首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 46 毫秒
1.
Abstract— Concentrations of the trace elements Mg, Al, Ca, Ti, V, Fe, Sr, Y, Zr, Ba and Ce were determined by ion microprobe mass spectrometry in 60 individual silicon carbide (SiC) grains (in addition, Nb and Nd were determined in 20 of them), from separate KJH (size range 3.4–5.9 μm) of the Murchison carbonaceous meteorite, whose C-, N- and Si-isotopic compositions have been measured before (Hoppe et al., 1994) and provide evidence that these grains are of stellar origin. The selected SiC grains represent all previously recognized subgroups: mainstream (20 < 12C/13C < 120; 200 < 14N/15N; Si isotopes on slope 1.34 line), grains A (12C/13C < 3.5), grains B (3.5 < 12C/13C < 10), grains X (15N excesses, large 28Si excesses) and grains Y (150 < 12C/13C < 260; Si isotopes on slope 0.35 line). Data on these grains are compared with measurements on fine-grained SiC fractions. Trace-element patterns reflect both the condensation behavior of individual elements and the source composition of the stellar atmospheres. A detailed discussion of the condensation of trace elements in SiC from C-rich stellar atmospheres is given in a companion paper by Lodders and Fegley (1995). Elements such as Mg, Al, Ca, Fe and Sr are depleted because their compounds are more volatile than SiC. Elements whose compounds are believed to be more refractory than SiC can also be depleted due to condensation and removal prior to SiC condensation. Among the refractory elements, however, the heavy elements from Y to Ce (and Nd) are systematically enriched relative to Ti and V, indicating enrichments by up to a factor of 14 of the s-process elements relative to elements lighter than Fe. Such enrichments are expected if N-type carbon stars (thermally pulsing AGB stars) are the main source of circumstellar SiC grains. Large grains are less enriched than small grains, possibly because they are from different AGB stars. The trace-element patterns of subgroups such as groups A and B and grains X can at least qualitatively be understood if grains A and B come from J-type carbon stars (known to be lacking in s-process enhancements shown by N-type carbon stars) or carbon stars that had not experienced much dredge-up of He-shell material and if grains X come from supernovae. However, a remaining puzzle is how stars become carbon stars without much accompanying dredge-up of s-process elements.  相似文献   

2.
Abstract— We investigate heterogeneous nucleation and growth of graphite on precondensed TiC grains in the gas outflows from carbon‐rich asymptotic giant branch (AGB) stars employing a newly‐derived heterogeneous nucleation rate taking into account of the chemical reactions at condensation. Competition between heterogeneous and homogeneous nucleations and growths of graphite is investigated to reveal the formation conditions of the TiC core‐graphite mantle spherules found in the Murchison meteorite. It is shown that no homogeneous graphite grain condenses whenever TiC condenses prior to graphite in the plausible ranges of the stellar parameters. Heterogeneous condensation of graphite occurs on the surfaces of growing TiC grains, and prevents the TiC cores from reaching the sizes realized if all available Ti atoms were incorporated into TiC grains. The physical conditions at the formation sites of the TiC core‐graphite mantle spherules observed in the Murchison meteorite are expressed by the relation 0.2 < n?0.1 (M5/ζ)?1/2L41/4 < 0.7, where v0.1 is the gas outflow velocity at the formation site in units of 0.1 km s?1, M5 the mass loss rate in 10?5 M⊙ year?1, L4 the stellar luminosity in 104 L⊙, and M/ζ is the effective mass loss rate taking account of non‐spherical symmetry of the gas outflows. The total gas pressures Pc at the formation sites for the effective mass loss rates M/ζ = 10?5‐10?3 M⊙ year?1 correspond to 0.01 < Pc < 0.9 dyn cm?2, implying that the observed TiC core‐graphite mantle spherules are formed not only at the superwind stage but also at the earlier stage of low mass loss rates. The constraint on the C/O abundance ratio, 1 < ? ? 1.03, is imposed to reproduce the observed sizes of the TiC cores. The derived upper limit of the C/O ratio is lower than the values estimated from the calculations without taking into account of heterogeneous condensation of graphite, and is close to the lower end of the C/O ratios inferred from the astronomical observations of carbon‐rich AGB stars. Brief discussion is given on other types of graphite spherules.  相似文献   

