Browsing by Subject "chemical"
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Item A Survey: Destruction of Chemical Agent Simulants in Supercritical Water Oxidadtion(University of Texas at Austin, 1992-10) Blank, M.R.; Gloyna, E.F.Item Chemical analyses of Texas rocks and minerals(University of Texas at Austin, 1918-03-05) Schoch, Eugene P.Item Chemical Laboratory(University of Texas at Austin, 1885-08) The University of Texas at AustinItem Contributions From the Chemical Laboratory(University of Texas at Austin, 1886) Everhart, EdgarItem Contributions from the Chemical Laboratory(University of Texas at Austin, 1887) Everhart, EdgarItem Elemental Abundances Of Solar Sibling Candidates(2014-06) Ramirez, Ivan; Bajkova, A. T.; Bobylev, V. V.; Roederer, Ian U.; Lambert, David L.; Endl, Michael; Cochran, William D.; MacQueen, Phillip J.; Wittenmyer, Robert A.; Bajkova, A. T.; Lambert, David L.; Endl, Michael; Cochran, William D.; MacQueen, Phillip J.Dynamical information along with survey data on metallicity and in some cases age have been used recently by some authors to search for candidates of stars that were born in the cluster where the Sun formed. We have acquired high-resolution, high signal-to-noise ratio spectra for 30 of these objects to determine, using detailed elemental abundance analysis, if they could be true solar siblings. Only two of the candidates are found to have solar chemical composition. Updated modeling of the stars' past orbits in a realistic Galactic potential reveals that one of them, HD 162826, satisfies both chemical and dynamical conditions for being a sibling of the Sun. Measurements of rare-element abundances for this star further confirm its solar composition, with the only possible exception of Sm. Analysis of long-term high-precision radial velocity data rules out the presence of hot Jupiters and confirms that this star is not in a binary system. We find that chemical tagging does not necessarily benefit from studying as many elements as possible but instead from identifying and carefully measuring the abundances of those elements that show large star-to-star scatter at a given metallicity. Future searches employing data products from ongoing massive astrometric and spectroscopic surveys can be optimized by acknowledging this fact.Item Exploring Anticorrelations And Light Element Variations In Northern Globular Clusters Observed By The APOGEE Survey(2015-05) Meszaros, Szabolcs.; Martell, Sarah L.; Shetrone, Matthew; Lucatello, Sara; Troup, Nicholas W.; Bovy, Jo; Cunha, Katia; Garcia-Hernandez, Domingo A.; Overbeek, Jamie C.; Prieto, Carlos Allende; Beers, Timothy C.; Frinchaboy, Peter M.; Perez, Ana E. Garcia; Hearty, Fred R.; Holtzman, Jon; Majewski, Steven R.; Nidever, David L.; Schiavon, Ricardo P.; Schneider, Donald P.; Sobeck, Jennifer S.; Smith, Verne V.; Zamora, Olga; Zasowski, Gail; Shetrone, Matthew D.We investigate the light-element behavior of red giant stars in northern globular clusters (GCs) observed by the SDSS-III Apache Point Observatory Galactic Evolution Experiment. We derive abundances of 9 elements (Fe, C, N, O, Mg, Al, Si, Ca, and Ti) for 428 red giant stars in 10 GCs. The intrinsic abundance range relative to measurement errors is examined, and the well-known C-N and Mg-Al anticorrelations are explored using an extreme-deconvolution code for the first time in a consistent way. We find that Mg and Al drive the population membership in most clusters, except in M107 and M71, the two most metal-rich clusters in our study, where the grouping is most sensitive to N. We also find a diversity in the abundance distributions, with some clusters exhibiting clear abundance bimodalities (for example M3 and M53) while others show extended distributions. The spread of Al abundances increases significantly as cluster average metallicity decreases as previously found by other works, which we take as evidence that low metallicity, intermediate mass AGB polluters were more common in the more metal-poor clusters. The statistically significant correlation of [Al/Fe] with [Si/Fe] in M15 suggests that Si-28 leakage has occurred in