The influences of convective overshooting and semiconvection on the chemical evolution of massive stars

In massive stars,convection in the interior is different from that of intermediate and small mass stars. In the main-sequence phase of small mass stars,there is a convective core and a radiative envelope,between which are the radiative intermediate layers with uneven chemical abundances. Semiconvection would occur in the intermediate layers between the convective core and the homogeneous envelope in massive stars. We treat core convective overshooting and semiconvection together as a process. We found that when decreasing overshooting,the semiconvection is more pronounced. In these two processes,we introduce one diffusive parameter D,which is different from other authors who have introduced different parameters for these two zones. The influences of the turbulent diffusion process on chemical evolution and other quantities of the stellar structure are shown in the present paper.     

A cometary aggregate interplanetary dust particle as an analog for comet Wild 2 grain chemistry preserved in silica‐rich Stardust glass

Abstract— Many of the nanometer‐scale grains from comet 81P/Wild 2 did not survive hypervelocity capture. Instead, they melted and interacted with silica melt derived from the aerogel used by the Stardust mission. Their petrological properties were completely modified, but their bulk chemistry was preserved in the chemical signatures of mostly vesicular Si‐rich glass with its typical Fe‐Ni‐S compound inclusions. Chondritic aggregate IDP L2011A9 that experienced atmospheric pre‐entry thermal modification was selected as an analog to investigate these Wild 2 chemical signatures. The chemical, petrologic, and mineralogical properties of the individual constituents in this aggregate IDP are presented and used to match the chemical signatures of these Wild 2 grains. Mixing of comet material and pure silica, which is used in a diagram that recognizes this mixing behavior, is used to constrain the probable petrologic and minerals that caused the Wild 2 signatures. The Wild 2 nanometer‐scale grain signatures in Si‐rich glass allocations from three different deceleration tracks resembled mixtures of ultrafine‐grained principal components and dense agglomerate‐like material, Mg‐rich silicates (<500 nm) and Fe,Ni‐sulfides (<100 nm), and Si‐rich amorphous material. Dust resembling the mixed matrix of common chondritic aggregate IDPs was present in Jupiter‐family comet Wild 2.     

CHEMICAL VARIATIONS AMONG L-CHONDRITES—IV. ANALYSES,WITH PETROGRAPHIC NOTES,OF 13 L-GROUP AND 3 LL-GROUP CHONDRITES

We review our procedures for selecting, preparing and analyzing meteorite samples, present new analyses of 16 ordinary chondrites, and discuss variations of Fe, S and Si in the L-group. A tendency for Fe/Mg, S/Mg and Si/Mg to be low in L chondrites of fades d to f testifies that post-metamorphic shock melting played a significant role in the chemical diversification of the L-group. However, these ratios also vary widely and sympathetically in melt-free chondrites, indicating that much of the L-group's chemical variation arose prior to thermal metamorphism and is in that sense primary. If all L-chondrites come from one parent body, type-correlated chemical trends suggest: 1) that the body had a traditional “onion skin” structure, with metamorphic intensity increasing with depth; and 2) that it formed from material that became more homogeneous, slightly poorer in iron, and significantly richer in sulfur as accretion proceeded.     

The primary liquid condensation model and the origin of barred olivine chondrules

Barred olivine (BO) chondrules are some of the most striking objects in chondrites. Their ubiquitous presence and peculiar texture caught the attention of researchers and, as a consequence, considerable effort has been expensed on unraveling their origin(s). Here we report on a detailed study of two types of chondrules: the Classic and the Multiple-Plate Type of BO chondrules from the Essebi (CM2), Bishunpur (LL3.1), Acfer 214 (CH3) and DAG 055 (C3-UNGR) chondrites, and discuss the petrographic and chemical data of their major mineral phases and glasses. Glasses occur as mesostasis or as glass inclusions, the latter either enclosed inside the olivine bars (plates) or still connected to the mesostasis. The chemical composition of all glasses, characterized by being Si-Al-Ca-rich and free of alkali elements, is similar to those of the constituents (the building blocks, such as chondrules, aggregates, inclusions, mineral fragments, etc.) of CR and CV3 chondrites. They all have high trace element contents (∼10×CI) with unfractionated CI-normalized abundances of refractory trace elements and depletions in moderately volatile and volatile elements with respect to the refractory trace elements. The presence of alkali elements (Na + K + Rb) is coupled with a low Ca content and is only observed in those glasses that have behaved as open systems. This result supports the previous finding that Ca was replaced by alkalis (e.g., Na-Ca exchange), presumably through a vapor-solid reaction. The glasses apparently are the quenched liquid from which the olivine plates crystallized. However, they do not show any chemical fractionation that could have resulted from the crystallization of the olivines, but rather have a constant chemical compositions throughout the formation of the chondrule. In a previous contribution we were able to demonstrate the role of these liquids in supporting crystal growth directly from the vapor. Here we extend application of the primary liquid condensation model to formulate a new model for the origin of BO chondrules. The primary liquid condensation model is based on the ability of dust-enriched solar-nebula gas to directly condense into a liquid, provided the gas/dust ratio is sufficiently low. Thus, we propose that chondrules can be formed by condensation of a liquid droplet directly from the solar nebula. The extensive variability in chemical composition of BO chondrules, which ranges from alkali-poor to alkali-rich, can be explained by elemental exchange reactions with the cooling nebula. We calculate the chemical composition of the initial liquid droplet from which BO chondrules could have formed and speculate about the physical and chemical conditions that prevail in the specific regions of the solar nebula that can promote creation of these objects.     

