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Chemical Evolution (chemical + evolution)
Selected AbstractsFactors controlling the chemical evolution of travertine-depositing rivers of the Barkly karst, northern AustraliaHYDROLOGICAL PROCESSES, Issue 15 2002Russell N. Drysdale Abstract Groundwaters feeding travertine-depositing rivers of the northeastern segment of the Barkly karst (NW Queensland, Australia) are of comparable chemical composition, allowing a detailed investigation of how the rate of downstream chemical evolution varies from river to river. The discharge, pH, temperature, conductivity and major-ion concentrations of five rivers were determined by standard field and laboratory techniques. The results show that each river experiences similar patterns of downstream chemical evolution, with CO2 outgassing driving the waters to high levels of calcite supersaturation, which in turn leads to widespread calcium carbonate deposition. However, the rate at which the waters evolve, measured as the loss of CaCO3 per kilometre, varies from river to river, and depends primarily upon discharge at the time of sampling and stream gradient. For example, Louie Creek (Q = 0·11 m3 s,1) and Carl Creek (Q = 0·50 m3 s,1) have identical stream gradients, but the loss of CaCO3 per kilometre for Louie Creek is twice that of Carl Creek. The Gregory River (Q = 3·07 m3 s,1), O'Shanassy River (Q = 0·57 m3 s,1) and Lawn Hill Creek (Q = 0·72 m3 s,1) have very similar gradients, but the rate of hydrochemical evolution of the Gregory River is significantly less than either of the other two systems. The results have major implications for travertine deposition: the stream reach required for waters to evolve to critical levels of calcite supersaturation will, all others things being equal, increase with increasing discharge, and the length of reach over which travertine is deposited will also increase with increasing discharge. This implies that fossil travertine deposits preserved well downstream of modern deposition limits are likely to have been formed under higher discharge regimes. Copyright © 2002 John Wiley & Sons, Ltd. [source] Thermal aging of a blend of high-performance fibersJOURNAL OF APPLIED POLYMER SCIENCE, Issue 5 2010Carlos Arrieta Abstract The focus of this work is the study of the thermal aging of high-performance fibers used in the making of fire protective garments. Accelerated thermal aging tests were carried out on fabric samples made up of a blend of Kevlar® (poly p -phenylene terephthalamide) and PBI (poly benzimidazole) staple fibers, as well as on yarns pulled from this fabric, by means of exposure to elevated temperatures, comprised between 190°C and 320°C. All samples underwent loss of breaking force retention. The material thermal life, defined as the time required for the fibers to attain a 50% reduction of the original breaking force, ranged between a dozen of days at the lowest exposure temperature, to less than an hour at the highest. Breaking force data were fitted using the Arrhenius model following two different approaches, namely the extrapolated thermal life value and the shift factors yielded by the time-temperature superposition (TTS). The Arrhenius model seemed to describe appropriately the overall aging process, as inferred from the excellent fit obtained when using both approaches, although activation energies provided from both approaches are different. To follow the chemical evolution of the material with thermal aging, Fourier-transform infrared (FTIR) analyses were conducted. The qualitative analysis of the FTIR spectra showed little evidence of chemical changes between the aged and the nonaged samples, indicating either that the aging process carries on without significant modification of the chemical structure of the fibers, or that FTIR is not an appropriate method to spot such a modification. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010 [source] A method for calculating effective bulk composition modification due to crystal fractionation in garnet-bearing schist: implications for isopleth thermobarometryJOURNAL OF METAMORPHIC GEOLOGY, Issue 6 2004T. P. Evans Abstract Quantitative P,T path determination in metamorphic rocks is commonly based on the variation in composition of growth-zoned garnet. However, some component of growth zoning in garnet is necessarily the result of an effective bulk composition change within the rock that has been generated by crystal fractionation of components into the core of garnet. Therefore, any quantitative calculation of the P,T regime of garnet growth should be completed using an accurate assessment of the composition of the chemical system from which garnet is growing. Consequently, a method for calculating the extent of crystal fractionation that provides a means of estimating the composition of the unfractionated rock at any stage during garnet growth is developed. The method presented here applies a Rayleigh fractionation model based on measured Mn content of garnet to generate composition v. modal proportion curves for garnet, and uses those curves to estimate the vectors of crystal fractionation. The technique is tested by calculating the precision of the equilibrium between three garnet compositional variables within the chemical system determined to be appropriate for each of a series of microprobe analyses from garnet. Application of the fractionation calculations in conjunction with the P,T estimates based on intersecting compositional isopleths provides a means of calculating