The Experts below are selected from a list of 120 Experts worldwide ranked by ideXlab platform
M Geha - One of the best experts on this subject based on the ideXlab platform.
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chemical analysis of the ultrafaint dwarf galaxy grus ii signature of high mass stellar nucleosynthesis
The Astrophysical Journal, 2020Co-Authors: Terese T Hansen, J L Marshall, J D Simon, R A Bernstein, A B Pace, P S Ferguson, D Q Nagasawa, K Kuehn, D Carollo, M GehaAbstract:We present a detailed abundance analysis of the three brightest member stars at the top of the giant branch of the ultrafaint dwarf (UFD) galaxy Grus II. All stars exhibit a higher than expected [Mg/Ca] ratio compared to metal-poor stars in other UFD galaxies and in the Milky Way (MW) halo. Nucleosynthesis in high-mass ($\geqslant $ 20 M ⊙) core-collapse supernovae has been shown to create this signature. The abundances of this small sample (three) stars suggests the chemical enrichment of Grus II could have occurred through substantial high-mass stellar evolution, and is consistent with the framework of a top-heavy initial mass function. However, with only three stars it cannot be ruled out that the abundance pattern is the result of a stochastic chemical enrichment at early times in the galaxy. The most metal-rich of the three stars also possesses a small enhancement in rapid neutron-capture (r-Process) Elements. The abundance pattern of the r-Process Elements in this star matches the scaled r-Process pattern of the solar system and r-Process enhanced stars in other dwarf galaxies and in the MW halo, hinting at a common origin for these Elements across a range of environments. All current proposed astrophysical sites of r-Process Element production are associated with high-mass stars, thus the possible top-heavy initial mass function of Grus II would increase the likelihood of any of these events occurring. The time delay between the α and r-Process Element enrichment of the galaxy favors a neutron star merger as the origin of the r-Process Elements in Grus II.
Sergey A. Korotin - One of the best experts on this subject based on the ideXlab platform.
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Neutron Star Mergers are the Dominant Source of the r -Process in the Early Evolution of Dwarf Galaxies
The Astrophysical Journal, 2018Co-Authors: Gina E. Duggan, Evan N. Kirby, Serge M. Andrievsky, Sergey A. Korotin, Gina Duggan, Evan KirbyAbstract:There are many candidate sites of the r-Process: core-collapse supernovae (CCSNe; including rare magnetorotational core-collapse supernovae), neutron star mergers (NSMs), and neutron star/black hole mergers. The chemical enrichment of galaxies—specifically dwarf galaxies—helps distinguish between these sources based on the continual build-up of r-Process Elements. This technique can distinguish between the r-Process candidate sites by the clearest observational difference—how quickly these events occur after the stars are created. The existence of several nearby dwarf galaxies allows us to measure robust chemical abundances for galaxies with different star formation histories. Dwarf galaxies are especially useful because simple chemical evolution models can be used to determine the sources of r-Process material. We have measured the r-Process Element barium with Keck/DEIMOS medium-resolution spectroscopy. We present the largest sample of barium abundances (almost 250 stars) in dwarf galaxies ever assembled. We measure [Ba/Fe] as a function of [Fe/H] in this sample and compare with existing [α/Fe] measurements. We have found that a large contribution of barium needs to occur at more delayed timescales than CCSNe in order to explain our observed abundances, namely the significantly more positive trend of the r-Process component of [Ba/Fe] versus [Fe/H] seen for [{Fe}/{{H}}]≲ -1.6 when compared to the [Mg/Fe] versus [Fe/H] trend. We conclude that NSMs are the most likely source of r-Process enrichment in dwarf galaxies at early times.
Terese T Hansen - One of the best experts on this subject based on the ideXlab platform.
