Presolar Silicate Grains Research Articles

Abundant presolar grains and primordial organics preserved in carbon-rich exogenous clasts in asteroid Ryugu.

Preliminary analyses of asteroid Ryugu samples show kinship to aqueously altered CI (Ivuna-type) chondrites, suggesting similar origins. We report identification of C-rich, particularly primitive clasts in Ryugu samples that contain preserved presolar silicate grains and exceptional abundances of presolar SiC and isotopically anomalous organic matter. The high presolar silicate abundance (104 ppm) indicates that the clast escaped extensive alteration. The 5 to 10 times higher abundances of presolar SiC (~235 ppm), N-rich organic matter, organics with N isotopic anomalies (1.2%), and organics with C isotopic anomalies (0.2%) in the primitive clasts compared to bulk Ryugu suggest that the clasts formed in a unique part of the protoplanetary disk enriched in presolar materials. These clasts likely represent previously unsampled outer solar system material that accreted onto Ryugu after aqueous alteration ceased, consistent with Ryugu's rubble pile origin.

Diverse assemblage of presolar and solar system materials in anhydrous interplanetary dust particles: Coordinated NanoSIMS and TEM analyses

A coordinated TEM and NanoSIMS isotopic imaging study of microtome sections of three anhydrous interplanetary dust particles (IDPs) revealed a diverse collection of primitive materials having disparate origins and histories. Presolar silicate grains that likely originated in asymptotic giant branch (AGB) stars were present in each IDP at abundances ranging from 140 (+320/-120) ppm to 2000 (+4600/-1700) ppm. A unique compound presolar grain was identified that consisted of a crystalline spinel core and amorphous silicate mantle having heterogeneous Fe content. This compound grain traces the changing conditions in the circumstellar region during condensation and is the first identified presolar spinel in an IDP. A presolar SiC grain, also rare in IDPs, was found to be enriched in 13C, 14N, and 28Si, consistent with mainstream SiC that originated in ∼ solar metallicity AGB stars. We determine presolar spinel and presolar SiC abundances of 760 (+1700/-630) ppm and 190 (+440/-160) ppm, respectively, in the individual IDPs.Two elongate whisker-like enstatite grains and one platy enstatite were found to have near-terrestrial O isotopic compositions (δ18O = -17 – 18 ‰) and show chemical evidence of equilibrium condensation from a high temperature gas. Two highly 16O-rich silicates with near-solar O isotopic compositions (δ18O = -79 ‰ and -83 ‰) were also identified and may represent the primordial dust reservoir. These silicates were crystalline equilibrated aggregates. The wide range of isotopic compositions observed in these silicate grains suggests they condensed from isotopically diverse reservoirs in the protoplanetary disk in different locations and/or times. The 16O-rich grains likely condensed in the inner solar system and were subsequently transported to the outer solar system, while grains having terrestrial O isotopic compositions likely condensed from the gas phase in the terrestrial planet forming region or beyond.The IDPs showed bulk 15N enrichments (δ15N = 15 – 129 ‰) and contained 15N-rich hotspots up to 1150 ‰, consistent with the presence of molecular cloud material. IDPs U2015D21 and W7013E17 had bulk O isotopic compositions that were offset from the carbonaceous chondrite anhydrous minerals line by ∼ 10 ‰ to more 17O-rich compositions. This 17O enrichment cannot be explained by the observed abundance of 17O-rich presolar grains in these particles and the source remains unknown. IDP W7027E6 had an unusual isotopically heavy bulk O isotopic composition (δ17,18O = 40 ‰, Δ17O = 19 ‰). W7027E6 lacked hydrous phases and was therefore not likely altered by isotopically heavy primordial water. We propose that the high temperature mineral assemblage in W7027E6 condensed in the inner solar system from an 16O-poor reservoir that existed prior to O isotope homogenization in the early nebula and was subsequently transported to the outer solar system.

Coordinated Analyses of an Altered Presolar Silicate Grain in the Miller Range 07687 Carbonaceous Chondrite

An abstract is not available for this content so a preview has been provided. As you have access to this content, a full PDF is available via the ‘Save PDF’ action button.