P,T conditions of garnet growth that is based on individual point-analyses on a garnet grain. Such spatially precise and easily obtainable P,T data allow for detailed parallel studies of the microstructural, the P,T, and the chemical evolution of metamorphosed pelites. This method provides a means of studying the dynamics of orogenic systems at a resolution that was previously unattainable. [source] Clay mineral-organic matter relationships in the early solar systemMETEORITICS & PLANETARY SCIENCE, Issue 12 2002Victoria K. Pearson These organic-rich meteorites provide a valuable and tangible record of the chemical steps taken towards the origin of life in the early solar system. Chondritic organic matter is present in the inorganic meteorite matrix which, in the CM and CI chondrites, contains evidence of alteration by liquid water on the parent asteroid. An unanswered and fundamental question is to what extent did the organic matter and inorganic products of aqueous alteration interact or display interdependence? We have used an organic labelling technique to reveal that the meteoritic organic matter is strongly associated with clay minerals. This association suggests that clay minerals may have had an important trapping and possibly catalytic role in chemical evolution in the early solar system prior to the origin of life on the early Earth. [source] Niobium and tantalum in carbonaceous chondrites: Constraints on the solar system and primitive mantle niobium/tantalum, zirconium/niobium, and niobium/uranium ratioMETEORITICS & PLANETARY SCIENCE, Issue 2 2000K. p. Jochum The abundance of Ta was determined in the same meteorites by radiochemical neutron activation analysis (RNAA). Precision of the MIC-SSMS and RNAA techniques is ,3% and , 5%, respectively. The new abundances for CI chondrites are: Nb = 0.247, Ta = 0.0142, Zr = 3.86, Y = 1.56 ,g/g; or 0.699, 0.0202, 11.2, and 4.64 atoms/106 Si atoms, respectively. The values agree with earlier compilations, but they are a factor of 2 more precise than earlier analyses. Trace element concentrations in the CM, CV, and CK chondrites are higher than in the CI chondrite Orgueil by about 37, 86, and 120%, respectively, in agreement with the variable absolute contents of refractory lithophile elements in different groups of carbonaceous chondrites. Of particular interest are the chondritic Nb/Ta, Zr/Nb, and Nb/U ratios, because these ratios are important tools for interpreting the chemical evolution of planetary bodies. We obtained Nb/Ta = 17.4 ± 0.5 for the carbonaceous chondrites and the Juvinas-type eucrites investigated. Though this value is similar to previous estimates, it is much more precise. The same is true for Zr/Nb (15.5 ± 0.2) and Zr/Y (2.32 ± 0.12). In combination with recently published MIC-SSMS U data for carbonaceous chondrites, we obtained a chondritic Nb/U ratio of 29 ± 2. Because Nb, Ta, Zr, Y, and U are refractory lithophile elements and presumably partitioned into the silicate phase of the Earth during core formation, the elemental ratios may also be used to constrain evolution of the Earth's primitive mantle and, with the more precise determinations fractionation of Nb and Ta during magmatic processes and mantle-crust interactions, can now be interpreted with greater confidence. [source] Global properties of the rich cluster ABCG 209 at z, 0.2. Spectroscopic and photometric catalogueMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2008A. Mercurio ABSTRACT This paper is aimed at giving an overview of the global properties of the rich cluster of galaxies ABCG 209. This is achieved by complementing the already available data with new medium-resolution spectroscopy and near-infrared (NIR) photometry which allow us to (i) analyse in detail the cluster dynamics, distinguishing among galaxies belonging to different substructures and deriving their individual velocity distributions, using a total sample of 148 galaxies in the cluster region, of which 134 belonging to the cluster; (ii) derive the cluster NIR luminosity function; (iii) study the Kormendy relation and the photometric plane of cluster early-type galaxies (ETGs). Finally we provide an extensive photometric (optical and NIR) and spectroscopic data set for such a complex system to be used in further analyses investigating the nature, formation and evolution of rich clusters of galaxies. The present study shows that the cluster is characterized by a very high value of the line-of-sight velocity dispersion: ,v= 1268+93,84 km s,1, that results in a virial mass of Mvir= 2.95+0.80,0.78× 1015 h,170 M, within Rvir= 3.42 h,170 Mpc. The analysis of the velocity dispersion profile shows that such high value of ,v is already reached in the central cluster region. There is evidence of three significant substructures, the primary one having a velocity dispersion of ,v= 847+52,49 km s,1, which makes it consistent with mass estimates from weak lensing analyses. This observational scenario confirms that ABCG 209 is presently undergoing strong dynamical evolution with the merging of two or more subclumps. This interpretation is also supported by the detection of a radio halo suggesting that there is a recent or ongoing merging. Cluster ETGs follow a Kormendy relation whose slope is consistent with previous studies both at optical and NIR wavelengths. We investigate the origin of the intrinsic scatter of the photometric plane due to trends of stellar populations, using line indices as indicators of age, metallicity and ,/Fe enhancement. We find that the chemical