3.
Transmission electron microscope (TEM) investigations have revealed Os, Ru, Mo‐rich refractory metal nuggets within four different presolar graphites, from both the high‐density (HD) Murchison (MUR) and low‐density (LD) Orgueil (ORG) fractions. Microstructural and chemical data suggest that these are direct condensates from the gas, rather than forming later by exsolution. The presolar refractory metal nugget (pRMN) compositions are variable (e.g., from 8 < Os atom% < 77), but follow the same chemical fractionation trends as isolated refractory metal nuggets (mRMNs) previously found in meteorites (Berg et al. 2009). From these compositions one can infer a temperature of last equilibration with the gas of 1405–1810 K (e.g., Berg et al. 2009 at approximately 100 dyne cm?2 pressure), which implies that the host graphites form over roughly the same range (in agreement with predictions) and that the pRMNs are chemically isolated from the gas when captured by graphite. Further, the pRMN compositions give evidence that HD graphites form at a higher T than LD ones. Chemical and phase similarities with the isolated mRMNs suggest that the mRMNs also condense directly from a gas, although from the early solar nebula rather than a presolar environment. Although the pRMNs themselves are too small for detection of isotopic anomalies, NanoSIMS isotopic measurements of their host graphites confirm a presolar origin for the assemblages. The two pRMN‐containing LD graphites show evidence of a supernova (SN) origin, whereas the stellar origins of the pRMNs in HD graphite are unclear, because only less‐diagnostic 12C enrichments are detectable (as is commonly true for HD graphites).  相似文献   

4.
Abstract— One hundred forty-three carbon grains, ranging in size from 2 to 8 μm, from two chemical and physical separates from the Murchison CM2 chondrite, were analyzed by ion microprobe mass spectrometry for their C- and N-isotopic compositions. Both separates are enriched in the exotic noble gas component Ne-E(L). Ninety grains were also analyzed for their H and O contents and 118, for Si. Thirteen grains were analyzed by micro-sampling laser Raman spectroscopy. Round grains have large C-isotopic anomalies with 12C/13C ratios ranging from 7 to 4500 (terrestrial ratio = 89). Nitrogen in these grains is also anomalous but shows much smaller deviations from the terrestrial composition, 14N/15N ratios ranging from 193 to 680 (terrestrial ratio = 272). Spherulitic aggregates and non-round compact grains have normal C-isotopic ratios but 15N excesses (up to 35%). Raman spectra of the analyzed grains indicate varying degrees of crystalline disorder of graphite with estimated in-plane crystallite dimensions varying from 18 Å (highly disordered, similar to terrestrial kerogen) to ～750 Å (well-crystallized graphite). Element contents of H, O, and Si are correlated with one another, and H and O are probably present in the form of organic molecules. On the basis of morphology, the round grains fall into two groups: grains with smooth, shell-like surfaces (“onions”) and grains that appear to be dense aggregates of small scales (“cauliflowers”). “Onions” tend to have lower trace element contents, isotopically light C (12C/13C > 89) and a high degree of crystalline order, whereas “cauliflowers” have a larger spread in trace element contents and C-isotopic ratios (they range from isotopically light to heavy) but tend to have a low degree of crystalline order. However, these differences exist only on average, and no clear distinction can be made for individual grains. A few limited conclusions can be drawn about the astrophysical origin of the carbon grains of this study. The 15N excesses in spherulitic aggregates and non-round grains can be explained as the result of ion-molecule reactions in molecular clouds. The round grains, on the other hand, must have formed in stellar atmospheres (circumstellar grains). Grains with isotopically light C must have formed in stellar environments characterized by He-burning, either in the atmosphere of Wolf-Rayet stars during the WC phase or in the He-burning, 12C-rich zone of a massive star, ejected by a supernova explosion. Isotopically heavy C is produced by H-burning in the CNO cycle. Possible sources for grains with heavy C are carbon stars (AGB stars during the thermally pulsing phase) or novae, but the detailed distribution of 12C/13C ratios agree neither with the distribution observed in carbon stars nor with theoretical predictions for these two types of stellar sources.  相似文献   