Computer simulation of chemical nucleation

The problem of nucleation at chemical instabilities is investigated by means of microscopic computer simulation. The first-order transition of interest involves a new kind of nucleation arising from chemical transformations rather than physical forces. Here it is the chemical state of matter, and not matter itself, which is spatially localized to form the nucleus for transition between different chemical states. First, the concepts of chemical instability, nonequilibrium phase transition, and dissipative structure are reviewed briefly. Then recently developed methods of reactive molecular dynamics are used to study chemical nucleation in a simple model chemical reaction. Finally, the connection of these studies to nucleation and condensation processes involving physical and chemical interactions is explored.Invited contribution to the Proceedings of a Workshop onThermodynamics and Kinetics of Dust Formation in the Space Medium held at the Lunar and Planetary Institute, Houston, 6–8 September, 1978.     

Searching for the birthplaces of open clusters with ages of several billion years

We discuss the possibility of finding the birthplaces of open clusters (OC) with ages of several billion years. The proposed method is based on the comparison of the results of the chemical evolution modeling of the Galactic disk with the parameters of the cluster. Five OCs older than 7 Gyr are known: NGC6791, BH176, Collinder 261, Berkeley 17, and Berkeley 39. The oxygen and iron abundances in NGC6791 and the oxygen abundance in BH176 are twice the solar level, the heavy-element abundances in other clusters are close to the corresponding solar values. According to chemical evolution models, at the time of the formation of the objects considered the regions where the oxygen and iron abundances reached the corresponding levels extended out to 5 kpc from the Galactic center.At present time theOCs considered are located several kpc from the Galactic center. Some of these clusters are located extremely high, about 1 kpc above the disk midplane, i.e., they have been subject to some mechanism that has carried them into orbits uncharacteristic of this type of objects. It follows from a comparison with the results of chemical evolution that younger clusters with ages of 4–5 Gyr, e.g., NGC1193,M67, and others, may have formed in a broad range of Galactocentric distances. Their large heights above the disk midplane is sufficient to suggest that these clusters have moved away from their likely birthplaces. Clusters are carried far away from the Galactic disk until the present time: about 40 clusters with ages from 0 to 2 Gyr are observed at heights ranging from 300 to 750 pc.     

Lessons of Comet Hale-Bopp for Coma Chemistry: Observations and Theory

Many new cometary molecules — both parents and daughters — were detected in the exceptionally productive comet C/1995 O1 (Hale-Bopp).The space distribution of several of these species could be investigated from radio interferometry or from long-slit spectroscopy in the infrared. The distinction between parent species — directly sublimated from nucleus ices — and secondary species — resulting from chemical processing in the coma or produced by a secondary source — is not always clear. It is important to assess whether or not observed minor species (HCOOCH₃, HCOOH...) could be synthesized by chemical reactions favoured by the high density of the coma of comet Hale-Bopp. Chemical modelling by Rodgers and Charnley suggests that this is notthe case. CO and H₂CO are abundant cometary species which partly come from distributed sources. The nature of these sources is still a mystery. A special case, now well documented, is that of HNC, for which the abundance evolution with heliocentric distance could be observed in comet Hale-Bopp and which was observed in several much less productive comets.     

