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B Sargent - One of the best experts on this subject based on the ideXlab platform.
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on the metallicity dependence of crystalline Silicates in oxygen rich asymptotic giant branch stars and red supergiants
Monthly Notices of the Royal Astronomical Society, 2012Co-Authors: O C Jones, B Sargent, G C Sloan, F Kemper, Iain Mcdonald, C Gielen, Paul M Woods, Martha L Boyer, A A ZijlstraAbstract:We investigate the occurrence of crystalline Silicates in oxygen-rich evolved stars across a range of metallicities and mass-loss rates. It has been suggested that the crystalline silicate feature strength increases with increasing mass-loss rate, implying a correlation between lattice structure and wind density. To test this, we analyse Spitzer Infrared Spectrograph and Infrared Space Observatory Short Wavelength Spectrometer spectra of 217 oxygen-rich asymptotic giant branch and 98 red supergiants in the Milky Way, the Large and Small Magellanic Clouds, and Galactic globular clusters. These encompass a range of spectral morphologies from the spectrally rich which exhibit a wealth of crystalline and amorphous silicate features to ‘naked’ (dust-free) stars. We combine spectroscopic and photometric observations with the GRAMS grid of radiative transfer models to derive (dust) mass-loss rates and temperature. We then measure the strength of the crystalline silicate bands at 23, 28 and 33µm. We detect crystalline Silicates in stars with dust mass-loss rates which span over 3 dex, down to rates ... ... ... ... ... ... ... ... ... – –
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on the metallicity dependence of crystalline Silicates in oxygen rich asymptotic giant branch stars and red supergiants
arXiv: Solar and Stellar Astrophysics, 2012Co-Authors: O C Jones, B Sargent, G C Sloan, F Kemper, Iain Mcdonald, C Gielen, Paul M Woods, Martha L Boyer, A A ZijlstraAbstract:We investigate the occurrence of crystalline Silicates in oxygen-rich evolved stars across a range of metallicities and mass-loss rates. It has been suggested that the crystalline silicate feature strength increases with increasing mass-loss rate, implying a correlation between lattice structure and wind density. To test this, we analyse Spitzer IRS and Infrared Space Observatory SWS spectra of 217 oxygen-rich asymptotic giant branch stars and 98 red supergiants in the Milky Way, the Large and Small Magellanic Clouds and Galactic globular clusters. These encompass a range of spectral morphologies from the spectrally-rich which exhibit a wealth of crystalline and amorphous silicate features to 'naked' (dust-free) stars. We combine spectroscopic and photometric observations with the GRAMS grid of radiative transfer models to derive (dust) mass-loss rates and temperature. We then measure the strength of the crystalline silicate bands at 23, 28 and 33 microns. We detect crystalline Silicates in stars with dust mass-loss rates which span over 3 dex, down to rates of ~10^-9 solar masses/year. Detections of crystalline Silicates are more prevalent in higher mass-loss rate objects, though the highest mass-loss rate objects do not show the 23-micron feature, possibly due to the low temperature of the forsterite grains or it may indicate that the 23-micron band is going into absorption due to high column density. Furthermore, we detect a change in the crystalline silicate mineralogy with metallicity, with enstatite seen increasingly at low metallicity.
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a spitzer infrared spectrograph detection of crystalline Silicates in a protostellar envelope
The Astrophysical Journal, 2011Co-Authors: Charles A Poteet, B Sargent, D M Watson, Thomas S Megeath, Nuria Calvet, Ian S Remming, M K Mcclure, William J Fischer, E FurlanAbstract:We present the Spitzer Space Telescope Infrared Spectrograph spectrum of the Orion A protostar HOPS-68. The mid-infrared spectrum reveals crystalline substructure at 11.1, 16.1, 18.8, 23.6, 27.9, and 33.6 μm superimposed on the broad 9.7 and 18 μm amorphous silicate features; the substructure is well matched by the presence of the olivine end-member forsterite (Mg2SiO4). Crystalline Silicates are often observed as infrared emission features around the circumstellar disks of Herbig Ae/Be stars and T Tauri stars. However, this is the first unambiguous detection of crystalline silicate absorption in a cold, infalling, protostellar envelope. We estimate the crystalline mass fraction along the line of sight by first assuming that the crystalline Silicates are located in a cold absorbing screen and secondly by utilizing radiative transfer models. The resulting crystalline mass fractions of 0.14 and 0.17, respectively, are significantly greater than the upper limit found in the interstellar medium (0.02-0.05). We propose that the amorphous Silicates were annealed within the hot inner disk and/or envelope regions and subsequently transported outward into the envelope by entrainment in a protostellar outflow.