5.
Abstract— We have studied 74 single presolar silicon carbide grains with sizes between 0.2 and 2.6 μm from the Murchison and Murray meteorites for Ba isotopic compositions using NanoSIMS. We also analyzed 7 SiC particles either consisting of sub‐micron‐size SiC grains or representing a morphologically and isotopically distinct subgroup. Of the 55 (likely) mainstream grains, originating from asymptotic giant branch (AGB) stars, 32 had high enough Ba contents for isotopic analysis. For 26 of them, CsHx interferences were either negligible or could be corrected with confidence. They exhibit typical s‐process Ba isotopic patterns with slightly higher than solar 134Ba/136Ba and lower than solar 135,137,138Ba/136Ba ratios. Results are generally well explained in the context of neutron capture nucleosynthesis in low mass (1–3 M⊙) AGB stars and provide constraints on AGB models, by reducing the needed 13C spread from factor of ～20 down to 2. Out of the 19 supernova X grains, three had sufficient concentrations for isotopic analysis. They tend to exhibit higher than solar 134Ba/136Ba and 138Ba/136Ba ratios, close to solar 137Ba/136Ba, and 135Ba/136Ba lower than solar but higher than in mainstream grains. This signature could indicate a mixture of n‐burst type Ba with either “normal Ba” more s‐process‐rich than solar, or normal Ba plus weak s‐process Ba. In the n‐burst component Cs may have to be separated from Ba at ～10 years after the SN explosion. Depending on predictions for its composition, another possibility is early separation (at ～1 year) coupled with addition of some unfractionated n‐burst matter. Abundances of trace elements (Sr, Zr, Cs, La, and Ce) analyzed along with Ba signify that implantation may have been an important process for their introduction.  相似文献   

6.
Abstract– An IDP nicknamed Andric, from a stratospheric dust collector targeted to collect dust from comet 55P/Tempel‐Tuttle, contains five distinct presolar silicate and/or oxide grains in 14 ultramicrotome slices analyzed, for an estimated abundance of approximately 700 ppm in this IDP. Three of the grains are 17O‐enriched and probably formed in low‐mass red giant or asymptotic giant branch (AGB) stars; the other two grains exhibit 18O enrichments and may have a supernova origin. Carbon and N isotopic analyses show that Andric also exhibits significant variations in its N isotopic composition, with numerous discrete 15N‐rich hotspots and more diffuse regions that are also isotopically anomalous. Three 15N‐rich hotspots also have statistically significant 13C enrichments. Auger elemental analysis shows that these isotopically anomalous areas consist largely of carbonaceous matter and that the anomalies may be hosted by a variety of components. In addition, there is evidence for dilution of the isotopically heavy components with an isotopically normal endmember; this may have occurred either as a result of extraterrestrial alteration or during atmospheric entry. Isotopically primitive IDPs such as Andric share many characteristics with primitive meteorites such as the CR chondrites, which also contain isotopically anomalous carbonaceous matter and abundant presolar silicate and oxide grains. Although comets are one likely source for the origin of primitive IDPs, the presence of similar characteristics in meteorites thought to come from the asteroid belt suggests that other origins are also possible. Indeed the distinction between cometary and asteroidal sources is somewhat blurred by recent observations of icy comet‐like planetesimals in the outer asteroid belt.  相似文献   

7.
Abstract— Carbon stars are an important source of presolar TiC, SiC, and graphite grains found in meteorites. The elemental abundances in the stellar sources of the SiC grains are inferred by using condensation calculations. These elemental abundances, together with C isotopic compositions, are used to identify possible groups of carbon stars that may have contributed SiC grains to the presolar dust cloud. The most likely parent stars of meteoritic SiC mainstream grains are N-type carbon stars and evolved subgiant CH stars. Both have s-process element abundances higher than solar and 10 < 12C/13C < 100 ratios. The J stars and giant CH stars, with solar and greater than solar abundances of s-process elements, respectively, are good candidate parents for the ‘A’ and ‘B’ SiC grains with low 12C/13C ratios. A special subgroup of CH giant stars with very large 12C/13C ratios could have parented the ‘Y’ SiC grains with 12C/13C ratios > 100. The carbon star population (e.g., N, R, J, CH groups) needed to provide the observed SiC grains is compared to the current population of carbon stars. This comparison suggests that low-metallicity CH stars may have been more abundant in the past (>4.5 Ga ago) than at present. This suggestion is also supported by condensation-chemistry modeling of the trace element patterns in the SiC grains that shows that subsolar Fe abundances may be required in the stellar sources for many SiC grains. The results of this study suggest that presolar SiC grains in meteorites can provide information about carbon stars during galactic evolution.  相似文献   

