +0.76, thus adding to the small number of known carbon-enhanced r-Process stars. All three stars show remarkable agreement with the scaled solar r-Process Pattern for elements above Ba, consistent with enrichment of the birth gas cloud by a neutron star merger. The abundances for Sr, Y, and Zr, however, deviate from the scaled solar Pattern. This indicates that more than one distinct r-Process site might be responsible for the observed neutron-capture element abundance Pattern. Thorium was detected in J1432-4125 and J2005-3057. Age estimates for J1432-4125 and J2005-3057 were adopted from one of two sets of initial production ratios each by assuming the stars are old. This yielded individual ages of 12+-6 Gyr and 10+-6 Gyr, respectively.
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r Process enrichment from a single event in an ancient dwarf galaxy
Nature, 2016Co-Authors: Anna Frebel, Anirudh Chiti, Joshua D SimonAbstract:Elements heavier than zinc are synthesized through the rapid (r) and slow (s) neutron-capture Processes. The main site of production of the r-Process elements (such as europium) has been debated for nearly 60 years. Initial studies of trends in chemical abundances in old Milky Way halo stars suggested that these elements are produced continually, in sites such as core-collapse supernovae. But evidence from the local Universe favours the idea that r-Process production occurs mainly during rare events, such as neutron star mergers. The appearance of a plateau of europium abundance in some dwarf spheroidal galaxies has been suggested as evidence for rare r-Process enrichment in the early Universe, but only under the assumption that no gas accretes into those dwarf galaxies; gas accretion favours continual r-Process enrichment in these systems. Furthermore, the universal r-Process Pattern has not been cleanly identified in dwarf spheroidals. The smaller, chemically simpler, and more ancient ultrafaint dwarf galaxies assembled shortly after the first stars formed, and are ideal systems with which to study nucleosynthesis events such as the r-Process. Reticulum II is one such galaxy. The abundances of non-neutron-capture elements in this galaxy (and others like it) are similar to those in other old stars. Here, we report that seven of the nine brightest stars in Reticulum II, observed with high-resolution spectroscopy, show strong enhancements in heavy neutron-capture elements, with abundances that follow the universal r-Process Pattern beyond barium. The enhancement seen in this 'r-Process galaxy' is two to three orders of magnitude higher than that detected in any other ultrafaint dwarf galaxy. This implies that a single, rare event produced the r-Process material in Reticulum II. The r-Process yield and event rate are incompatible with the source being ordinary core-collapse supernovae, but consistent with other possible sources, such as neutron star mergers.
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R-Process enrichment from a single event in an ancient dwarf galaxy
Nature, 2016Co-Authors: Anna Frebel, Anirudh Chiti, Joshua D SimonAbstract:It has long been debated whether elements heavier than zinc are formed continually, for example in core-collapse supernovae, or in rare events, such as neutron star mergers; here, studies of element abundances in a local ultrafaint dwarf galaxy provide evidence that these elements are formed during rare yet prolific stellar events. Elements heavier than zinc are synthesized through the rapid (r) and slow (s) neutron-capture Processes^ 1 , 2 . The main site of production of the r-Process elements (such as europium) has been debated for nearly 60 years^ 2 . Initial studies of trends in chemical abundances in old Milky Way halo stars suggested that these elements are produced continually, in sites such as core-collapse supernovae^ 3 , 4 . But evidence from the local Universe favours the idea that r-Process production occurs mainly during rare events, such as neutron star mergers^ 5 , 6 . The appearance of a plateau of europium abundance in some dwarf spheroidal galaxies has been suggested as evidence for rare r-Process enrichment in the early Universe^ 7 , but only under the assumption that no gas accretes into those dwarf galaxies; gas accretion^ 8 favours continual r-Process enrichment in these systems. Furthermore, the universal r-Process Pattern^ 1 , 9 has not been cleanly identified in dwarf spheroidals. The smaller, chemically simpler, and more ancient ultrafaint dwarf galaxies assembled shortly after the first stars formed, and are ideal systems with which to study nucleosynthesis events such as the r-Process^ 10 , 11 . Reticulum II is one such galaxy^ 12 , 13 , 14 . The abundances of non-neutron-capture elements in this galaxy (and others like it) are similar to those in other old stars^ 15 . Here, we report that seven of the nine brightest stars in Reticulum II, observed with high-resolution spectroscopy, show strong enhancements in heavy neutron-capture elements, with abundances that follow the universal r-Process Pattern beyond barium. The enhancement seen in this ‘r-Process galaxy’ is two to three orders of magnitude higher than that detected in any other ultrafaint dwarf galaxy^ 11 , 16 , 17 . This implies that a single, rare event produced the r-Process material in Reticulum II. The r-Process yield and event rate are incompatible with the source being ordinary core-collapse supernovae^ 18 , but consistent with other possible sources, such as neutron star mergers^ 19 . The recently discovered Milky Way satellite Reticulum II, an ultra-faint dwarf galaxy, provides an ideal model for the study of stellar nucleosynthesis, the creation of heavier elements from hydrogen, helium and other lighter elements and particles. It has long been debated whether elements heavier than zinc are formed continually, for example in core-collapse supernovae, or in rare events, such as neutron star mergers. Alexander Ji et al . used high-resolution spectroscopy to determine element abundances in nine young stars in Reticulum II, and find that seven of the nine show strong enhancements in heavy neutron-capture elements with abundances that follow the universal r-Process Pattern above barium. The enhancement is several orders of magnitude greater than that seen in other ultra-faint dwarf galaxies, implying that a single rare event produced the r-Process material.