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dust processing and grain growth in protoplanetary disks in the taurus auriga star forming region
Astrophysical Journal Supplement Series, 2009Co-Authors: B Sargent, W J Forrest, C Tayrien, Melissa Mcclure, D M Watson, G C Sloan, P Manoj, C J Bohac, Elise Furlan, K H KimAbstract:Mid-infrared spectra of 65 T Tauri stars (TTS) taken with the Infrared Spectrograph (IRS) on board the Spitzer Space Telescope are modeled using populations of optically thin warm and cool grains to probe the radial variation in dust composition in the uppermost layers of protoplanetary disks. Most spectra with narrow emission features associated with crystalline Silicates require Mg-rich minerals and silica, while a few spectra with these features suggest the presence of other components. IRS spectra indicating the presence of large amounts of warm enstatite of ~400-500 K require crystalline Silicates (enstatite or forsterite) at temperatures lower than the median temperature of the cool dust in the models, ~127 K; spectra showing a high abundance of other crystalline Silicates (forsterite or silica) typically do not. A few spectra show 10 ?m complexes of very small equivalent width. They are fit well using abundant crystalline Silicates but very few large grains, inconsistent with the expectation that a low peak-to-continuum ratio of the 10 ?m complex always indicates grain growth. Most of the spectra in our sample are fit well without using the opacities of large crystalline silicate grains. If large grains grow by agglomeration of submicron grains of all dust types, the amorphous silicate components of these aggregates must typically be more abundant than the crystalline silicate components. We also find that the more there is of one crystalline dust species, the more there is of the others. This could suggest that crystalline Silicates are processed directly from amorphous Silicates, whether through evaporation of the amorphous grains and condensation in chemical equilibrium or by annealing of the amorphous precursors. Alternatively, if one kind of crystalline silicate transforms into another kind, it suggests that the intermediate species transforms into the end-product species at a slower rate than the precursor transforms into the intermediate species; otherwise, there would be a negligible abundance of intermediate species. It is also found that the crystalline silicate abundance is correlated tightly with disk geometry, in the sense of higher crystallinity accompanying more-settled disks, which are commonly associated with growth and settling of grains. The abundance of large grains is also correlated with disks that are more highly settled, but with a wide range of large grain abundance for a given degree of settling. We interpret this range as that the settling of large grains is sensitive to individual disk properties. We also find that lower-mass stars have higher abundances of large grains in their inner regions.
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dust processing and grain growth in protoplanetary disks in the taurus auriga star forming region
arXiv: Astrophysics, 2008Co-Authors: B Sargent, W J Forrest, C Tayrien, Melissa Mcclure, D M Watson, G C Sloan, P Manoj, C J Bohac, Elise Furlan, K H KimAbstract:Mid-infrared spectra of 65 T Tauri stars (TTS) taken with the Infrared Spectrograph (IRS) on board the Spitzer Space Telescope are modeled using dust at two temperatures to probe the radial variation in dust composition in the uppermost layers of protoplanetary disks. Most spectra indicating crystalline Silicates require Mg-rich minerals and silica, but a few suggest otherwise. Spectra indicating abundant enstatite at higher temperatures also require crystalline Silicates at temperatures lower than those required for spectra showing high abundance of other crystalline Silicates. A few spectra show 10 micron complexes of very small equivalent width. They are fit well using abundant crystalline Silicates but very few large grains, inconsistent with the expectation that low peak-to-continuum ratio of the 10 micron complex always indicates grain growth. Most spectra in our sample are fit well without using the opacities of large crystalline silicate grains. If large grains grow by agglomeration of submicron grains of all dust types, the amorphous silicate components of these aggregates must typically be more abundant than the crystalline silicate components. Crystalline silicate abundances correlate positively with other such abundances, suggesting that crystalline Silicates are processed directly from amorphous Silicates and that neither forsterite, enstatite, nor silica are intermediate steps when producing either of the other two. Disks with more dust settling typically have greater crystalline abundances. Large-grain abundance is somewhat correlated with greater settling of disks. The lack of strong correlation is interpreted to mean that settling of large grains is sensitive to individual disk properties. Lower-mass stars have higher abundances of large grains in their inner regions.