this cluster. We also present C, N, and O abundances for stars cooler than 4500 K and examine the behavior of A(C+N+O) in each cluster as a function of temperature and [Al/Fe]. The scatter of A(C+N+O) is close to its estimated uncertainty in all clusters and independent of stellar temperature. A(C+N+O) exhibits small correlations and anticorrelations with [Al/Fe] in M3 and M13, but we cannot be certain about these relations given the size of our abundance uncertainties. Star-to-star variations of alpha-element (Si, Ca, Ti) abundances are comparable to our estimated errors in all clusters.Item Identifying Contributions To The Stellar Halo From Accreted, Kicked-Out, And In Situ Populations(2012-12) Sheffield, Allyson A.; Majewski, Steven R.; Johnston, Kathryn V.; Cunha, Katia; Smith, Verne V.; Cheung, Andrew M.; Hampton, Christina M.; David, T. J.; Wagner-Kaiser, Rachel.; Johnson, Marshall C.; Kaplan, Evan; Miller, Jacob; Patterson, Richard J.; Johnson, Marshall C.We present a medium-resolution spectroscopic survey of late-type giant stars at mid-Galactic latitudes of (30 degrees < | b| < 60 degrees), designed to probe the properties of this population to distances of similar to 9 kpc. Because M giants are generally metal-rich and we have limited contamination from thin disk stars by the latitude selection, most of the stars in the survey are expected to be members of the thick disk (<[Fe/H]> similar to -0.6) with some contribution from the metal-rich component of the nearby halo. Here we report first results for 1799 stars. The distribution of radial velocity (RV) as a function of l for these stars shows (1) the expected thick disk population and (2) local metal-rich halo stars moving at high speeds relative to the disk, which in some cases form distinct sequences in RV-l space. High-resolution echelle spectra taken for 34 of these "RV outliers" reveal the following patterns across the [Ti/Fe]-[Fe/H] plane: 17 of the stars have abundances reminiscent of the populations present in dwarf satellites of the Milky Way, 8 have abundances coincident with those of the Galactic disk and a more metal-rich halo, and 9 of the stars fall on the locus defined by the majority of stars in the halo. The chemical abundance trends of the RV outliers suggest that this sample consists predominantly of stars accreted from infalling dwarf galaxies. A smaller fraction of stars in the RV outlier sample may have been formed in the inner Galaxy and subsequently kicked to higher eccentricity orbits, but the sample is not large enough to distinguish conclusively between this interpretation and the alternative that these stars represent the tail of the velocity distribution of the thick disk. Our data do not rule out the possibility that a minority of the sample could have formed from gas in situ on their current orbits. These results are consistent with scenarios where the stellar halo, at least as probed by M giants, arises from multiple formation mechanisms; however, when taken at face value, our results for metal-rich halo giants suggest a much higher proportion to be accreted than found by Carollo et al. and more like the fraction suggested in the analysis by Nissen & Schuster and Schuster et al. We conclude that M giants with large RVs can provide particularly fruitful samples to mine for accreted structures and that some of the velocity sequences may indeed correspond to real physical associations resulting from recent accretion events.Item Interstellar CN And CH+ In Diffuse Molecular Clouds: C-12/C-13 Ratios And Cn Excitation(2011-02) Ritchey, A. M.; Federman, S. R.; Lambert, David L.; Ritchey, A. M.; Federman, S. R.; Lambert, David L.We present very high signal-to-noise ratio absorption-line observations of CN and CH+ along 13 lines of sight through diffuse molecular clouds. The data are examined to extract precise isotopologic ratios of (CN)-C-12/(CN)-C-13 and (CH+)-C-12/(CH+)-C-13 in order to assess predictions of diffuse cloud chemistry. Our results on (CH+)-C-12/(CH+)-C-13 confirm that this ratio does not deviate from the ambient C-12/C-13 ratio in local interstellar clouds, as expected if