Bich Thuy Dong - One of the best experts on this subject based on the ideXlab platform.
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ICSEA - Modeling Process Patterns and Their Application
International Conference on Software Engineering Advances (ICSEA 2007), 2007Co-Authors: Hanh Nhi Tran, Bernard Coulette, Bich Thuy DongAbstract:Process Pattern is an emerging approach for Process reuse. Representing Process models based on Process Patterns to explicit Process solutions and factor recurrent Process constituents is useful for Process understanding as well as Process modeling. This subject, however, is still a challenge for the software Process technology community. In this paper, we present a UML-based Process meta-model that allows explicit representation of Process Patterns in Process models. The novel of our proposition is to enable the application of different Process knowledge kinds not only for building but also for improving Process models.
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A UML-based Process meta-model integrating a rigorous Process Patterns definition
Lecture Notes in Computer Science, 2006Co-Authors: Hanh Nhi Tran, Bernard Coulette, Bich Thuy DongAbstract:Process Pattern is an emergent approach to reuse Process knowledge. However, in practice this concept still remains difficult to be exploited due to the lack of formalization and supporting methodology. In this paper, we propose a way to formalize the Process Pattern concept by introducing it into a Process meta-model. We provide a general definition to cover various kinds of Process-related Patterns in different domains. We define rigorously Process concepts and their relations to allow representing Processes based on Process Patterns and to facilitate the development of supporting tools. By distinguishing Process Patterns at different abstraction levels, we aim to develop a systematic approach to define and apply Process Patterns.
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PROFES - A UML-based Process meta-model integrating a rigorous Process Patterns definition
Product-Focused Software Process Improvement, 2006Co-Authors: Hanh Nhi Tran, Bernard Coulette, Bich Thuy DongAbstract:Process Pattern is an emergent approach to reuse Process knowledge. However, in practice this concept still remains difficult to be exploited due to the lack of formalization and supporting methodology. In this paper, we propose a way to formalize the Process Pattern concept by introducing it into a Process meta-model. We provide a general definition to cover various kinds of Process-related Patterns in different domains. We define rigorously Process concepts and their relations to allow representing Processes based on Process Patterns and to facilitate the development of supporting tools. By distinguishing Process Patterns at different abstraction levels, we aim to develop a systematic approach to define and apply Process Patterns.
Hanh Nhi Tran - One of the best experts on this subject based on the ideXlab platform.
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SAC - Automatic reuse of Process Patterns in Process modeling
Proceedings of the 2011 ACM Symposium on Applied Computing - SAC '11, 2011Co-Authors: Hanh Nhi Tran, Bernard Coulette, Dan Thu TranAbstract:Process Pattern concept was proposed as a promising approach to reuse Process knowledge. However, their general and practical use has been handicapped by the lack of formalization and supporting tools. In order to facilitate Process Patterns reuse, we aim at an (semi)automatic approach that can assure correct Patterns' applications and reduce Process modeling time. In our previous works, we developed a UML-based meta-model allowing Pattern-based Process modeling. In this paper, we present a set of reuse operators that enable automatic applications of Process Patterns for generating and (re)structuring Process models. These operators are defined with operational semantics and implemented in a prototype of Pattern-based Process modeling tool.
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ICSEA - Modeling Process Patterns and Their Application
International Conference on Software Engineering Advances (ICSEA 2007), 2007Co-Authors: Hanh Nhi Tran, Bernard Coulette, Bich Thuy DongAbstract:Process Pattern is an emerging approach for Process reuse. Representing Process models based on Process Patterns to explicit Process solutions and factor recurrent Process constituents is useful for Process understanding as well as Process modeling. This subject, however, is still a challenge for the software Process technology community. In this paper, we present a UML-based Process meta-model that allows explicit representation of Process Patterns in Process models. The novel of our proposition is to enable the application of different Process knowledge kinds not only for building but also for improving Process models.