The Burnwell,Kentucky, low iron oxide chondrite fall: Description,classification and origin

Abstract— The Burnwell, Kentucky, meteorite fell as a single stone on 1990 September 4. The Burnwell meteorite has lower Fa in olivine (15.8 mol%), Fs in orthopyroxene (13.4 mol%), Co in kamacite (0.36 wt%), FeO from bulk chemical analysis (9.43 wt%), and Δ¹⁷O (0.51 ± 0.02%), and higher Fe, Ni, Co metal (19.75 wt% from bulk wet chemical analysis) than observed in H chondrites. The Burnwell meteorite plots on extensions of H-L-LL chondrite trends for each of these properties towards more reducing compositions than in H chondrites. Extensions of this trend have been previously suggested in the case of other low-FeO chondrites or silicate inclusions in the HE iron Netschaëvo, but interpretation of the evidence in these meteorites is complicated by terrestrial weathering, chemical disequilibrium or reduction. In contrast, the Burn-well meteorite is an equilibrated fall that exhibits no evidence for reduction. As such, it provides the first definitive evidence for extension of the H-L-LL ordinary chondrite trend beyond typical H values towards more reducing compositions.     

Clues on the importance of comets in the origin and evolution of the atmospheres of Titan and Earth

Earth and Titan are two planetary bodies formed far from each other. Nevertheless the chemical composition of their atmospheres exhibits common indications of being produced by the accretion, plus ulterior in-situ processing of cometary materials. This is remarkable because while the Earth formed in the inner part of the disk, presumably from the accretion of rocky planetesimals depleted in oxygen and exhibiting a chemical similitude with enstatite chondrites, Titan formed within Saturn's sub-nebula from oxygen- and volatile-rich bodies, called cometesimals. From a cosmochemical and astrobiological perspective, the study of the H, C, N, and O isotopes on Earth and Titan could be the key to decipher the processes occurred in the early stages of formation of both planetary bodies. The main goal of this paper is to quantify the presumable ways of chemical evolution of both planetary bodies, in particular the abundance of CO and N₂ in their early atmospheres. In order to do that the primeval atmospheres and evolution of Titan and Earth have been analyzed from a thermodynamic point of view. The most relevant chemical reactions involving these species and presumably important at their early stages are discussed. Then, we have interpreted the results of this study in light of the results obtained by the Cassini–Huygens mission on these species and their isotopes. Given that H, C, N, and O were preferentially depleted from inner disk materials that formed our planet, the observed similitude of their isotopic fractionation, and subsequent close evolution of Earth's and Titan's atmospheres points towards a cometary origin of Earth atmosphere. Consequently, our scenario also supports the key role of late veneers (comets and water-rich carbonaceous asteroids) enriching the volatile content of the Earth at the time of the late heavy bombardment of terrestrial planets.     

A new Picture for the Galaxy Formation

The observational constraints are of fundamental importance to build a realistic chemical evolution model. With respect to these constraints the last years have been of crucial importance and, in the case of the Milky Way, the new observational data required a revision of the previous chemical evolution models (see Chiappini et al., 1997 – CMG97) and Pagel and Tautvaisiene, 1995 – PT95). The results obtained by CMG97 from a careful comparison between model predictions and observational constraints strongly suggest that the previously adopted picture for the Galaxy formation in which the gas shed from the halo was the main contributor to the thin disk formation, is not valid anymore. With our detailed chemical evolution model we are able to put some constraints on the IMF variation and on the Deuterium primordial value. This revised version was published online in July 2006 with corrections to the Cover Date.     

Our astrochemical heritage

Our Sun and planetary system were born about 4.5 billion years ago. How did this happen, and what is the nature of our heritage from these early times? This review tries to address these questions from an astrochemical point of view. On the one hand, we have some crucial information from meteorites, comets and other small bodies of the Solar System. On the other hand, we have the results of studies on the formation process of Sun-like stars in our Galaxy. These results tell us that Sun-like stars form in dense regions of molecular clouds and that three major steps are involved before the planet-formation period. They are represented by the prestellar core, protostellar envelope and protoplanetary disk phases. Simultaneously with the evolution from one phase to the other, the chemical composition gains increasing complexity. In this review, we first present the information on the chemical composition of meteorites, comets and other small bodies of the Solar System, which is potentially linked to the first phases of the Solar System??s formation. Then we describe the observed chemical composition in the prestellar core, protostellar envelope and protoplanetary-disk phases, including the processes that lead to them. Finally, we draw together pieces from the different objects and phases to understand whether and how much we inherited chemically from the time of the Sun??s birth.     