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chemical analysis of the ultrafaint dwarf galaxy grus ii signature of high mass stellar nucleosynthesis
The Astrophysical Journal, 2020Co-Authors: Terese T Hansen, J L Marshall, J D Simon, R A Bernstein, A B Pace, P S Ferguson, D Q Nagasawa, K Kuehn, D Carollo, M GehaAbstract:We present a detailed abundance analysis of the three brightest member stars at the top of the giant branch of the ultrafaint dwarf (UFD) galaxy Grus II. All stars exhibit a higher than expected [Mg/Ca] ratio compared to metal-poor stars in other UFD galaxies and in the Milky Way (MW) halo. Nucleosynthesis in high-mass ($\geqslant $ 20 M ⊙) core-collapse supernovae has been shown to create this signature. The abundances of this small sample (three) stars suggests the chemical enrichment of Grus II could have occurred through substantial high-mass stellar evolution, and is consistent with the framework of a top-heavy initial mass function. However, with only three stars it cannot be ruled out that the abundance pattern is the result of a stochastic chemical enrichment at early times in the galaxy. The most metal-rich of the three stars also possesses a small enhancement in rapid neutron-capture (r-Process) Elements. The abundance pattern of the r-Process Elements in this star matches the scaled r-Process pattern of the solar system and r-Process enhanced stars in other dwarf galaxies and in the MW halo, hinting at a common origin for these Elements across a range of environments. All current proposed astrophysical sites of r-Process Element production are associated with high-mass stars, thus the possible top-heavy initial mass function of Grus II would increase the likelihood of any of these events occurring. The time delay between the α and r-Process Element enrichment of the galaxy favors a neutron star merger as the origin of the r-Process Elements in Grus II.
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the r Process alliance discovery of the first metal poor star with a combined r and s Process Element signature
arXiv: Solar and Stellar Astrophysics, 2018Co-Authors: Maude Gull, Anna Frebel, Vinicius M Placco, Amanda I Karakas, Terese T Hansen, Madelyn Cain, C Abate, Rana Ezzeddine, Charli M SakariAbstract:We present a high-resolution (R~35,000), high signal-to-noise (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- or 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.a
Vinicius M Placco - One of the best experts on this subject based on the ideXlab platform.
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the r Process alliance discovery of the first metal poor star with a combined r and s Process Element signature
arXiv: Solar and Stellar Astrophysics, 2018Co-Authors: Maude Gull, Anna Frebel, Vinicius M Placco, Amanda I Karakas, Terese T Hansen, Madelyn Cain, C Abate, Rana Ezzeddine, Charli M SakariAbstract:We present a high-resolution (R~35,000), high signal-to-noise (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- or 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.a
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the chemical abundances of stars in the halo cash project iii a new classification scheme for carbon enhanced metal poor stars with s Process Element enhancement
The Astrophysical Journal, 2015Co-Authors: Julie K Hollek, Christopher Sneden, Anna Frebel, Vinicius M Placco, Amanda I Karakas, Matthew D Shetrone, Norbert ChristliebAbstract: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 (CASH) Project. Its spectroscopic stellar parameters are Teff = 4863 K, log g = 1.25, vmic = 2.20 km/s, 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 "CEMP-r/s" star. Based on abundance comparisons with 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 1-D 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 Msun mass AGB star and a late-time mass transfer, that transformed HE 0414-0343 into a CEMP-sC star. We also find the [Y/Ba] ratio well parametrizes the classification and can thus be used to easily classify any future such stars.
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[O/Fe] Estimates for Carbon-Enhanced Metal-Poor Stars from Near-IR Spectroscopy
Astronomical Journal, 2011Co-Authors: Catherine R. Kennedy, Vinicius M Placco, Norbert Christlieb, Thirupathi Sivarani, Timothy C. Beers, Young Sun Lee, Silvia Rossi, Falk Herwig, Bertrand PlezAbstract:We report on oxygen abundances determined from medium-resolution near-IR spectroscopy for a sample of 57 carbon-enhanced metal-poor (CEMP) stars selected from the Hamburg/ESO survey. The majority of our program stars exhibit oxygen-to-iron ratios in the range +0.5 < [O/Fe]< +2.0. The [O/Fe] values for this sample are statistically compared to available high-resolution estimates for known CEMP stars, as well as to high-resolution estimates for a set of carbon-normal metal-poor stars. Carbon, nitrogen, and oxygen abundance patterns for a sub-sample of these stars are compared to yield predictions for very metal-poor asymptotic giant-branch abundances in the recent literature. We find that the majority of our sample exhibit patterns that are consistent with previously studied CEMP stars having s-Process-Element enhancements, and thus have very likely been polluted by carbon- and oxygen-enhanced material transferred from a metal-poor asymptotic giant-branch companion.