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SEKE - Broadening the Use of Process Patterns for Modeling Processes.
2007Co-Authors: Hanh Nhi Tran, Bernard Coulette, Dong Thi Bich ThuyAbstract:Generally, Process Patterns are considered as Patterns capturing reusable development activities, and serve as building blocks for constructing new Processes. However, such a definition is not adequate to represent the original idea of Process Patterns that aims to capture and reuse diverse Process knowledge. In this work, we broaden the definition and application of Process Patterns to take more advantage of them for different Process modeling needs. First, we formalized the Process Pattern concept so that it can capture various kinds of Process knowledge. Then, we proposed different ways for reusing Process Patterns to generate or improve Process models. These propositions were rigorously defined in a UML-based meta-model to permit describing Processes based on Process Patterns with standard notations, and to facilitate the development of supporting tools.
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A UML-based Process meta-model integrating a rigorous Process Patterns definition
Lecture Notes in Computer Science, 2006Co-Authors: Hanh Nhi Tran, Bernard Coulette, Bich Thuy DongAbstract:Process Pattern is an emergent approach to reuse Process knowledge. However, in practice this concept still remains difficult to be exploited due to the lack of formalization and supporting methodology. In this paper, we propose a way to formalize the Process Pattern concept by introducing it into a Process meta-model. We provide a general definition to cover various kinds of Process-related Patterns in different domains. We define rigorously Process concepts and their relations to allow representing Processes based on Process Patterns and to facilitate the development of supporting tools. By distinguishing Process Patterns at different abstraction levels, we aim to develop a systematic approach to define and apply Process Patterns.
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PROFES - A UML-based Process meta-model integrating a rigorous Process Patterns definition
Product-Focused Software Process Improvement, 2006Co-Authors: Hanh Nhi Tran, Bernard Coulette, Bich Thuy DongAbstract:Process Pattern is an emergent approach to reuse Process knowledge. However, in practice this concept still remains difficult to be exploited due to the lack of formalization and supporting methodology. In this paper, we propose a way to formalize the Process Pattern concept by introducing it into a Process meta-model. We provide a general definition to cover various kinds of Process-related Patterns in different domains. We define rigorously Process concepts and their relations to allow representing Processes based on Process Patterns and to facilitate the development of supporting tools. By distinguishing Process Patterns at different abstraction levels, we aim to develop a systematic approach to define and apply Process Patterns.
Joshua D Simon - One of the best experts on this subject based on the ideXlab platform.
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r Process enrichment from a single event in an ancient dwarf galaxy
Nature, 2016Co-Authors: Anna Frebel, Anirudh Chiti, Joshua D SimonAbstract:Elements heavier than zinc are synthesized through the rapid (r) and slow (s) neutron-capture Processes. The main site of production of the r-Process elements (such as europium) has been debated for nearly 60 years. Initial studies of trends in chemical abundances in old Milky Way halo stars suggested that these elements are produced continually, in sites such as core-collapse supernovae. But evidence from the local Universe favours the idea that r-Process production occurs mainly during rare events, such as neutron star mergers. The appearance of a plateau of europium abundance in some dwarf spheroidal galaxies has been suggested as evidence for rare r-Process enrichment in the early Universe, but only under the assumption that no gas accretes into those dwarf galaxies; gas accretion favours continual r-Process enrichment in these systems. Furthermore, the universal r-Process Pattern has not been cleanly identified in dwarf spheroidals. The smaller, chemically simpler, and more ancient ultrafaint dwarf galaxies assembled shortly after the first stars formed, and are ideal systems with which to study nucleosynthesis events such as the r-Process. Reticulum II is one such galaxy. The abundances of non-neutron-capture elements in this galaxy (and others like it) are similar to those in other old stars. Here, we report that seven of the nine brightest stars in Reticulum II, observed with high-resolution spectroscopy, show strong enhancements in heavy neutron-capture elements, with abundances that follow the universal r-Process Pattern beyond barium. The enhancement seen in this 'r-Process galaxy' is two to three orders of magnitude higher than that detected in any other ultrafaint dwarf galaxy. This implies that a single, rare event produced the r-Process material in Reticulum II. The r-Process yield and event rate are incompatible with the source being ordinary core-collapse supernovae, but consistent with other possible sources, such as neutron star mergers.