Chemical abundances determined from meteor spectra: I. Ratios of the main chemical elements

Abstract— Relative chemical abundances of 13 meteoroids were determined by averaging the composition of the radiating gas along the fireball path that originated during their penetration into the Earth's atmosphere. Mg, Fe, Ni, Cr, Mn, and Co abundances, relative to Si, are similar to those reported for CI and CM carbonaceous chondrites and interplanetary dust particles. In contrast, relative abundances of Ca and Ti in meteor spectra indicate that these elements suffer incomplete evaporation processes. The chemical composition of all meteoroids studied in this work differs from that of 1P/Halley dust.     

Transition from Gaseous Compounds to Aerosols in Titan's Atmosphere

We investigate the chemical transition of simple molecules like C₂H₂ and HCN into aerosol particles in the context of Titan's atmosphere. Experiments that synthesize analogs (tholins) for these aerosols can help illuminate and constrain these polymerization mechanisms. Using information available from these experiments, we suggest chemical pathways that can link simple molecules to macromolecules, which will be the precursors to aerosol particles: polymers of acetylene and cyanoacetylene, polycyclic aromatics, polymers of HCN and other nitriles, and polyynes. Although our goal here is not to build a detailed kinetic model for this transition, we propose parameterizations to estimate the production rates of these macromolecules, their C/N and C/H ratios, and the loss of parent molecules (C₂H₂, HCN, HC₃N and other nitriles, and C₆H₆) from the gas phase to the haze. We use a one-dimensional photochemical model of Titan's atmosphere to estimate the formation rate of precursor macromolecules. We find a production zone slightly lower than 200 km altitude with a total production rate of 4×10⁻¹⁴ g cm⁻² s⁻¹ and a C/N?4. These results are compared with experimental data, and to microphysical model requirements. The Cassini/Huygens mission will bring a detailed picture of the haze distribution and properties, which will be a great challenge for our understanding of these chemical processes.     

Spectroscopic evidence for protostellar infall

The observational evidence for infall associated with star formation is discussed. Whilst spectral energy distributions of young protostellar objects are consistent with infall, the best direct evidence comes from millimetre and sub-millimetre spectral line observations. Considerations of the formation of the line profiles and the chemical effects of gas-grain interactions suggest that there is only a very short ‘window’ in the evolutionary track of a protostellar object during which infall is directly observable. This may explain why so few infall candidates have been detected. It is argued that self-consistent models of the dynamical and chemical evolution of collapsing cores, coupled to multiple high resolution line observations, will provide definitive evidence for the presence of infall in these objects.     

A new extraction technique for in situ analyses of amino and carboxylic acids on Mars by gas chromatography mass spectrometry

In order to target key organic compounds in the Martian regolith using gas chromatography mass spectrometry (GC-MS), we have developed a new extraction procedure coupled with chemical derivatization. This new technique was tested on a Mars analog soil sample collected from the Atacama Desert in Chile. We found that amino and carboxylic acids can be extracted from the Atacama soil in a 1:1 mixture of isopropanol and water after ultrasonic treatment for 30 min. The extracted organic compounds were then derivatized in a single-step reaction using N-methyl-N-(tert-butyldimethylsilyl)-trifluoroacetamide (MTBSTFA) as the silylating agent in order to transform these compounds into volatile species that can then be detected by GC-MS. We are currently developing a miniaturized reaction cell suited for spaceflight, where both organic extraction and chemical derivatization processes can take place in a single step.     

Using the seismology of non-magnetic chemically peculiar stars as a probe of dynamical processes in stellar interiors

Chemical composition is a good tracer of the hydrodynamical processes that occur in stars as they often lead to mixing and particle transport. By comparing abundances predicted by models and those observed in stars we can infer some constraints on those mixing processes. As pulsations in the stars are often very sensitive to chemical composition, we can use asteroseismology to probe the internal chemical composition of stars where no direct observations are possible. In this paper I focus on main sequence stars Am, λ Bootis, and HgMn stars and discuss what we can learn of mixing processes in these stars from seismology.     