the formation of CH+ involves nonthermal processes. We find that (CN)-C-12/(CN)-C-13, however, can be significantly fractionated away from the ambient value. The dispersion in our sample of (CN)-C-12/(CN)-C-13 ratios is similar to that found in recent surveys of (CO)-C-12/(CO)-C-13. For sight lines where both ratios have been determined, the (CN)-C-12/(CN)-C-13 ratios are generally fractionated in the opposite sense compared to (CO)-C-12/(CO)-C-13. Chemical fractionation in CO results from competition between selective photodissociation and isotopic charge exchange (ICE). An inverse relationship between (CN)-C-12/(CN)-C-13 and (CO)-C-12/(CO)-C-13 follows from the coexistence of CN and CO in diffuse cloud cores. However, an ICE reaction with CN may mitigate the enhancements in (CN)-C-12/(CN)-C-13 for lines of sight with low (CO)-C-12/(CO)-C-13 ratios. For two sight lines with high values of (CO)-C-12/(CO)-C-13, our results indicate that about 50% of the carbon is locked up in CO, which is consistent with the notion that these sight lines probe molecular cloud envelopes where the transition from C+ to CO is expected to occur. An analysis of CN rotational excitation yields a weighted mean value for T-01((CN)-C-12) of 2.754 +/- 0.002 K, which implies an excess over the temperature of the cosmic microwave background (CMB) of only 29 +/- 3 mK. This modest excess eliminates the need for a local excitation mechanism beyond electron and neutral collisions. The rotational excitation temperatures in (CN)-C-13 show no excess over the temperature of the CMB.Item Manganese Abundances In The Globular Cluster Omega Centauri(2010-07) Cunha, Katia; Smith, Verne V.; Bergemann, Maria; Suntzeff, Nicholas B.; Lambert, David L.; Lambert, David L.We present manganese abundances in 10 red giant members of the globular cluster omega Centauri; eight stars are from the most metal-poor population (RGB MP and RGB MInt1) while two targets are members of the more metal-rich groups (RGB MInt2 and MInt3). This is the first time Mn abundances have been studied in this peculiar stellar system. The LTE values of [Mn/Fe] in omega Cen overlap those of Milky Way stars in the metal-poor. Cen populations ([Fe/H] similar to -1.5 to -1.8), however unlike what is observed in Milky Way halo and disk stars, [Mn/Fe] declines in the two more metal-rich RGB MInt2 and MInt3 targets. Non-LTE calculations were carried out in order to derive corrections to the LTE Mn abundances. The non-LTE results for omega Cen in comparison with the non-LTE [Mn/Fe] versus [Fe/H] trend obtained for the Milky Way confirm and strengthen the conclusion that the manganese behavior in omega Cen is distinct. These results suggest that low-metallicity supernovae (with metallicities <= -2) of either Type II or Type Ia dominated the enrichment of the more metal-rich stars in omega Cen. The dominance of low-metallicity stars in the chemical evolution of omega Cen has been noted previously in the s-process elements where enrichment from metal-poor asymptotic giant branch stars is indicated. In addition, copper, which also has metallicity-dependent yields, exhibits lower values of [Cu/Fe] in the RGB MInt2 and MInt3 omega Cen populations.Item Programmable Three-Dimensional Self-Assembly(2015) Glazener, William Stanton; Willson, Carlton GrantThe development of modern technology has emphasized the utility of nanomaterials and efficient design of nanostructures. Self-assembly is a powerful, bottom-up approach for the fabrication of nanomaterials. The Hammer and Tongs method that we have developed signifies the advent of a holistic approach to programmable, three-dimensional self-assembly. This method integrates biochemistry and chemical engineering techniques to create DNA-mediated, self-assembling polymer cubes. Selective functionalization of cubic faces with complimentary DNA sequences provides a programmable route for self-assembly. The development of this method and its process steps demonstrates the potential to form complex structures on extremely small scales.Item The Stellar Content Of The Hamburg/ESO Survey V. The Metallicity Distribution Function