Ann N Nguyen - One of the best experts on this subject based on the ideXlab platform.
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fe and mg isotopic analyses of isotopically unusual presolar silicate grains
LPI, 2011Co-Authors: Ann N Nguyen, S Messenger, Motoo Ito, Zia RahmanAbstract:Interstellar and circumstellar silicate grains are thought to be Mg-rich and Fe-poor, based on astronomical observations and equilibrium condensation models of silicate dust formation in stellar outflows. On the other hand, presolar Silicates isolated from meteorites have surprisingly high Fe contents and few Mg-rich grains are observed. The high Fe contents in meteoritic presolar Silicates may indicate they formed by a non-equilibrium condensation process. Alternatively, the Fe in the stardust grains could have been acquired during parent body alteration. The origin of Fe in presolar Silicates may be deduced from its isotopic composition. Thus far, Fe isotopic measurements of presolar Silicates are limited to the Fe-54/Fe-56 ratios of 14 grains. Only two slight anomalies (albeit solar within error) were observed. However, these measurements suffered from contamination of Fe from the adjacent meteorite matrix, which diluted any isotopic anomalies. We have isolated four presolar Silicates having unusual O isotopic compositions by focused ion beam (FIB) milling and obtained their undiluted Mg and Fe isotopic compositions. These compositions help to identify the grains stellar sources and to determine the source of Fe in the grains.
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coordinated analyses of presolar grains in the allan hills 77307 and queen elizabeth range 99177 meteorites
The Astrophysical Journal, 2010Co-Authors: Ann N Nguyen, Larry R Nittler, F J Stadermann, Rhonda M Stroud, Conel Od M AlexanderAbstract:We report the identification of presolar Silicates ({approx}177 ppm), presolar oxides ({approx}11 ppm), and one presolar SiO{sub 2} grain in the Allan Hills (ALHA) 77307 chondrite. Three grains having Si-isotopic compositions similar to SiC X and Z grains were also identified, though the mineral phases are unconfirmed. Similar abundances of presolar Silicates ({approx}152 ppm) and oxides ({approx}8 ppm) were also uncovered in the primitive CR chondrite Queen Elizabeth Range (QUE) 99177, along with 13 presolar SiC grains and one presolar silicon nitride. The O-isotopic compositions of the presolar Silicates and oxides indicate that most of the grains condensed in low-mass red giant and asymptotic giant branch stars. Interestingly, unlike presolar oxides, few presolar silicate grains have isotopic compositions pointing to low-metallicity, low-mass stars (Group 3). The {sup 18}O-rich (Group 4) Silicates, along with the few Group 3 Silicates that were identified, likely have origins in supernova outflows. This is supported by their O- and Si-isotopic compositions. Elemental compositions for 74 presolar silicate grains were determined by scanning Auger spectroscopy. Most of the grains have non-stoichiometric elemental compositions inconsistent with pyroxene or olivine, the phases commonly used to fit astronomical spectra, and have comparable Mg and Fe contents. Non-equilibrium condensation and/ormore » secondary alteration could produce the high Fe contents. Transmission electron microscopic analysis of three silicate grains also reveals non-stoichiometric compositions, attributable to non-equilibrium or multistep condensation, and very fine scale elemental heterogeneity, possibly due to subsequent annealing. The mineralogies of presolar Silicates identified in meteorites thus far seem to differ from those in interplanetary dust particles.« less
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coordinated analyses of presolar grains in the allan hills 77307 and queen elizabeth range 99177 meteorites
arXiv: Solar and Stellar Astrophysics, 2010Co-Authors: Ann N Nguyen, Larry R Nittler, F J Stadermann, Rhonda M Stroud, Conel Od M AlexanderAbstract:We report the identification of presolar Silicates (~177 ppm), presolar oxides (~11 ppm), and one presolar SiO2 grain in the Allan Hills (ALHA) 77307 chondrite. Three grains having Si isotopic compositions similar to SiC X and Z grains were also identified, though the mineral phases are unconfirmed. Similar abundances of presolar Silicates (~152 ppm) and oxides (~8 ppm) were also uncovered in the primitive CR chondrite Queen Elizabeth Range (QUE) 99177, along with 13 presolar SiC grains and one presolar silicon nitride. The O isotopic compositions of the presolar Silicates and oxides indicate that most of the grains condensed in low-mass red giant and asymptotic giant branch stars. Interestingly, unlike presolar oxides, few presolar silicate grains have isotopic compositions pointing to low-metallicity, low-mass stars (Group 3). The 18O-rich (Group 4) Silicates, along with the few Group 3 Silicates that were identified, likely have origins in supernova outflows. This is supported by their O and Si isotopic compositions. Elemental compositions for 74 presolar silicate grains were determined by scanning Auger spectroscopy. Most of the grains have non-stoichiometric elemental compositions inconsistent with pyroxene or olivine, the phases commonly used to fit astronomical spectra, and have comparable Mg and Fe contents. Non-equilibrium condensation and/or secondary alteration could produce the high Fe contents. Transmission electron microscopic analysis of three silicate grains also reveals non-stoichiometric compositions, attributable to non-equilibrium or multistep condensation, and very fine scale elemental heterogeneity, possibly due to subsequent annealing. The mineralogies of presolar Silicates identified in meteorites thus far seem to differ from those in interplanetary dust particles.