Ordinary chondritic micrometeorites from the Indian Ocean

Extraterrestrial particulate materials on the Earth can originate in the form of collisional debris from the asteroid belt, cometary material, or as meteoroid ablation spherules. Signatures that link them to their parent bodies become obliterated if the frictional heating is severe during atmospheric entry. We investigated 481 micrometeorites isolated from ~300 kg of deep sea sediment, out of which 15 spherules appear to have retained signatures of their provenance, based on their textures, bulk chemical compositions, and relict grain compositions. Seven of these 15 spherules contain chromite grains whose compositions help in distinguishing subgroups within the ordinary chondrite sources. There are seven other spherules which comprise either entirely of dusty olivines or contain dusty olivines as relict grains. Two of these spherules appear to be chondrules from an unequilibrated ordinary chondrite. In addition, a porphyritic olivine pyroxene (POP) chondrule‐like spherule is also recovered. The bulk chemical composition of all the spherules, in combination with trace elements, the chromite composition, and presence of dusty olivines suggest an ordinary chondritic source. These micrometeorites have undergone minimal frictional heating during their passage through the atmosphere and have retained these features. These micrometeorites therefore also imply there is a significant contribution from ordinary chondritic sources to the micrometeorite flux on the Earth.     

Fluid mobilization of rare earth elements,Th, and U during the terrestrial alteration of H chondrites

The chemical effects of terrestrial alteration, with a particular focus on lithophile trace elements, were studied for a set of H chondrites displaying various degrees of weathering from fresh falls to altered finds collected from hot deserts. According to their trace element distributions, a considerable fraction of rare earth elements (REEs), Th, and U resides within cracks observed in weathered meteorite specimens. These cracks appear to accumulate unbound REEs locally accompanied by Th and U relative to the major element abundances, especially P and Si. The deposition of Ce is observed in cracks in the case of most of the weathered samples. Trace element maps visually confirm the accumulation of these elements in such cracks, as previously inferred based on chemical leaching experiments. Because the positive Ce anomalies and unbound REE depositions in cracks occur in all weathered samples studied here while none of such features are observed in less altered samples including falls (except for altered fall sample Nuevo Mercurio), these features are interpreted to have been caused by terrestrial weathering following chemical leaching. However, the overall effects on the bulk chemical composition remain limited as the data for all Antarctic meteorites studied in this work (except for heavily weathered sample A 09516, H6) are in good agreement with published data for unaltered meteorites.     

SPECTROSCOPY OF A GEMINID FIREBALL: ITS SIMILARITY TO COMETARY METEOROIDS AND THE NATURE OF ITS PARENT BODY

A detailed analysis of a photographic spectrum of a Geminid fireball obtained in December 14, 1961 at the Ondrejov Observatory is presented. We have computed a synthetic spectrum for the fireball and compared it with the observed spectrum assuming chemical equilibrium in the meteor head. In this way we have determined relative chemical abundances in meteor vapors. Comparing the relative chemical abundances of this Geminid meteoroid with those obtained from meteoroids associated with comets 55P/Tempel-Tuttle and 109P/Swift-Tuttle we found no significant chemical differences in main rock-forming elements. Despite of this similarity, the deepest penetration of the Geminid meteoroids and their ability to reach high rotation rates in space without fragmentation suggest that thermal processing is affecting their physical properties. We suggest that as consequence of space weathering a high-strength envelope is produced around these particles. In this picture, heating processes of the mineral phases could result in the peculiar properties observed during atmospheric entry of the Geminid meteoroids, especially their strength, which is evidenced by its resistance to ablation. Finally, although one meteoroid cannot be obviously considered as representative of the composition of its parent body, we conclude that 3200 Phaethon is able to produce millimetre-size debris nearly chondritic in composition, but the measured slight overabundance of Na would support a cometary origin for this body.     

Yamato 792947, 793408 and 82038: The most primitive H chondrites,with abundant refractory inclusions

Abstract— In this paper we report petrological and chemical data of the unusual chondritic meteorites Yamato (Y)‐792947, Y‐93408 and Y‐82038. The three meteorites are very similar in texture and chemical composition, suggesting that they are pieces of a single fall. The whole‐rock oxygen isotopes and the chemical compositions are indicative of H chondrites. In addition, the mineralogy, and the abundances of chondrule types, opaque minerals and matrices suggest that these meteorites are H3 chondrites. They were hardly affected by thermal and shock metamorphism. The degree of weathering is very low. We conclude that these are the most primitive H chondrites, H3.2–3.4 (S1), known to date. On the other hand, these chondrites contain extraordinarily high amounts of refractory inclusions, intermediate between those of ordinary and carbonaceous chondrites. The distribution of the inclusions may have been highly heterogeneous in the primitive solar nebula. The mineralogy, chemistry and oxygen isotopic compositions of inclusions studied here are similar to those in CO and E chondrites.