Coordinated chemical and microstructural analyses of presolar silicate grains from AGB/RGB stars and supernovae in the CO3.0 chondrite Dominion Range 08006

AbstractWe report the structural and chemical analyses of six presolar silicate grains identified in situ in the CO3.0 carbonaceous chondrite Dominion Range (DOM) 08006. Two of the grains have O‐isotopic compositions consistent with origins in the circumstellar envelopes of low‐mass (<2M☉) asymptotic giant branch (AGB)/red giant branch (RGB) stars, although without Mg‐isotopic data, origins in supernovae (SNe) cannot be ruled out. The other four grains have O‐isotopic compositions consistent with origins in the ejecta of type‐II SNe. Transmission electron microscopy analyses reveal that all grains are crystalline (single crystal or polycrystalline) and have varied compositions. The analyzed AGB/RGB grains include an Fe‐rich crystalline olivine with an Fe‐sulfide inclusion and a chemically zoned olivine grain that also contains an Fe‐oxide rim. The grains derived from SNe include two polycrystalline assemblages with structures that overlap with both olivine and pyroxene, an assemblage composed of both a single crystal of forsterite and polycrystalline forsterite, and an orthopyroxene grain with an embedded Fe‐sulfide crystal. The thermodynamic origins of both AGB/RGB and SN grains are also diverse. The structure and compositions of two grains are consistent with equilibrium thermodynamic predictions of condensation, whereas four are not, suggesting formation through nonequilibrium or multistep processes. Our observations of presolar silicate grains suggest that the circumstellar envelopes of AGB/RGB stars and the ejecta of SNe can produce grains with comparable structures and compositions.

Microstructural and Chemical Investigations of Presolar Silicates from Diverse Stellar Environments

Abstract We report the structural and chemical investigation of nine presolar silicate grains from the CH3/CBb3 chondrite Isheyevo and CR2 chondrite Northwest Africa (NWA) 801. Five of these grains belong to group 1, likely condensed in low- to intermediate-mass asymptotic giant branch (AGB) stars, super-AGB stars, or core-collapse supernovae, while the remaining four grains belong to group 4 and have a supernova origin. The advanced transmission electron microscopy and associated electron spectroscopy analyses show a diverse range of chemical and structural compositions for presolar silicates. Two GEMS (glass with embedded metal and sulfide)-like silicates, each from different groups, condensed under nonequilibrium conditions in stellar outflows. Two nonstoichiometric silicates from group 1 have dissimilar formation and alteration histories. An amorphous silicate from group 1 with olivine-like [(Mg,Fe)2SiO4] composition likely formed as a crystalline olivine that subsequently amorphized in the interstellar medium. An oldhamite (CaS) grain within a stoichiometric enstatite (MgSiO3) from group 1 probably formed by heterogeneous condensation in circumstellar outflows. Of the two crystalline grains from group 4, one is an antigorite [(Mg,Fe)3Si2O5(OH)4], while the other is a nontronite [Na,Fe2(Si,Al)4O10(OH)2.nH2O], both formed as a crystalline forsterite and later altered to have hydrated silicate composition. A group-4 silicate has a chemical composition similar to a low Ca-pyroxene [(Ca,Mg)(Si,Al)2O6]. Our data imply that presolar grains from different groups can have a similar range of grain-formation conditions.

Isotope Systematics of Presolar Silicate Grains: New Insights from Magnesium and Silicon