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characterization of presolar silicate and oxide grains in primitive carbonaceous chondrites
The Astrophysical Journal, 2007Co-Authors: Ernst Zinner, Ann N Nguyen, F J Stadermann, Rhonda M Stroud, Conel Od M Alexander, Larry R NittlerAbstract:Raster ion imaging of the oxygen isotopes with the NanoSIMS ion microprobe has been used to identify presolar grains in two primitive meteorites. Eleven presolar Silicates and eight presolar oxides were identified in the primitive carbonaceous chondrite Acfer 094 for abundances of 325 and 360 parts per million (ppm), respectively. In addition, nine presolar Silicates and five presolar oxide grains were identified in the CO3 chondrite ALHA 77307, for abundancesof 320and200ppm,respectively.Theseabundances,whicharematrix-normalizedandcorrectedforinstrumental detectionefficiencies,aremuchhigherthanthoseofotherpresolarphases,withtheexceptionof nanodiamonds,although the latter may not all be presolar. The chemical compositions of six presolar silicate grains from ALHA 77307 were elucidated by Auger spectroscopy. Transmission electron microscopy (TEM) analysis of one presolar silicate grain revealed a nonstoichiometric composition and an amorphous structure as indicated by the diffuse electron diffraction pattern. The oxygen isotopic compositions of the presolar Silicates indicate origins in red giant and asymptotic giant branch stars. Analysis of the Si isotopic compositions of 10 presolar Silicates provides further constraints on the effects of Galactic chemical evolution. Subject headingg circumstellar matter — dust, extinction — Galaxy: evolution — nuclear reactions, nucleosynthesis, abundances — stars: evolution
Conel Od M Alexander - One of the best experts on this subject based on the ideXlab platform.
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coordinated analyses of presolar grains in the allan hills 77307 and queen elizabeth range 99177 meteorites
The Astrophysical Journal, 2010Co-Authors: Ann N Nguyen, Larry R Nittler, F J Stadermann, Rhonda M Stroud, Conel Od M AlexanderAbstract:We report the identification of presolar Silicates ({approx}177 ppm), presolar oxides ({approx}11 ppm), and one presolar SiO{sub 2} grain in the Allan Hills (ALHA) 77307 chondrite. Three grains having Si-isotopic compositions similar to SiC X and Z grains were also identified, though the mineral phases are unconfirmed. Similar abundances of presolar Silicates ({approx}152 ppm) and oxides ({approx}8 ppm) were also uncovered in the primitive CR chondrite Queen Elizabeth Range (QUE) 99177, along with 13 presolar SiC grains and one presolar silicon nitride. The O-isotopic compositions of the presolar Silicates and oxides indicate that most of the grains condensed in low-mass red giant and asymptotic giant branch stars. Interestingly, unlike presolar oxides, few presolar silicate grains have isotopic compositions pointing to low-metallicity, low-mass stars (Group 3). The {sup 18}O-rich (Group 4) Silicates, along with the few Group 3 Silicates that were identified, likely have origins in supernova outflows. This is supported by their O- and Si-isotopic compositions. Elemental compositions for 74 presolar silicate grains were determined by scanning Auger spectroscopy. Most of the grains have non-stoichiometric elemental compositions inconsistent with pyroxene or olivine, the phases commonly used to fit astronomical spectra, and have comparable Mg and Fe contents. Non-equilibrium condensation and/ormore » secondary alteration could produce the high Fe contents. Transmission electron microscopic analysis of three silicate grains also reveals non-stoichiometric compositions, attributable to non-equilibrium or multistep condensation, and very fine scale elemental heterogeneity, possibly due to subsequent annealing. The mineralogies of presolar Silicates identified in meteorites thus far seem to differ from those in interplanetary dust particles.« less