evolution of galaxies could be responsible for the intrinsic dispersion of the photometric plane. [source] Life in the last lane: star formation and chemical evolution in an extremely gas rich dwarfMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2008Ayesha Begum ABSTRACT We present an analysis of H i, H, and oxygen abundance data for NGC 3741. This galaxy has a very extended gas disc (,8.8 times the Holmberg radius), and a dark-to-luminous (i.e. stellar) mass ratio of ,149, which makes it one of the ,darkest' dwarf irregular galaxies known. However, its ratio of baryon (i.e. gas + stellar) mass to dark mass is typical of that in galaxies. Our new high-resolution H i images of the galaxy show evidence for a large-scale (purely gaseous) spiral arm and central bar. From our H i data, a rotation curve can be derived out to ,37,44 disc scalelengths in the J and B bands, respectively. This is just slightly short of the radius at which one would expect a Navarro,Frenk,White type rotation curve to start falling. The galaxy has an integrated star formation rate (SFR) of ,0.0034 M, yr,1, while the average SFR within the optical disc is ,0.0049 M, yr,1 kpc,2. Despite the gaseous spiral feature and the ongoing star formation, we find that the global gas density in NGC 3741 is significantly lower than the Toomre instability criterion. This is consistent with the behaviour seen in other dwarf galaxies. We also find that the SFR is consistent with that expected from the observed correlations between H i mass and SFR and the global Kennicutt,Schmidt law, respectively. We measure the oxygen abundance to be 12 + log(O/H) = 7.66 ± 0.10, which is consistent with that expected from the metallicity,luminosity relation, despite its extreme gas mass ratio. We also examine the issue of chemical evolution of NGC 3741 in the context of the closed-box model of chemical evolution. The effective oxygen yield of NGC 3741 is consistent with recent model estimates of closed-box yields, provided one assumes that the gas has been efficiently mixed all the way to the edge of the H i disc (i.e. greater than eight times the optical radius). This seems a priori unlikely. On the other hand, using a sample of galaxies with both interferometric H i maps and chemical abundance measurements, we find that the effective yield is anticorrelated with the total dynamical mass, as expected in leaky box models. [source] Revisiting two local constraints of the Galactic chemical evolutionMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2006M. Haywood ABSTRACT I review the uncertainties in two observational local constraints of the Galactic disc chemical evolution: the metallicity distribution of long-lived dwarfs and the age,metallicity relation. Analysing most recent data, it is shown first that the observed metallicity distribution at solar galactocentric radius, designed with standard methods, is more fit to a closed-box model than to the infall metallicity distribution. We argue that this is due to the specific contribution of the thick-disc population, which has been overlooked both in the derivation of the observed metallicity distribution and in the standard chemical evolution models. Although this agreement disqualifies the metallicity distribution as the best supportive (indirect) evidence for infall, we argue that the evolution must be more complex than described by either the closed-box or the standard infall models. It is then shown that recent determinations of the age,metallicity distribution (AMD) from large Strömgren photometric surveys are dominated by noise resulting from systematic biases in metallicities and effective temperatures. These biases are evaluated and a new AMD is obtained, where particularities of the previous determinations are phased out. The new age,metallicity relation shows a mean increase limited to about a factor of 2 in Z over the disc age. It is shown that below 3 Gyr, the dispersion in metallicity is about 0.1 dex, which, given the observational uncertainties in the derived metallicities, is compatible with the small cosmic dispersion measured on the interstellar medium and meteoritic pre-solar dust grains. A population that is progressively older and more metal rich arises at a metallicity greater than that of the Hyades, to reach [Fe/H],+0.5 dex at ages greater than 5 Gyr. We suggest that this is best explained by radial migration. A symmetrical widening of the metallicity interval towards lower values is seen at about the same age, which is attributed to a similar cause. Finally, the new derived ages are sufficiently consistent that an age,metallicity relation within the thick disc is confirmed. These new features altogether draw a picture of the chemical evolution in the solar neighbourhood where dynamical effects and complexity in the AMD dominate, rather than a generalized high dispersion at all ages. [source] Dwarf elliptical galaxies: structure, star formation and colour,magnitude diagramsMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2001Giovanni Carraro The aim of this paper is to cast light on the formation and evolution of elliptical galaxies by means of N -body hydrodynamical simulations that include star formation, feedback and chemical evolution. Particular attention is paid to the case of dwarf spheroidals of the Local Group which, thanks to their proximity and modern ground-based and space instrumentation, can be resolved into single stars so that independent determinations of their age and star formation history can be derived. Indeed, the