Abstract We report on Mg and Si isotope data of 86 presolar silicate grains identified through NanoSIMS oxygen ion imaging in thin sections of carbonaceous and ordinary chondrites. The O, Mg, and Si isotope data of 106 presolar silicates (including grains studied previously by our group) suggest division of O isotope Group 1 grains into four subpopulations: (i) “normal,” (ii) 25Mg-rich, (iii) 26Mg-rich, and (iv) 25Mg-poor. Normal Group 1 grains (∼60% of Group 1 grains) formed in the winds of low-mass asymptotic giant branch (AGB) stars, with Mg and Si defining linear arrays with slopes of ∼0.9 and 1.37, respectively, in three-isotope representations, most likely representing Galactic chemical evolution (GCE). The 25Mg-rich grains (∼25%) show enrichments in 25Mg of up to a factor 2.4 relative to solar composition and most likely formed in supernova (SN) ejecta or the winds of intermediate-mass AGB stars. The 26Mg-rich and 25Mg-poor Group 1 grains lie below the Mg GCE line and their isotopic compositions favor origins from supergiants or SNe. The O isotope Group 2 grains show a wide range of Mg-isotopic compositions, similar to Group 1 grains, with likely origins from massive AGB stars, super-AGB stars, supergiants, and SNe. The Mg- and Si-isotopic compositions of Group 4 grains are compatible with previously proposed SN origins. Our results suggest that >30% of presolar silicates formed in the winds of supergiants and in SN ejecta, and that low-mass AGB stars appear to have contributed only some 50% to presolar silicates, less than previously thought.

Presolar Silicate and Oxide Grains Found in Lithic Clasts from Isheyevo and the Fine-grained Matrix of Northwest Africa 801

Abstract We report on the discovery of 33 oxygen-anomalous grains from the CH3/CBb3 chondrite Isheyevo and the CR2 chondrite Northwest Africa (NWA) 801. Oxygen isotopic compositions indicate the origin of the majority grains in stellar outflows of low-mass (∼1.2 to ∼2.2 M ⊙), solar-metallicity red giant or asymptotic giant branch stars, while highly 17O-enriched grains probably have nova origins. Isotopic compositions of the eight 18O-rich grains, including an extremely 18O-rich grain (∼16 times solar 18O/16O ratio), are reproduced by zone mixing of SNe II ejecta. Close-to-normal silicon, magnesium, and calcium isotopic compositions of grains are consistent with the isotope exchange in the interstellar medium or the meteorite parent body, while two grains with Si isotopic anomalies and one grain with Mg isotopic anomalies reflect the Galactic chemical evolution. An Isheyevo clast showed several hot spots with moderate to high 15N enrichments, including a hot spot with an extreme 15N excess of (7225 ± 316)‰. However, no correlation between 15N enrichment and presolar oxygen-rich grain abundance is found. Grains with elliptical shapes probably indicate primary condensation feature. Two complex grains possibly display decoupling of the isotopic and elemental compositions in the grain formation environments. The low silicate-to-oxide abundance ratio for the fine-grained chondrule rims in NWA 801 likely reflects the preferential destruction of silicates due to terrestrial weathering. In NWA 801, the presolar O-rich grain abundance in fine-grained chondrule rims is higher than in the interchondrule matrix, similar to the trend observed for some aqueously altered chondrites of petrologic type 2.

NanoSIMS isotopic investigation of xenolithic carbonaceous clasts from the kapoeta howardite

We report on the investigation of presolar grains and organic matter (OM) in 14 xenolithic carbonaceous clasts (C-clasts) identified in the Kapoeta howardite based on high-resolution NanoSIMS hydrogen, carbon, nitrogen, and oxygen isotopic imaging data. The 14 C-clasts are ∼50–200 μm in size and consist of one CM-like and 13 CI-like clasts, which are classified based on their mineralogies. The clasts from this study are likely sourced from an ice-bearing parent body, either an icy asteroid or a comet, originating from the outer solar system according to the following mineralogical observations: (1) in two CI-like clasts, embayments of magnetite grains between the C-clast and the host howardite point to aqueous alteration occurring on Vesta as a result of melting the ice embedded in the C-clasts; (2) all of the C-clasts, especially the 13 CI-like clasts, likely originated from the same parent body, because (i) the 14 C-clasts are clustered in the thin section, and (ii) the clasts show a much higher ratio of CI-like to CM-like clasts with respect to those reported in the literature. Four presolar silicon carbide (SiC) and two presolar silicate grains were identified in the C-clasts. In addition, all the C-clasts contain moderate bulk D- and 15N-enrichments with the presence of sub-micron to micron-sized D and 15N hotspots, indicating the presence of primitive organic material. Comparison of the abundances and isotopic compositions of presolar grains and OM in these C-clasts with literature data for different samples of primitive extraterrestrial material provides support for (1) the genetic linkage of xenolithic C-clasts to highly aqueously altered but minimally heated carbonaceous chondritic materials and (2) a homogeneous distribution of circumstellar and interstellar materials in the protoplanetary disk. The low amounts of heat experienced by the C-clasts suggest that they arrived at Vesta and/or Vestoids at low speeds after the late heavy bombardment in the inner solar system ∼3.5–4.0 Gyr ago.