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coordinated analyses of presolar grains in the allan hills 77307 and queen elizabeth range 99177 meteorites
arXiv: Solar and Stellar Astrophysics, 2010Co-Authors: Ann N Nguyen, Larry R Nittler, F J Stadermann, Rhonda M Stroud, Conel Od M AlexanderAbstract:We report the identification of presolar Silicates (~177 ppm), presolar oxides (~11 ppm), and one presolar SiO2 grain in the Allan Hills (ALHA) 77307 chondrite. Three grains having Si isotopic compositions similar to SiC X and Z grains were also identified, though the mineral phases are unconfirmed. Similar abundances of presolar Silicates (~152 ppm) and oxides (~8 ppm) were also uncovered in the primitive CR chondrite Queen Elizabeth Range (QUE) 99177, along with 13 presolar SiC grains and one presolar silicon nitride. The O isotopic compositions of the presolar Silicates and oxides indicate that most of the grains condensed in low-mass red giant and asymptotic giant branch stars. Interestingly, unlike presolar oxides, few presolar silicate grains have isotopic compositions pointing to low-metallicity, low-mass stars (Group 3). The 18O-rich (Group 4) Silicates, along with the few Group 3 Silicates that were identified, likely have origins in supernova outflows. This is supported by their O and Si isotopic compositions. Elemental compositions for 74 presolar silicate grains were determined by scanning Auger spectroscopy. Most of the grains have non-stoichiometric elemental compositions inconsistent with pyroxene or olivine, the phases commonly used to fit astronomical spectra, and have comparable Mg and Fe contents. Non-equilibrium condensation and/or secondary alteration could produce the high Fe contents. Transmission electron microscopic analysis of three silicate grains also reveals non-stoichiometric compositions, attributable to non-equilibrium or multistep condensation, and very fine scale elemental heterogeneity, possibly due to subsequent annealing. The mineralogies of presolar Silicates identified in meteorites thus far seem to differ from those in interplanetary dust particles.
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characterization of presolar silicate and oxide grains in primitive carbonaceous chondrites
The Astrophysical Journal, 2007Co-Authors: Ernst Zinner, Ann N Nguyen, F J Stadermann, Rhonda M Stroud, Conel Od M Alexander, Larry R NittlerAbstract:Raster ion imaging of the oxygen isotopes with the NanoSIMS ion microprobe has been used to identify presolar grains in two primitive meteorites. Eleven presolar Silicates and eight presolar oxides were identified in the primitive carbonaceous chondrite Acfer 094 for abundances of 325 and 360 parts per million (ppm), respectively. In addition, nine presolar Silicates and five presolar oxide grains were identified in the CO3 chondrite ALHA 77307, for abundancesof 320and200ppm,respectively.Theseabundances,whicharematrix-normalizedandcorrectedforinstrumental detectionefficiencies,aremuchhigherthanthoseofotherpresolarphases,withtheexceptionof nanodiamonds,although the latter may not all be presolar. The chemical compositions of six presolar silicate grains from ALHA 77307 were elucidated by Auger spectroscopy. Transmission electron microscopy (TEM) analysis of one presolar silicate grain revealed a nonstoichiometric composition and an amorphous structure as indicated by the diffuse electron diffraction pattern. The oxygen isotopic compositions of the presolar Silicates indicate origins in red giant and asymptotic giant branch stars. Analysis of the Si isotopic compositions of 10 presolar Silicates provides further constraints on the effects of Galactic chemical evolution. Subject headingg circumstellar matter — dust, extinction — Galaxy: evolution — nuclear reactions, nucleosynthesis, abundances — stars: evolution
Larry R Nittler - One of the best experts on this subject based on the ideXlab platform.