C. C. Worley - One of the best experts on this subject based on the ideXlab platform.
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The AMBRE Project: r-Process Element abundances in the Milky Way thin and thick discs
Proceedings of the International Astronomical Union, 2017Co-Authors: Guillaume Guiglion, Patrick De Laverny, Alejandra Recio-blanco, C. C. WorleyAbstract:AbstractChemical evolution of r-Process Elements in the Milky Way disc is still a matter of debate. We took advantage of high resolution HARPS spectra from the ESO archive in order to derive precise chemical abundances of 3 r-Process Elements Eu, Dy & Gd for a sample of 4 355 FGK Milky Way stars. The chemical analysis has been performed thanks to the automatic optimization pipeline GAUGUIN. Based on the [α/Fe] ratio, we chemically characterized the thin and the thick discs, and present here results of these 3 r-Process Element abundances in both discs. We found an unexpected Gadolinium and Dysprosium enrichment in the thick disc stars compared to Europium, while these three Elements track well each other in the thin disc.
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s‐Process Element Abundance Results for 47 Tuc Stars Using SALT
AIP Conference Proceedings, 2008Co-Authors: C. C. Worley, P. L. Cottrell, E. C. Wylie De BoerAbstract:Eleven giant branch stars in 47 Tucanae were observed using the Robert Stobie Spectro‐graph (RSS) on the Southern African Large Telescope during the performance verification phase of this instrument. These stars were analysed as part of a quest to investigate s‐Process Element abundances throughout the colour‐magnitude diagram of 47 Tucanae. No abundance variations of Zr, Ba, Nd and Eu were found in these eleven stars, such that [X/Fe] = 0.0±0.5 3dex. Further, theoretical analysis indicates that the maximum resolution on RSS and AAOmega on the Anglo‐Australian Telescope is sufficient to detect s‐Process Element abundance variations. More detailed discussion on the abundance analysis of these stars and the theoretical analysis of spectrograph resolution can be found in [l].
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s- and r-Process Element abundances in the CMD of 47 Tucanae using the Robert Stobie Spectrograph on SALT
Publications of the Astronomical Society of Australia, 2008Co-Authors: C. C. Worley, P. L. Cottrell, E. C. Wylie De BoerAbstract:A recent study by Wylie et al 2006 has revealed that s-Process Element abundances are enhanced relative to iron in both red giant branch and asymptotic giant branch stars of 47 Tucanae. A more detailed investigation into s-Process Element abundances throughout the colour-magnitude diagram of 47 Tucanae is vital in order to determine whether the observed enhancements are intrinsic to the cluster. This paper explores this possibility through observational and theoretical means. The visibility of s- and r-Process Element lines in synthetic spectra of giant and dwarf stars throughout the colour magnitude diagram of 47 Tucanae has been explored. It was determined that a resolving power of 10 000 was sufficient to observe s-Process Element abundance variations in globular cluster giant branch stars. These synthetic results were compared with the spectra of eleven 47 Tucanae giant branch stars observed during the performance verification of the Robert Stobie Spectrograph on the Southern African Large Telescope. Three s-Process Elements, Zr, Ba, Nd, and one r-Process Element, Eu, were investigated. No abundance variations were found such that [X/Fe] = 0.0 +/- 0.5 dex. It was concluded that this resolving power, R ~ 5000, was not sufficient to obtain exact abundances but upper limits on the s-Process Element abundances could be determined.