8.
Abstract— Thirteen presolar silicon carbide grains—three of supernova (SN) origin and ten of asymptotic giant branch (AGB) star origin—were examined with time‐of‐flight‐secondary ion mass spectrometry (TOF‐SIMS). The grains had been extracted from two different meteorites—Murchison and Tieschitz—using different acid residue methods. At high lateral resolution of ～300 nm, isotopic and elemental heterogeneities within the micrometer‐sized grains were detected. The trace elemental abundances, when displayed in two‐element correlation plots, of Li, Mg, K, and Ca show a clear distinction between the two different meteoritic sources. The different concentrations might be attributed to differences of the host meteorites and/or of extraction methods whereas the stellar source seems to be less decisive. In one SN grain with 26Mg‐enrichment from extinct 26Al, the acid treatment, as part of the grain separation procedure, affected the Mg/Al ratio in the outer rim and therefore the inferred initial 26Al/27Al ratio. A second SN grain exhibits a lateral heterogeneity in 26Al/27Al, which either is due to residual Al‐rich contamination on the grain surface or to the condensation chemistry in the SN ejecta.  相似文献   

9.
We aim to compare properties of early‐type post‐asymptotic giant‐branch (post‐AGB) stars, including normal first‐time B‐type post‐AGB stars, and extreme helium stars (EHes). Hipparcos photometry for 12 post‐AGB stars and 7 EHe stars has been analyzed; 5 post‐AGB stars are clearly variable. The Hipparcos data are not sufficiently sensitive to detect variability in any of the EHes. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

10.
Abstract– The composition of the most primitive solar system condensates, such as calcium‐aluminum‐rich inclusions (CAIs) and micron‐sized corundum grains, show that short‐lived radionuclides (SLR), e.g., 26Al, were present in the early solar system. Their abundances require a local or stellar origin, which, however, is far from being understood. We present for the first time the abundances of several SLR up to 60Fe predicted from stars with initial mass in the range approximately 7–11 M⊙. These stars evolve through core H, He, and C burning. After core C burning they go through a “Super”‐asymptotic giant branch (Super‐AGB) phase, with the H and He shells activated alternately, episodic thermal pulses in the He shell, a very hot temperature at the base of the convective envelope (approximately 108 K), and strong stellar winds driving the H‐rich envelope into the surrounding interstellar medium. The final remnants of the evolution of Super‐AGB stars are mostly O–Ne white dwarfs. Our Super‐AGB models produce 26Al/27Al yield ratios approximately 0.02–0.26. These models can account for the canonical value of the 26Al/27Al ratio using dilutions with the solar nebula of the order of 1 part of Super‐AGB mass per several 102 to several 103 of solar nebula mass, resulting in associated changes in the O‐isotope composition in the range Δ17O from 3 to 20‰. This is in agreement with observations of the O isotopic ratios in primitive solar system condensates, which do not carry the signature of a stellar polluter. The radionuclides 41Ca and 60Fe are produced by neutron captures in Super‐AGB stars and their meteoritic abundances are also matched by some of our models, depending on the nuclear and stellar physics uncertainties as well as the meteoritic experimental data. We also expect and are currently investigating Super‐AGB production of SLR heavier than iron, such as 107Pd.  相似文献   

11.
Abstract— Primitive meteorites contain a few parts per million (ppm) of pristine interstellar grains that provide information on nuclear and chemical processes in stars. Their interstellar origin is proven by highly anomalous isotopic ratios, varying more than 1000-fold for elements such as C and N. Most grains isolated thus far are stable only under highly reducing conditions (C/O > 1), and apparently are “stardust” formed in stellar atmospheres. Microdiamonds, of median size ～ 10 Å, are most abundant (～ 400–1800 ppm) but least understood. They contain anomalous noble gases including Xe-HL, which shows the signature of the r- and p-processes and thus apparently is derived from supernovae. Silicon carbide, of grain size 0.2–10 μm and abundance ～ 6 ppm, shows the signature of the s-process and apparently comes mainly from red giant carbon (AGB) stars of 1–3 solar masses. Some grains appear to be ≥109 a older than the Solar System. Graphite spherules, of grain size 0.8–7 μm and abundance <2 ppm, contain highly anomalous C and noble gases, as well as large amounts of fossil 26Mg from the decay of extinct 26Al. They seem to come from at least three sources, probably AGB stars, novae, and Wolf-Rayet stars.  相似文献   