The presolar grain inventory of fine‐grained chondrule rims in the Mighei‐type (CM) chondrites

AbstractWe investigated the inventory of presolar silicate, oxide, and silicon carbide (SiC) grains of fine‐grained chondrule rims in six Mighei‐type (CM) carbonaceous chondrites (Banten, Jbilet Winselwan, Maribo, Murchison, Murray and Yamato 791198), and the CM‐related carbonaceous chondrite Sutter's Mill. Sixteen O‐anomalous grains (nine silicates, six oxides) were detected, corresponding to a combined matrix‐normalized abundance of ~18 ppm, together with 21 presolar SiC grains (~42 ppm). Twelve of the O‐rich grains are enriched in 17O, and could originate from low‐mass asymptotic giant branch stars. One grain is enriched in 17O and significantly depleted in 18O, indicative of additional cool bottom processing or hot bottom burning in its stellar parent, and three grains are of likely core‐collapse supernova origin showing enhanced 18O/16O ratios relative to the solar system ratio. We find a presolar silicate/oxide ratio of 1.5, significantly lower than the ratios typically observed for chondritic meteorites. This may indicate a higher degree of aqueous alteration in the studied meteorites, or hint at a heterogeneous distribution of presolar silicates and oxides in the solar nebula. Nevertheless, the low O‐anomalous grain abundance is consistent with aqueous alteration occurring in the protosolar nebula and/or on the respective parent bodies. Six O‐rich presolar grains were studied by Auger Electron Spectroscopy, revealing two Fe‐rich silicates, one forsterite‐like Mg‐rich silicate, two Al‐oxides with spinel‐like compositions, and one Fe‐(Mg‐)oxide. Scanning electron and transmission electron microscopic investigation of a relatively large silicate grain (490 nm × 735 nm) revealed that it was crystalline åkermanite (Ca2Mg[Si2O7]) or a an åkermanite‐diopside (MgCaSi2O6) intergrowth.

Presolar grains in primitive ungrouped carbonaceous chondrite Northwest Africa 5958

AbstractWe report a correlated NanoSIMS‐transmission electron microscopy study of the ungrouped carbonaceous chondrite Northwest Africa (NWA) 5958. We identified 10 presolar SiC grains, 2 likely presolar graphite grains, and 20 presolar silicate and/or oxide grains in NWA 5958. We suggest a slight modification of the commonly used classification system for presolar oxides and silicates that better reflects the grains’ likely stellar origins. The matrix‐normalized presolar SiC abundance in NWA 5958 is ppm (2σ) similar to that seen in many classes of unmetamorphosed chondrites. In contrast, the matrix‐normalized abundance of presolar O‐rich phases (silicates and oxides) is ppm (2σ), much lower than seen in interplanetary dust particles and the least‐altered CR, CO, and ungrouped C chondrites, but close to that reported for CM chondrites. NanoSIMS mapping also revealed an unusual 13C‐enriched (δ13C≈100–200‰) carbonaceous rim surrounding a 1.4 μm diameter phyllosilicate grain. Transmission electron microscopy (TEM) analysis of two presolar grains with a likely origin in asymptotic giant branch stars identified one as enstatite and one as Al‐Mg spinel with minor Cr. The enstatite grain amorphized rapidly under the electron beam, suggesting partial hydration. TEM data of NWA 5958 matrix confirm that it has experienced aqueous alteration and support the suggestion of Jacquet et al. (34) that this meteorite has affinities to CM2 chondrites.