Of The Galactic Halo(2009-11) Schorck, T.; Christlieb, N.; Cohen, J. G.; Beers, T. C.; Shectman, S.; Thompson, I.; McWilliam, A.; Bessell, M. S.; Norris, J. E.; Melendez, J.; Ramirez, S.; Haynes, D.; Cass, P.; Hartley, M.; Russell, K.; Watson, F.; Zickgraf, F. J.; Behnke, B.; Fechner, C.; Fuhrmeister, B.; Barklem, P. S.; Edvardsson, B.; Frebel, A.; Wisotzki, L.; Reimers, D.; Frebel, A.We determine the metallicity distribution function (MDF) of the Galactic halo by means of a sample of 1638 metal-poor stars selected from the Hamburg/ESO objective-prism survey (HES). The sample was corrected for minor biases introduced by the strategy for spectroscopic follow-up observations of the metal-poor candidates, namely >best and brightest stars first>. Comparison of the metallicities [Fe/H] of the stars determined from moderate-resolution (i.e., R similar to 2000) follow-up spectra with results derived from abundance analyses based on high-resolution spectra (i.e., R > 20 000) shows that the [Fe/H] estimates used for the determination of the halo MDF are accurate to within 0.3 dex, once highly C-rich stars are eliminated. We determined the selection function of the HES, which must be taken into account for a proper comparison between the HES MDF with MDFs of other stellar populations or those predicted by models of Galactic chemical evolution. The latter show a reasonable agreement with the overall shape of the HES MDF for [Fe/H] > -3.6, but only a model of Salvadori et al. (2007) with a critical metallicity for low-mass star formation of Z(cr) = 10(-3.4) Z(circle dot) reproduces the sharp drop at [Fe/H] similar to -3.6 present in the HES MDF. Although currently about ten stars at [Fe/H] < -3.6 are known, the evidence for the existence of a tail of the halo MDF extending to [Fe/H] similar to -5.5 is weak from the sample considered in this paper, because it only includes two stars [Fe/H] < -3.6. Therefore, a comparison with theoretical models has to await larger statistically complete and unbiased samples. A comparison of the MDF of Galactic globular clusters and of dSph satellites to the Galaxy shows qualitative agreement with the halo MDF, derived from the HES, once the selection function of the latter is included. However, statistical tests show that the differences between these are still highly significant.Item Supercritical Water Oxidation of Dimethyl Methylphosphonate and Thiodiglycol(University of Texas at Austin, 1994-12) Turner, M.D.; Gloyna, E.F.Item Vector Spring 2014(Student Engineering Council, 2014) University of Texas at AustinVector is the student engineering magazine on campus. Published by the Student Engineering Council, Vector is completely written, managed, and designed by students for students. With issues dating back to 1971, the magazine has a long-standing tradition of serving as the voice for engineering students at The University of Texas at Austin. The Vector staff publishes two issues per semester. For more information regarding the Vector magazine, please contact us at vector@ sec.engr.utexas.edu.Item Vector Winter 2012(Student Engineering Council, 2012) University of Texas at AustinVector is the student engineering magazine on campus. Published by the Student Engineering Council, Vector is completely written, managed, and designed by students for students. With issues dating back to 1971, the magazine has a long-standing tradition of serving as the voice for engineering students at The University of Texas at Austin. The Vector staff publishes two issues per semester. For more information regarding the Vector magazine, please contact us at vector@ sec.engr.utexas.edu.Item VLT/FLAMES Spectroscopy Of Red Giant Branch Stars In The Fornax Dwarf Spheroidal Galaxy(2014-12) Lemasle, B.; de Boer, T. J. L.; Hill, V.; Tolstoy, E.; Irwin, M. J.; Jablonka, P.; Venn, K.; Battaglia, G.; Starkenburg, E.; Shetrone, M.; Letarte, B.; Francois, P.; Helmi, A.; Primas, F.; Kaufer, A.; Szeifert, T.; Shetrone, Matthew D.Context. Fornax is one of the most massive dwarf spheroidal galaxies in the Local Group. The Fornax field star population is dominated by intermediate age