12.
Abstract— We performed in situ morphological and isotopic studies of graphite in the primitive chondrites Khohar (L3), Mezö‐Madaras (L3), Inman (L3), Grady (H3), Acfer 182 (CH3), Acfer 207 (CH3), Acfer 214 (CH3), and St. Marks (EH5). Various graphite morphologies were identified, including book, veins, fibrous, fine‐grained, spherulitic, and granular graphite, and cliftonite. SIMS measurements of H, C, N, and O isotopic compositions of the graphites revealed large variations in the isotopic ratios of these four elements. The δ15N and δ13C values show significant variations among the different graphite types without displaying any strict correlation between the isotopic composition and morphology. In the Khohar vein graphites, large 15N excesses are found, with δ15Nmax ～+955‰, confirming previous results. Excesses in 15N are also detected in fine‐grained graphites in chondrites of the CH clan, Acfer 182, Acfer 207, and Acfer 214, with δ15N ranging up to +440‰. The 15N excesses are attributed to ion‐molecule reactions at low temperatures in the interstellar molecular cloud (IMC) from which the solar system formed, though the largest excesses seem to be incompatible with the results of some recent calculation. Significant variations in the carbon isotopic ratios are detected between graphite from different chondrite groups, with a tendency for a systematic increase in δ13C from ordinary to enstatite to carbonaceous chondrites. These variations are interpreted as being due to small‐ and large‐scale carbon isotopic variations in the solar nebula.  相似文献   

13.
Abstract— We report measurements of isotopic ratios of C, N, Mg, Si, Ca, Ti, Cr, and Fe in bulk samples (aggregates of many grains) of up to seven different fractions of silicon carbide (SiC), ranging from 0.38 to 3.0μm in diameter, from the Murchison CM2 carbonaceous chondrite. Ratios of 12C/13C range from 37 to 42 and 14N/15N ratios from 370 to 520, within the range of single‐grain measurements on coarser samples and in agreement with an asymptotic giant branch (AGB) star origin of most of the grains. Variations among size fractions do not show any simple trend and can be explained by varying contamination with isotopically normal material. Silicon isotopic ratios vary only little and, with one exception, lie to the right of the singlegrain mainstream correlation line. This might indicate a higher percentage of the minor populations Y and Z among finer grain‐size fractions. All bulk samples have large 26Mg excesses attributed to the presence of short‐lived 26Al at the time of grain formation. Inferred 26Al/27Al ratios are much larger than those measured in single larger mainstream grains. This is probably because of the presence of SiC grains of type X; we obtain an estimate of 0.4 for their 26Al/27Al ratio. Our Ca‐isotopic measurements, the first made on presolar SiC grains, show excesses in 42Ca and 43Ca, which is in general agreement with theoretical expectations for AGB stars. Calcium‐44 excesses are much larger than expected and are probably because of X grains, which have high44Ca excesses because of the decay of short‐lived 44Ti produced in supernova explosions. We arrive at an estimate of 0.014 for the initial 44Ti/48Ti ratio of the X grains, within the range obtained from previous single X grain measurements. The Ti‐isotopic ratios of the bulk samples show a V‐shaped pattern with excesses of all isotopes relative to 48Ti. Isotopes 46Ti, 47Ti, and 50Ti show excesses relative to the correlation between Ti and Si ratios for single grains and are in general agreement with theoretical models of s‐process nucleosynthesis in AGB stars. In contrast, 49Ti does not show any excess relative to the singlegrain data; it also fails to agree with theory, which predicts much larger excesses than observed. Measured 53Cr/52Cr and 57Fe/56Fe ratios are normal within errors. The first result is expected even for Cr in AGB star envelopes, but the second result suggests that most of the Fe analyzed originates from contamination. We have found no simple trends in isotopic composition with respect to grain size that can be interpreted in terms of nucleosynthetic origin, unlike the results for Kr, Xe, Ba, and Sr.  相似文献   