Survivability of presolar oxygen isotopic signature of amorphous silicate dust in the protosolar disk

AbstractOxygen isotope exchange experiments between tens of nanometer‐sized amorphous enstatite grains and water vapor were carried out under a condition of protoplanetary disk‐like low water vapor pressure in order to investigate the survivability of distinct oxygen isotope signatures of presolar silicate grains in the protosolar disk. Oxygen isotope exchange between amorphous enstatite and water vapor proceeded at 923–1003 K and 0.3 Pa of water vapor through diffusive isotope exchange in the amorphous structure. The rate of diffusive isotope exchange is given by D (m2 s–1) = (5.0 ± 0.2) × 10–21 exp[–161.3 ± 1.7 (kJ mol–1) R–1 (1/T–1/1200)]. The activation energy for the diffusive isotope exchange for amorphous enstatite is the same as that for amorphous forsterite within the analytical uncertainties, but the isotope exchange rate is ~30 times slower in amorphous enstatite because of the difference in frequency factor of the reaction. The reaction kinetics indicates that 0.1–1 μm‐sized presolar amorphous silicate dust with enstatite and forsterite compositions would avoid oxygen isotope exchange with protosolar disk water vapor only if they were kept at temperatures below ~500–650 K within the lifetime of the disk gas.

High abundances of presolar grains and 15N-rich organic matter in CO3.0 chondrite Dominion Range 08006

NanoSIMS C-, N-, and O-isotopic mapping of matrix in CO3.0 chondrite Dominion Range (DOM) 08006 revealed it to have in its matrix the highest abundance of presolar O-rich grains (257 +76/−96 ppm, 2σ) of any meteorite. It also has a matrix abundance of presolar SiC of 35 (+25/−17, 2σ) ppm, similar to that seen across primitive chondrite classes. This provides additional support to bulk isotopic and petrologic evidence that DOM 08006 is the most primitive known CO meteorite. Transmission electron microscopy of five presolar silicate grains revealed one to have a composite mineralogy similar to larger amoeboid olivine aggregates and consistent with equilibrium condensation, two non-stoichiometric amorphous grains, and two olivine grains, though one is identified as such solely based on its composition. We also found insoluble organic matter (IOM) to be present primarily as sub-micron inclusions with ranges of C- and N-isotopic anomalies similar to those seen in primitive CR chondrites and interplanetary dust particles. In contrast to other primitive extraterrestrial materials, H isotopic imaging showed normal and homogeneous D/H. Most likely, DOM 08006 and other CO chondrites accreted a similar complement of primitive and isotopically anomalous organic matter to that found in other chondrite classes and IDPs, but the very limited amount of thermal metamorphism experienced by DOM 08006 has caused loss of D-rich organic moieties, while not substantially affecting either the molecular carriers of C and N anomalies or most inorganic phases in the meteorite. One C-rich grain that was highly depleted in 13C and 15N was identified; we propose it originated in the Sun’s parental molecular cloud.

A study of presolar material in hydrated lithic clasts from metal‐rich carbonaceous chondrites

AbstractWe report on the investigation of presolar grain inventories of hydrated lithic clasts in three metal‐rich carbonaceous chondrites from the CR clan, Acfer 182 (CH3), Isheyevo (CH3/CBb3), and Lewis Cliff (LEW) 85332 (C3‐un), as well as the carbon‐ and nitrogen‐isotopic compositions of the fine‐grained clast material. Eleven presolar silicate grains as well as nine presolar silicon carbide (SiC) grains were identified in the clasts. Presolar silicate abundances range from 4 to 22 parts per million (ppm), significantly lower than in pristine meteorites and interplanetary dust particles (IDP), and comparable to recent findings for CM2s and CR2 interchondrule matrix. SiC concentrations lie between 9 and 23 ppm, and are comparable to the values for CI, CM, and CR chondrites. The results of our investigation suggest similar alteration pathways for the clast material, the interchondrule matrix of the CR2 chondrites, and the fine‐grained fraction of CM2 chondrites. Fine‐grained matter of all three meteorites contains moderate to high 15N‐enrichments (~50‰ ≤ δ15N ≤ ~1600‰) compared to the terrestrial value, indicating the presence of primitive organic material. We observed no correlation between 15N‐enrichments and presolar dust concentrations in the clasts. This is in contrast to the findings from a suite of primitive IDPs, which display in several cases enhanced bulk 15N/14N ratios and high presolar grain abundances of several hundred or even thousand ppm. The bulk 15N/14N ratios of the clasts are comparable to the range for primitive IDPs, suggesting a nitrogen carrier less susceptible to destruction by aqueous alteration than silicate stardust.