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coordinated analyses of presolar grains in the allan hills 77307 and queen elizabeth range 99177 meteorites
The Astrophysical Journal, 2010Co-Authors: Ann N Nguyen, Larry R Nittler, F J Stadermann, Rhonda M Stroud, Conel Od M AlexanderAbstract:We report the identification of presolar Silicates ({approx}177 ppm), presolar oxides ({approx}11 ppm), and one presolar SiO{sub 2} grain in the Allan Hills (ALHA) 77307 chondrite. Three grains having Si-isotopic compositions similar to SiC X and Z grains were also identified, though the mineral phases are unconfirmed. Similar abundances of presolar Silicates ({approx}152 ppm) and oxides ({approx}8 ppm) were also uncovered in the primitive CR chondrite Queen Elizabeth Range (QUE) 99177, along with 13 presolar SiC grains and one presolar silicon nitride. The O-isotopic compositions of the presolar Silicates and oxides indicate that most of the grains condensed in low-mass red giant and asymptotic giant branch stars. Interestingly, unlike presolar oxides, few presolar silicate grains have isotopic compositions pointing to low-metallicity, low-mass stars (Group 3). The {sup 18}O-rich (Group 4) Silicates, along with the few Group 3 Silicates that were identified, likely have origins in supernova outflows. This is supported by their O- and Si-isotopic compositions. Elemental compositions for 74 presolar silicate grains were determined by scanning Auger spectroscopy. Most of the grains have non-stoichiometric elemental compositions inconsistent with pyroxene or olivine, the phases commonly used to fit astronomical spectra, and have comparable Mg and Fe contents. Non-equilibrium condensation and/ormore » secondary alteration could produce the high Fe contents. Transmission electron microscopic analysis of three silicate grains also reveals non-stoichiometric compositions, attributable to non-equilibrium or multistep condensation, and very fine scale elemental heterogeneity, possibly due to subsequent annealing. The mineralogies of presolar Silicates identified in meteorites thus far seem to differ from those in interplanetary dust particles.« less
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coordinated analyses of presolar grains in the allan hills 77307 and queen elizabeth range 99177 meteorites
arXiv: Solar and Stellar Astrophysics, 2010Co-Authors: Ann N Nguyen, Larry R Nittler, F J Stadermann, Rhonda M Stroud, Conel Od M AlexanderAbstract:We report the identification of presolar Silicates (~177 ppm), presolar oxides (~11 ppm), and one presolar SiO2 grain in the Allan Hills (ALHA) 77307 chondrite. Three grains having Si isotopic compositions similar to SiC X and Z grains were also identified, though the mineral phases are unconfirmed. Similar abundances of presolar Silicates (~152 ppm) and oxides (~8 ppm) were also uncovered in the primitive CR chondrite Queen Elizabeth Range (QUE) 99177, along with 13 presolar SiC grains and one presolar silicon nitride. The O isotopic compositions of the presolar Silicates and oxides indicate that most of the grains condensed in low-mass red giant and asymptotic giant branch stars. Interestingly, unlike presolar oxides, few presolar silicate grains have isotopic compositions pointing to low-metallicity, low-mass stars (Group 3). The 18O-rich (Group 4) Silicates, along with the few Group 3 Silicates that were identified, likely have origins in supernova outflows. This is supported by their O and Si isotopic compositions. Elemental compositions for 74 presolar silicate grains were determined by scanning Auger spectroscopy. Most of the grains have non-stoichiometric elemental compositions inconsistent with pyroxene or olivine, the phases commonly used to fit astronomical spectra, and have comparable Mg and Fe contents. Non-equilibrium condensation and/or secondary alteration could produce the high Fe contents. Transmission electron microscopic analysis of three silicate grains also reveals non-stoichiometric compositions, attributable to non-equilibrium or multistep condensation, and very fine scale elemental heterogeneity, possibly due to subsequent annealing. The mineralogies of presolar Silicates identified in meteorites thus far seem to differ from those in interplanetary dust particles.