analysis of the colour,magnitude diagram of their stellar content allows us to infer the past history of star formation and chemical enrichment, thus setting important constraints on galactic models. Dwarf galaxies are known to exhibit complicated histories of star formation ranging from a single very old episode to a series of bursts over most of the Hubble time. By understanding the physical process driving star formation in these objects, we might be able to infer the mechanism governing star formation in more massive elliptical galaxies. Given these premises, we start from virialized haloes of dark matter, and follow the infall of gas into the potential wells and the formation of stars. We find that in objects of the same total mass, different star formation histories are possible, if the collapse phase started at different initial densities. We predict the final structure of dwarf spheroidal galaxies, their kinematics, their large-scale distribution of gas and stars, and their detailed histories of the star formation and metal enrichment. Using a population synthesis technique, star formation and metal enrichment rates are then adopted to generate the present colour,magnitude diagrams of the stellar populations hosted by dwarf spheroidal galaxies. The simulations are made assuming the redshift of galaxy formation and varying the cosmological parameters H0 and q0. The resulting colour,magnitude diagrams are then compared with the observational ones for some dwarf spheroidals of the Local Group. [source] Constraining the star formation histories of spiral bulgesMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2000R. N. Proctor Stellar populations in spiral bulges are investigated using the Lick system of spectral indices. Long-slit spectroscopic observations of line strengths and kinematics made along the minor axes of four spiral bulges are reported. Comparisons are made between central line strengths in spiral bulges and those in other morphological types [elliptical, spheroidal (Sph) and S0]. The bulges investigated are found to have central line strengths comparable to those of single stellar populations of approximately solar abundance or above. Negative radial gradients are observed in line strengths, similar to those exhibited by elliptical galaxies. The bulge data are also consistent with correlations between Mg2, Mg2 gradient and central velocity dispersion observed in elliptical galaxies. In contrast to elliptical galaxies, central line strengths lie within the loci defining the range of ,Fe, and Mg2 achieved by Worthey's solar abundance ratio, single stellar populations (SSPs). The implication of solar abundance ratios indicates significant differences in the star formation histories of spiral bulges and elliptical galaxies. A ,single zone with infall' model of galactic chemical evolution, using Worthey's SSPs, is used to constrain the possible star formation histories of our sample. We show that the ,Fe,, Mg2 and H, line strengths observed in these bulges cannot be reproduced using primordial collapse models of formation but can be reproduced by models with extended infall of gas and star formation (2,17 Gyr) in the region modelled. One galaxy (NGC 5689) shows a central population with a luminosity-weighted average age of ,5 Gyr, supporting the idea of extended star formation. Kinematic substructure, possibly associated with a central spike in metallicity, is observed at the centre of the Sa galaxy NGC 3623. [source] H2 reformation in post-shock regionsMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY: LETTERS (ELECTRONIC), Issue 1 2010H. M. Cuppen ABSTRACT H2 formation is an important process in post-shock regions, since H2 is an active participant in the cooling and shielding of the environment. The onset of H2 formation therefore has a strong effect on the temperature and chemical evolution in the post-shock regions. We recently developed a model for H2 formation on a graphite surface in warm conditions. The graphite surface acts as a model system for grains containing large areas of polycyclic aromatic hydrocarbon structures. Here, this model is used to obtain a new description of the H2 formation rate as a function of gas temperature that can be implemented in molecular shock models. The H2 formation rate is substantially higher at high gas temperatures as compared to the original implementation of this rate in shock models, because of the introduction of H atoms which are chemically bonded to the grain (chemisorption). Since H2 plays such a key role in the cooling, the increased rate is found to have a substantial effect on the predicted line fluxes of an important coolant in dissociative shocks [O i] at 63.2 and 145.5 ,m. With the new model, a better agreement between the model and observations is obtained. Since one of the goals of Herschel/PACS will be to observe these lines with higher spatial resolution and sensitivity than the former observations by Infrared Space Observatory -LWS, this more accurate model is very timely to help with the interpretation of these future results. [source] Stellar archaeology: Exploring the Universe with metal-poor starsASTRONOMISCHE NACHRICHTEN, Issue 5 2010A. Frebel Abstract The abundance patterns of the most metal-poor stars in the Galactic halo and small dwarf galaxies provide us with a wealth of information about the early Universe. In particular, these old survivors allow us to study the nature of the first stars and supernovae, the relevant nucleosynthesis processes responsible for