The Application of Auger Spectroscopy in Planetary Science: Elemental Analysis of Presolar Silicate Grains.

The Application of Auger Spectroscopy in Planetary Science: Elemental Analysis of Presolar Silicate Grains.

Presolar silicates in the matrix and fine-grained rims around chondrules in primitive CO3.0 chondrites: Evidence for pre-accretionary aqueous alteration of the rims in the solar nebula

To investigate the origin of fine-grained rims around chondrules (FGRs), we compared presolar grain abundances, elemental compositions and mineralogies in fine-grained interstitial matrix material and individual FGRs in the primitive CO3.0 chondrites Allan Hills A77307, LaPaz Icefield 031117 and Dominion Range 08006. The observation of similar overall O-anomalous (∼155ppm) and C-anomalous grain abundances (∼40ppm) in all three CO3.0 chondrites suggests that they all accreted from a nebular reservoir with similar presolar grain abundances. The presence of presolar silicate grains in FGRs combined with the observation of similar estimated porosity between interstitial matrix regions and FGRs in LAP 031117 and ALHA77307, as well as the identification of a composite FGR (a small rimmed chondrule within a larger chondrule rim) in ALHA77307, all provide evidence for a formation of FGRs by accretion of dust grains onto freely-floating chondrules in the solar nebula before their aggregation into their parent body asteroids. Our study also shows systematically lower abundances of presolar silicate grains in the FGRs than in the matrix regions of CO3 chondrites, while the abundances of SiC grains are the same in all areas, within errors. This trend differs from CR2 chondrites in which the presolar silicate abundances are higher in the FGRs than in the matrix, but similar to each other within 2σ errors. This observation combined with the identification of localized (micrometer-scaled) aqueous alteration in a FGR of LAP 031117 suggests that the lower abundance of presolar silicates in FGRs reflects pre-accretionary aqueous alteration of the fine-grained material in the FGRs. This pre-accretionary alteration could be due to either hydration and heating of freely floating rimmed chondrules in icy regions of the solar nebula or melted water ice associated with 26Al-related heating inside precursor planetesimals, followed by aggregation of FGRs into the CO chondrite parent-body.

Diffusion of Oxygen Isotopes in Thermally Evolving Planetesimals and Size Ranges of Presolar Silicate Grains

Abstract Presolar grains are small particles found in meteorites through their isotopic compositions, which are considerably different from those of materials in the solar system. If some isotopes in presolar grains diffused out beyond their grain sizes when they were embedded in parent bodies of meteorites, their isotopic compositions could be washed out, and hence the grains could no longer be identified as presolar grains. We explore this possibility for the first time by self-consistently simulating the thermal evolution of planetesimals and the diffusion length of 18O in presolar silicate grains. Our results show that presolar silicate grains smaller than ∼0.03 μm cannot keep their original isotopic compositions even if the host planetesimals experienced a maximum temperature as low as 600 °C. Since this temperature corresponds to that experienced by petrologic type 3 chondrites, isotopic diffusion can constrain the size of presolar silicate grains discovered in such chondrites to be larger than ∼0.03 μm. We also find that the diffusion length of 18O reaches ∼0.3–2 μm in planetesimals that were heated up to 700–800°C. This indicates that, if the original size of presolar grains spans a range from ∼0.001 μm to ∼0.3 μm like that in the interstellar medium, then the isotopic records of the presolar grains may be almost completely lost in such highly thermalized parent bodies. We propose that isotopic diffusion could be a key process to control the size distribution and abundance of presolar grains in some types of chondrites.