14.
Abstract— We report isotopic abundances for C, N, Mg‐Al, Si, Ca‐Ti, and Fe in 99 presolar silicon carbide (SiC) grains of type X (84 grains from this work and 15 grains from previous studies) from the Murchison CM2 meteorite, ranging in size from 0.5 to 1.5 μm. Carbon was measured in 41 X grains, n in 37 grains, Mg‐Al in 18 grains, Si in 87 grains, Ca‐Ti in 25 grains, and Fe in 8 grains. These X grains have 12C/13C ratios between 18 and 6800, 14N/15n ratios from 13 to 200, δ29Si/28Si between ?750 and +60%0, δ30Si/28Si from ?770 to ?10%0, and 54Fe/56Fe ratios that are compatible with solar within the analytical uncertainties of several tens of percent. Many X grains carry large amounts of radiogenic 26Mg (from the radioactive decay of 26Al, half‐life ? 7 times 105 years) and radiogenic 44Ca (from the radioactive decay of 44Ti, half‐life = 60 years). While all X grains but one have radiogenic 26Mg, only ～20% of them have detectable amounts of radiogenic 44Ca. Initial 26Al/27Al ratios of up to 0.36 and initial 44Ti/48Ti ratios of up to 0.56 can be inferred. The isotopic data are compared with those expected from the potential stellar sources of SiC dust. Carbon stars, Wolf‐Rayet stars, and novae are ruled out as stellar sources of the X grains. The isotopic compositions of C and Fe and abundances of extinct 44Ti are well explained both by type Ia and type II supernova (SN) models. The same holds for 26Al/27Al ratios, except for the highest 26Al/27Al ratios of >0.2 in some X grains. Silicon agrees qualitatively with SN model predictions, but the observed 29Si/30Si ratios in the X grains are in most cases too high, pointing to deficiencies in the current understanding of the production of Si in SN environments. The measured 14n/15n ratios are lower than those expected from SN mixing models. This problem can be overcome in a 15 Modot; type II SN if rotational mixing, preferential trapping of N, or both from 15n‐rich regions in the ejecta are considered. The isotopic characteristics of C, N, Si, and initial 26Al/27Al ratios in small X grains are remarkably similar to those of large X grains (2–10 μm). Titanium‐44 concentrations are generally much higher in smaller grains, indicative of the presence of Ti‐bearing subgrains that might have served as condensation nuclei for SiC. The fraction of X grains among presolar SiC is largely independent of grain size. This implies similar grain‐size distributions for SiC from carbon stars (mainstream grains) and supernovae (X grains), a surprising conclusion in view of the different conditions for dust formation in these two types of stellar sources.  相似文献   

15.
Abstract— A wide range of stellar nucleosynthetic sources has been analyzed to derive their contributions of short‐lived and stable nuclei to the presolar cloud. This detailed study is required to infer the most plausible source(s) of short‐lived nuclei through a critical comparison among the various stellar sources that include AGB stars, novae, supernovae II, Ia, and Wolf‐Rayet stars that evolved to supernovae Ib/c. In order to produce the canonical value of 26Al/27Al in the early solar system, almost all stellar sources except low‐mass AGB stars imply large isotopic anomalies in Ca‐Al‐rich inclusions (CAIs). This is contrary to the observed isotopic compositions of CAIs. The discrepancy could impose stringent constraints on the formation and thermal evolution of CAIs from different chondrites. Among the various stellar scenarios, the injection of short‐lived nuclei into the previously formed solar protoplanetary disc by a massive star of an ad hoc chosen high‐injection mass cut is a possible scenario. There is a possibility of the contribution of short‐lived nuclides by a 1.5–3 M⊙ AGB star as it implies the smallest shift in stable isotopes. A low‐mass AGB star of relatively low metallicity would be even a better source of short‐lived nuclei. However, this scenario would require independent gravitational collapse of the presolar cloud coupled with ambipolar diffusion of magnetic flux. Alternatively, numerous scenarios can be postulated that involve distant (≥10 pc) massive stars can contribute 60Fe to the presolar cloud and can trigger its gravitational collapse. These scenarios would require production of 26Al and 41Ca by irradiation in the early solar system. Significant production of 26Al and 60Fe can be explained if massive, rotating Wolf‐Rayet stars that evolved to supernovae Ib/c were involved.  相似文献   