the formation and evolution of the elements, early star- and galaxy formation processes, as well as the assembly process of the stellar halo from dwarf galaxies a long time ago. This review presents the current state of the field of "stellar archaeology" , the diverse use of metal-poor stars to explore the high-redshift Universe and its constituents. In particular, the conditions for early star formation are discussed, how these ultimately led to a chemical evolution, and what the role of the most iron-poor stars is for learning about Population III supernovae yields. Rapid neutron-capture signatures found in metal-poor stars can be used to obtain stellar ages, but also to constrain this complex nucleosynthesis process with observational measurements. Moreover, chemical abundances of extremely metal-poor stars in different types of dwarf galaxies can be used to infer details on the formation scenario of the halo and the role of dwarf galaxies as Galactic building blocks. I conclude with an outlook as to where this field may be heading within the next decade. A table of ~ 1000 metal-poor stars and their abundances as collected from the literature is provided in electronic format (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] The effect of clouds on the dynamical and chemical evolution of gas-rich dwarf galaxiesASTRONOMISCHE NACHRICHTEN, Issue 9-10 2009S. Recchi Abstract We study the effects of clouds on the dynamical and chemical evolution of gas-rich dwarf galaxies, in particular focusing on two model galaxies similar to IZw18 and NGC 1569. We consider both scenarios, clouds put at the beginning of the simulation and continuously created infalling ones. Due to dynamical processes and thermal evaporation, the clouds survive only a few tens of Myr, but during this time they act as an additional cooling agent and the internal energy ofcloudy models is typically reduced by 20,40% in comparison with models without clouds. The clouds delay the development of large-scale outflows, therefore helping to retain a larger amount of gas inside the galaxy. However, especially in models with continuous creation of infalling clouds, their bullet effect can pierce the expanding supershell and create holes through which the superbubble can vent freshly produced metals. Moreover, assuming a pristine chemical composition for the clouds, their interaction with the superbubble dilutes the gas, reducing the metallicity (by up to ,0.4 dex) with respect to the one attained by diffuse models (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] The evolution of spheroidal galaxies in different environmentsASTRONOMISCHE NACHRICHTEN, Issue 9-10 2009A. Fritz Abstract We analyse the kinematic and chemical evolution of 203 distant spheroidal (elliptical and S0) galaxies at 0.2 < z < 0.8 which are located in different environments (rich clusters, low-mass clusters and in the field). VLT/FORS and CAHA/MOSCA spectra with intermediate-resolution have been acquired to measure the internal kinematics and stellar populations of the galaxies. From HST/ACS and WFPC2 imaging, surface brightness profiles and structural parameters were derived for half of the galaxy sample. The scaling relations of the Faber-Jackson relation and Kormendy relation as well as the Fundamental Plane indicate a moderate evolution for the whole galaxy population in each density regime. In all environments, S0 galaxies show a faster evolution than elliptical galaxies. For the cluster galaxies a slight radial dependence of the evolution out to one virial radius is found. Dividing the samples with respect to their mass, a mass dependent evolution with a stronger evolution of lower-mass galaxies (M < 2 × 1011 M,) is detected. Evidence for recent star formation is provided by blue colours and weak [OII] emission or strong H, absorption features in the spectra. The results are consistent with a down-sizing formation scenario which is independent from the environment of the galaxies (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Nucleosynthesis and early chemical evolution in galaxiesASTRONOMY & GEOPHYSICS, Issue 2 2003Stelios A Tsangarides Nucleosynthesis and galactic chemical evolution are inter-linked topics that merge various fields in astronomy and physics. Speakers at the RAS discussion meeting of January 2003 combined theory and observation in understanding these fields. Stelios A Tsangarides, Enrico Arnone and Sean G Ryan report. [source] The Possible Influence of L -Histidine on the Origin of the First Peptides on the Primordial EarthCHEMISTRY & BIODIVERSITY, Issue 6 2006Hannes Reiner Abstract One of the most unsettled problems of prebiotic evolution and the origin of life is the explanation why one enantiomeric form of biomolecules prevailed. In the experiments presented in this paper, the influence of L -histidine on the peptide formation in the Salt-Induced Peptide Formation (SIPF) reaction of the enantiomeric forms of valine, proline, serine, lysine, and tryptophan, and the catalytic effects in this first step toward the first building blocks of proteins on the primordial earth were investigated. In the majority of the produced dipeptides, a remarkable increase of yields was shown, and the preference of the L - amino acids in the peptide formation in most cases cannot be denied. In summary, our data provide further experimental evidence for the plausibility of the SIPF reaction and point at a possible important role of L -histidine in the chemical evolution on the primordial earth. [source] | |||||||||||||