MINERALOGY OF PRESOLAR SILICATE AND OXIDE GRAINS OF DIVERSE STELLAR ORIGINS

ABSTRACT We report the chemical and structural analysis of nine presolar silicate grains and one presolar oxide grain from the ungrouped chondrite Acfer 094 and the CR chondrite Queen Alexandra Range 99177. Oxygen isotopic analyses indicate that five of these grains condensed in the outflows of asymptotic giant branch (AGB) stars, four have supernova (SN) origins, and one grain likely has a nova origin. Transmission electron microscopy studies show that most of the grains are amorphous with widely varying non-stoichiometric chemical compositions. Three crystalline AGB grains were identified: a clinoenstatite-containing grain assemblage, a Fe-rich olivine grain, and a nanocrystalline enstatite grain encased in an amorphous silicate shell. An amorphous stoichiometric enstatite (MgSiO3) SN grain likely condensed as a crystal and was later rendered amorphous. We do not observe a systematic difference in the chemistries and mineralogies of presolar silicates from different stellar sources, suggesting that the grains formed under a similar range of conditions.

Presolar silicate grains: Abundances, isotopic and elemental compositions, and the effects of secondary processing

Presolar silicate grains: Abundances, isotopic and elemental compositions, and the effects of secondary processing

CAN STAR CLUSTER ENVIRONMENT AFFECT DUST INPUT FROM MASSIVE AGB STARS?

We examine the fraction of massive asymptotic giant branch (AGB) stars remaining bound in their parent star clusters and the effect of irradiation of these stars by intracluster ultraviolet (UV) field. We employ a set of N-body models of dynamical evolution of star clusters rotating in a galactic potential at the solar galactocentric radius. The cluster models are combined with stellar evolution formulae, a library of stellar spectra, and simple models for SiO photodissociation in circumstellar environment (CSE). The initial stellar masses of clusters are varied from $50\rm M_\odot$ to $10^{5}\rm M_\odot$. Results derived for individual clusters are combined using a mass distribution function for young star clusters. We find that about 30% of massive AGB stars initially born in clusters become members of the field population, while the rest evolves in star clusters. They are irradiated by strong intracluster UV radiation resulting in the decrease of the photodissociation radius of SiO molecules, in many stars down to the dust formation zone. In absence of dust shielding, the UV photons penetrate in the CSE deeper than $10R_*$ in 64% and deeper than $2 R_*$ in 42% of all massive AGB stars. If this suppresses following dust formation, the current injection rate of silicate dust from AGB stars in the local Galaxy decreases from $2.2 \times 10^{-4}\rm M_\odot\,kpc^{-2}\,Gyr^{-1}$ to $1.8 \times 10^{-4}\rm M_\odot\,kpc^{-2}\,Gyr^{-1}$ at most. A lower revised value of 40% for the expected fraction of presolar silicate grains from massive AGB stars is still high to explain the non-detection of these grains in meteorites.

Water and organics in interplanetary dust particles

AbstractInterplanetary dust particles (IDPs) and larger micrometeorites (MMs) impinge on the upper atmosphere where they decelerate at 90 km altitude and settle to the Earths surface. Comets and asteroids are the major sources and the flux, 30,000-40,000 tons/yr, is comparable to the mass of larger meteorites impacting the Earths surface. The sedimentary record suggests that the flux was much higher on the early Earth. The chondritic porous (CP) subset of IDPs together with their larger counterparts, ultracarbonaceous micrometeorites (UCMMs), appear to be unique among known meteoritic materials in that they are composed almost exclusively of anhydrous minerals, some of them contain >> 50% organic carbon by volume as well as the highest abundances of presolar silicate grains including GEMS. D/H and 15N abundances implicate the Oort Cloud or presolar molecular cloud as likely sources of the organic carbon. Prior to atmospheric entry, IDPs and MMs spend 104-105 year lifetimes in solar orbit where their surfaces develop amorphous space weathered rims from exposure to the solar wind (SW). Similar rims are observed on lunar soil grains and on asteroid Itokawa regolith grains. Using valence electron energy-loss spectroscopy (VEELS) we have detected radiolytic water in the rims on IDPs formed by the interaction of solar wind protons with oxygen in silicate minerals. Therefore, IDPs and MMs continuously deliver both water and organics to the earth and other terrestrial planets. The interaction of protons with oxygen-rich minerals to form water is a universal process.