stars but star formation was going on over almost its entire history. It has been proposed that Fornax experienced a minor merger event. Aims. Despite recent progress, only the high metallicity end of Fornax field stars ([Fe/H] > -1.2 dex) has been sampled in larger number via high resolution spectroscopy. We want to better understand the full chemical evolution of this galaxy by better sampling the whole metallicity range, including more metal poor stars. Methods. We use the VLT-FLAMES multi-fibre spectrograph in high-resolution mode to determine the abundances of several a, iron-peak and neutron-capture elements in a sample of 47 individual red giant branch stars in the Fornax dwarf spheroidal galaxy. We combine these abundances with accurate age estimates derived from the age probability distribution from the colour-magnitude diagram of Fornax. Results. Similar to other dwarf spheroidal galaxies, the old, metal-poor stars of Fornax are typically alpha-rich while the young metal-rich stars are a-poor. In the classical scenario of the time delay between Type II (SNe II) and Type Ia Supernovae (SNe la), we confirm that SNe la started to contribute to the chemical enrichment at [Fe/H] between -2.0 and -1.8 dex. We find that the onset of SNe Ia took place between 12-10 Gyr ago. The high values of [Ba/Fe], [La/Fe] reflect the influence of SNe Ia and AGB stars in the abundance pattern of the younger stellar population of Fornax. Conclusions. Our findings of low [alpha/Fe] and enhanced [Eu/Mg] are compatible with an initial mass function that lacks the most massive stars and with star formation that kept going on throughout the whole history of Fornax. We find that massive stars kept enriching the interstellar medium in alpha-elements, although they were not the main contributor to the iron enrichment.Item VLT/FLAMES Spectroscopy Of Red Giant Branch Stars In The Fornax Dwarf Spheroidal Galaxy(2014-12) Lemasle, B.; de Boer, T. J. L.; Hill, V.; Tolstoy, E.; Irwin, M. J.; Jablonka, P.; Venn, K.; Battaglia, G.; Starkenburg, E.; Shetrone, M.; Letarte, B.; Francois, P.; Helmi, A.; Primas, F.; Kaufer, A.; Szeifert, T.; Shetrone, Matthew D.Context. Fornax is one of the most massive dwarf spheroidal galaxies in the Local Group. The Fornax field star population is dominated by intermediate age stars but star formation was going on over almost its entire history. It has been proposed that Fornax experienced a minor merger event. Aims. Despite recent progress, only the high metallicity end of Fornax field stars ([Fe/H] > -1.2 dex) has been sampled in larger number via high resolution spectroscopy. We want to better understand the full chemical evolution of this galaxy by better sampling the whole metallicity range, including more metal poor stars. Methods. We use the VLT-FLAMES multi-fibre spectrograph in high-resolution mode to determine the abundances of several a, iron-peak and neutron-capture elements in a sample of 47 individual red giant branch stars in the Fornax dwarf spheroidal galaxy. We combine these abundances with accurate age estimates derived from the age probability distribution from the colour-magnitude diagram of Fornax. Results. Similar to other dwarf spheroidal galaxies, the old, metal-poor stars of Fornax are typically alpha-rich while the young metal-rich stars are a-poor. In the classical scenario of the time delay between Type II (SNe II) and Type Ia Supernovae (SNe la), we confirm that SNe la started to contribute to the chemical enrichment at [Fe/H] between -2.0 and -1.8 dex. We find that the onset of SNe Ia took place between 12-10 Gyr ago. The high values of [Ba/Fe], [La/Fe] reflect the influence of SNe Ia and AGB stars in the abundance pattern of the younger stellar population of Fornax. Conclusions. Our findings of low [alpha/Fe] and enhanced [Eu/Mg] are compatible with an initial mass function that lacks the most massive stars and with star formation that kept going on throughout the whole history of Fornax. We find that massive stars kept enriching the interstellar medium in alpha-elements, although they were not the main contributor to the iron enrichment.