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R-Process enrichment from a single event in an ancient dwarf galaxy
Nature, 2016Co-Authors: Anna Frebel, Anirudh Chiti, Joshua D SimonAbstract:It has long been debated whether elements heavier than zinc are formed continually, for example in core-collapse supernovae, or in rare events, such as neutron star mergers; here, studies of element abundances in a local ultrafaint dwarf galaxy provide evidence that these elements are formed during rare yet prolific stellar events. Elements heavier than zinc are synthesized through the rapid (r) and slow (s) neutron-capture Processes^ 1 , 2 . The main site of production of the r-Process elements (such as europium) has been debated for nearly 60 years^ 2 . Initial studies of trends in chemical abundances in old Milky Way halo stars suggested that these elements are produced continually, in sites such as core-collapse supernovae^ 3 , 4 . But evidence from the local Universe favours the idea that r-Process production occurs mainly during rare events, such as neutron star mergers^ 5 , 6 . The appearance of a plateau of europium abundance in some dwarf spheroidal galaxies has been suggested as evidence for rare r-Process enrichment in the early Universe^ 7 , but only under the assumption that no gas accretes into those dwarf galaxies; gas accretion^ 8 favours continual r-Process enrichment in these systems. Furthermore, the universal r-Process Pattern^ 1 , 9 has not been cleanly identified in dwarf spheroidals. The smaller, chemically simpler, and more ancient ultrafaint dwarf galaxies assembled shortly after the first stars formed, and are ideal systems with which to study nucleosynthesis events such as the r-Process^ 10 , 11 . Reticulum II is one such galaxy^ 12 , 13 , 14 . The abundances of non-neutron-capture elements in this galaxy (and others like it) are similar to those in other old stars^ 15 . Here, we report that seven of the nine brightest stars in Reticulum II, observed with high-resolution spectroscopy, show strong enhancements in heavy neutron-capture elements, with abundances that follow the universal r-Process Pattern beyond barium. The enhancement seen in this ‘r-Process galaxy’ is two to three orders of magnitude higher than that detected in any other ultrafaint dwarf galaxy^ 11 , 16 , 17 . This implies that a single, rare event produced the r-Process material in Reticulum II. The r-Process yield and event rate are incompatible with the source being ordinary core-collapse supernovae^ 18 , but consistent with other possible sources, such as neutron star mergers^ 19 . The recently discovered Milky Way satellite Reticulum II, an ultra-faint dwarf galaxy, provides an ideal model for the study of stellar nucleosynthesis, the creation of heavier elements from hydrogen, helium and other lighter elements and particles. It has long been debated whether elements heavier than zinc are formed continually, for example in core-collapse supernovae, or in rare events, such as neutron star mergers. Alexander Ji et al . used high-resolution spectroscopy to determine element abundances in nine young stars in Reticulum II, and find that seven of the nine show strong enhancements in heavy neutron-capture elements with abundances that follow the universal r-Process Pattern above barium. The enhancement is several orders of magnitude greater than that seen in other ultra-faint dwarf galaxies, implying that a single rare event produced the r-Process material.
David L Nidever - One of the best experts on this subject based on the ideXlab platform.
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detailed chemical abundances in the r Process rich ultra faint dwarf galaxy reticulum 2
The Astronomical Journal, 2016Co-Authors: Ian U Roederer, Mario Mateo, John I Bailey, Yingyi Song, Eric F Bell, Jeffrey D Crane, Sarah Loebman, David L NideverAbstract:The ultra-faint dwarf galaxy Reticulum 2 (Ret 2) was recently discovered in images obtained by the Dark Energy Survey. We have observed the four brightest red giants in Ret 2 at high spectral resolution using the Michigan/Magellan Fiber System. We present detailed abundances for as many as 20 elements per star, including 12 elements heavier than the Fe group. We confirm previous detection of high levels of r-Process material in Ret 2 (mean [Eu/Fe]=+1.69+/-0.05) found in three of these stars (mean [Fe/H]=-2.88+/-0.10). The abundances closely match the r-Process Pattern found in the well-studied metal-poor halo star CS22892-052. Such r-Process-enhanced stars have not been found in any other ultra-faint dwarf galaxy, though their existence has been predicted by at least one model. The fourth star in Ret 2 ([Fe/H]=-3.42+/-0.20) contains only trace amounts of Sr ([Sr/Fe]=-1.73+/-0.43) and no detectable heavier elements. One r-Process enhanced star is also enhanced in C (natal [C/Fe]=+1.1). This is only the third such star known, which suggests that the nucleosynthesis sites leading to C and r-Process enhancements are decoupled. The r-Process-deficient star is enhanced in Mg ([Mg/Fe]=+0.81+/-0.14), and the other three stars show normal levels of alpha-enhancement (mean [Mg/Fe]=+0.34+/-0.03). The abundances of other alpha and Fe-group elements closely resemble those in ultra-faint dwarf galaxies and metal-poor halo stars, suggesting that the nucleosynthesis that led to the large r-Process enhancements either produced no light elements or produced light-element abundance signatures indistinguishable from normal supernovae.