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characterization of presolar silicate and oxide grains in primitive carbonaceous chondrites
The Astrophysical Journal, 2007Co-Authors: Ernst Zinner, Ann N Nguyen, F J Stadermann, Rhonda M Stroud, Conel Od M Alexander, Larry R NittlerAbstract:Raster ion imaging of the oxygen isotopes with the NanoSIMS ion microprobe has been used to identify presolar grains in two primitive meteorites. Eleven presolar Silicates and eight presolar oxides were identified in the primitive carbonaceous chondrite Acfer 094 for abundances of 325 and 360 parts per million (ppm), respectively. In addition, nine presolar Silicates and five presolar oxide grains were identified in the CO3 chondrite ALHA 77307, for abundancesof 320and200ppm,respectively.Theseabundances,whicharematrix-normalizedandcorrectedforinstrumental detectionefficiencies,aremuchhigherthanthoseofotherpresolarphases,withtheexceptionof nanodiamonds,although the latter may not all be presolar. The chemical compositions of six presolar silicate grains from ALHA 77307 were elucidated by Auger spectroscopy. Transmission electron microscopy (TEM) analysis of one presolar silicate grain revealed a nonstoichiometric composition and an amorphous structure as indicated by the diffuse electron diffraction pattern. The oxygen isotopic compositions of the presolar Silicates indicate origins in red giant and asymptotic giant branch stars. Analysis of the Si isotopic compositions of 10 presolar Silicates provides further constraints on the effects of Galactic chemical evolution. Subject headingg circumstellar matter — dust, extinction — Galaxy: evolution — nuclear reactions, nucleosynthesis, abundances — stars: evolution
G C Sloan - One of the best experts on this subject based on the ideXlab platform.
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on the metallicity dependence of crystalline Silicates in oxygen rich asymptotic giant branch stars and red supergiants
Monthly Notices of the Royal Astronomical Society, 2012Co-Authors: O C Jones, B Sargent, G C Sloan, F Kemper, Iain Mcdonald, C Gielen, Paul M Woods, Martha L Boyer, A A ZijlstraAbstract:We investigate the occurrence of crystalline Silicates in oxygen-rich evolved stars across a range of metallicities and mass-loss rates. It has been suggested that the crystalline silicate feature strength increases with increasing mass-loss rate, implying a correlation between lattice structure and wind density. To test this, we analyse Spitzer Infrared Spectrograph and Infrared Space Observatory Short Wavelength Spectrometer spectra of 217 oxygen-rich asymptotic giant branch and 98 red supergiants in the Milky Way, the Large and Small Magellanic Clouds, and Galactic globular clusters. These encompass a range of spectral morphologies from the spectrally rich which exhibit a wealth of crystalline and amorphous silicate features to ‘naked’ (dust-free) stars. We combine spectroscopic and photometric observations with the GRAMS grid of radiative transfer models to derive (dust) mass-loss rates and temperature. We then measure the strength of the crystalline silicate bands at 23, 28 and 33µm. We detect crystalline Silicates in stars with dust mass-loss rates which span over 3 dex, down to rates ... ... ... ... ... ... ... ... ... – –
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on the metallicity dependence of crystalline Silicates in oxygen rich asymptotic giant branch stars and red supergiants
arXiv: Solar and Stellar Astrophysics, 2012Co-Authors: O C Jones, B Sargent, G C Sloan, F Kemper, Iain Mcdonald, C Gielen, Paul M Woods, Martha L Boyer, A A ZijlstraAbstract:We investigate the occurrence of crystalline Silicates in oxygen-rich evolved stars across a range of metallicities and mass-loss rates. It has been suggested that the crystalline silicate feature strength increases with increasing mass-loss rate, implying a correlation between lattice structure and wind density. To test this, we analyse Spitzer IRS and Infrared Space Observatory SWS spectra of 217 oxygen-rich asymptotic giant branch stars and 98 red supergiants in the Milky Way, the Large and Small Magellanic Clouds and Galactic globular clusters. These encompass a range of spectral morphologies from the spectrally-rich which exhibit a wealth of crystalline and amorphous silicate features to 'naked' (dust-free) stars. We combine spectroscopic and photometric observations with the GRAMS grid of radiative transfer models to derive (dust) mass-loss rates and temperature. We then measure the strength of the crystalline silicate bands at 23, 28 and 33 microns. We detect crystalline Silicates in stars with dust mass-loss rates which span over 3 dex, down to rates of ~10^-9 solar masses/year. Detections of crystalline Silicates are more prevalent in higher mass-loss rate objects, though the highest mass-loss rate objects do not show the 23-micron feature, possibly due to the low temperature of the forsterite grains or it may indicate that the 23-micron band is going into absorption due to high column density. Furthermore, we detect a change in the crystalline silicate mineralogy with metallicity, with enstatite seen increasingly at low metallicity.
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dust processing and grain growth in protoplanetary disks in the taurus auriga star forming region
Astrophysical Journal Supplement Series, 2009Co-Authors: B Sargent, W J Forrest, C Tayrien, Melissa Mcclure, D M Watson, G C Sloan, P Manoj, C J Bohac, Elise Furlan, K H KimAbstract:Mid-infrared spectra of 65 T Tauri stars (TTS) taken with the Infrared Spectrograph (IRS) on board the Spitzer Space Telescope are modeled using populations of optically thin warm and cool grains to probe the radial variation in dust composition in the uppermost layers of protoplanetary disks. Most spectra with narrow emission features associated with crystalline Silicates require Mg-rich minerals and silica, while a few spectra with these features suggest the presence of other components. IRS spectra indicating the presence of large amounts of warm enstatite of ~400-500 K require crystalline Silicates (enstatite or forsterite) at temperatures lower than the median temperature of the cool dust in the models, ~127 K; spectra showing a high abundance of other crystalline Silicates (forsterite or silica) typically do not. A few spectra show 10 ?m complexes of very small equivalent width. They are fit well using abundant crystalline Silicates but very few large grains, inconsistent with the expectation that a low peak-to-continuum ratio of the 10 ?m complex always indicates grain growth. Most of the spectra in our sample are fit well without using the opacities of large crystalline silicate grains. If large grains grow by agglomeration of submicron grains of all dust types, the amorphous silicate components of these aggregates must typically be more abundant than the crystalline silicate components. We also find that the more there is of one crystalline dust species, the more there is of the others. This could suggest that crystalline Silicates are processed directly from amorphous Silicates, whether through evaporation of the amorphous grains and condensation in chemical equilibrium or by annealing of the amorphous precursors. Alternatively, if one kind of crystalline silicate transforms into another kind, it suggests that the intermediate species transforms into the end-product species at a slower rate than the precursor transforms into the intermediate species; otherwise, there would be a negligible abundance of intermediate species. It is also found that the crystalline silicate abundance is correlated tightly with disk geometry, in the sense of higher crystallinity accompanying more-settled disks, which are commonly associated with growth and settling of grains. The abundance of large grains is also correlated with disks that are more highly settled, but with a wide range of large grain abundance for a given degree of settling. We interpret this range as that the settling of large grains is sensitive to individual disk properties. We also find that lower-mass stars have higher abundances of large grains in their inner regions.
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dust processing and grain growth in protoplanetary disks in the taurus auriga star forming region
arXiv: Astrophysics, 2008Co-Authors: B Sargent, W J Forrest, C Tayrien, Melissa Mcclure, D M Watson, G C Sloan, P Manoj, C J Bohac, Elise Furlan, K H KimAbstract:Mid-infrared spectra of 65 T Tauri stars (TTS) taken with the Infrared Spectrograph (IRS) on board the Spitzer Space Telescope are modeled using dust at two temperatures to probe the radial variation in dust composition in the uppermost layers of protoplanetary disks. Most spectra indicating crystalline Silicates require Mg-rich minerals and silica, but a few suggest otherwise. Spectra indicating abundant enstatite at higher temperatures also require crystalline Silicates at temperatures lower than those required for spectra showing high abundance of other crystalline Silicates. A few spectra show 10 micron complexes of very small equivalent width. They are fit well using abundant crystalline Silicates but very few large grains, inconsistent with the expectation that low peak-to-continuum ratio of the 10 micron complex always indicates grain growth. Most spectra in our sample are fit well without using the opacities of large crystalline silicate grains. If large grains grow by agglomeration of submicron grains of all dust types, the amorphous silicate components of these aggregates must typically be more abundant than the crystalline silicate components. Crystalline silicate abundances correlate positively with other such abundances, suggesting that crystalline Silicates are processed directly from amorphous Silicates and that neither forsterite, enstatite, nor silica are intermediate steps when producing either of the other two. Disks with more dust settling typically have greater crystalline abundances. Large-grain abundance is somewhat correlated with greater settling of disks. The lack of strong correlation is interpreted to mean that settling of large grains is sensitive to individual disk properties. Lower-mass stars have higher abundances of large grains in their inner regions.