16.
Abstract— We have determined the recoil losses from silicon carbide (SiC) grain‐size fractions of spallation Ne produced by irradiation with 1.6 GeV protons. During the irradiation, the SiC grains were dispersed in paraffin wax in order to avoid reimplantation into neighboring grains. Analysis for spallogenic 21Ne of grain‐size separates in the size range 0.3 to 6 μm and comparison with the 22Na activity of the SiC + paraffin mixture indicates an effective recoil range of 2–3 μm with no apparent effect from acid treatments, which are routinely used in the isolation of meteoritic SiC grains. Our results indicate that the majority of presolar SiC grains in primitive meteorites, which are micrometer‐sized, will have lost essentially all spallogenic Ne produced by cosmic‐ray interaction in the interstellar medium. This argues against the validity of previously published presolar ages of Murchison SiC (～10 to ～130 Ma, increasing with grain size; Lewis et al., 1994), where recoil losses had been based on calculated recoil energies. It is argued that the observed variations in meteoritic SiC grain‐size fractions of 21Ne/22Ne ratios are more likely due to the effects of nucleosynthesis in the He‐burning shell of the parent AGB stars which imposes new boundary conditions on nuclear parameters and stellar models. It is suggested that spallation‐Xe produced on the abundant Ba and REE in presolar SiC, rather than spallogenic Ne, may be a promising approach to the presolar age problem. There is a hint in the currently available Xe data (Lewis et al., 1994) that the large (>1 μm) grains may be younger than the smaller (<1 μm) grains. The retention of spallogenic 21Ne produced by the bombardment of SiC grains of different grain sizes with 1.6 GeV protons, avoiding reimplantation into neighboring grains by dispersing the SiC grains in paraffin wax, has been derived from a comparison of mass spectrometrically determined 21Ne, retained in the grains, with the 22Na activity of the grains‐plus‐paraffin mixture. Compared to estimates of retention used in previous attempts to determine presolar ages for SiC (Tang and Anders, 1988b; Lewis et al., 1990, 1994), the results indicate significantly lower values. They do, however, agree with retention as expected from previous measurements of recoil ranges in similar systems (Nyquist et al., 1973; Steinberg and Winsberg, 1974). The prime reason for the discrepancy must lie in the energy of the recoiling nuclei entering in the calculation of retention by Tang and Anders (1988b), which is based on considerations by Ray and Völk (1983). Based on the results, it appears questionable that spallation contributes significantly to the observed variations of 21Ne/22Ne ratios among various SiC grain‐size separates (Lewis et al., 1994). We rather suggest that the variations, just as it has been observed for Kr and Ba already (Lewis et al., 1994; Prombo et al., 1993), have a nucleosynthetic origin. Confirmation needs input of improved nuclear data and stellar models into new network calculations of the nucleosynthesis in AGB stars of elements in the Ne region. Finally we argue that, to determine presolar system irradiation effects, spallation Xe is more favorable than is Ne, primarily because of smaller recoil losses for Xe. Although preliminary estimates hint at the possibility that the larger (>1 μm) grains are younger than the smaller (<1 μm) ones, the major uncertainty for a quantitative evaluation lies in the exact composition of the Xe‐N component thought to originate from the envelope of the SiC grains' parent stars.  相似文献   

17.
Abstract— We demonstrate that a massive asymptotic giant branch (AGB) star is a good candidate as the main source of short‐lived radionuclides in the early solar system. Recent identification of massive (4–8 M⊙) AGB stars in the galaxy, which are both lithium‐ and rubidium‐rich, demonstrates that these stars experience proton captures at the base of the convective envelope (hot bottom burning), together with high‐neutron density nucleosynthesis with 22Ne as a neutron source in the He shell and efficient dredge‐up of the processed material. A model of a 6.5 M⊙ star of solar metallicity can simultaneously match the abundances of 26Al, 41Ca, 60Fe, and 107Pd inferred to have been present in the solar nebula by using a dilution factor of 1 part of AGB material per 300 parts of original solar nebula material, and taking into account a time interval between injection of the short‐lived nuclides and consolidation of the first meteorites equal to 0.53 Myr. Such a polluting source does not overproduce 53Mn, as supernova models do, and only marginally affects isotopic ratios of stable elements. It is usually argued that it is unlikely that the short‐lived radionuclides in the early solar system came from an AGB star because these stars are rarely found in star forming regions, however, we think that further interdisciplinary studies are needed to address the fundamental problem of the birth of our solar system.  相似文献   

18.
Abstract— We have measured the titanium isotopic compositions of 23 silicon carbide grains from the Orgueil (CI) carbonaceous chondrites for which isotopic compositions of silicon, carbon, and nitrogen and aluminum‐magnesium systematics had been measured previously. Using the 16 most‐precise measurements, we estimate the relative contributions of stellar nucleosynthesis during the asymptotic giant branch (AGB) phase and the initial compositions of the parent stars to the compositions of the grains. To do this, we compare our data to the results of several published stellar models that employ different values for some important parameters. Our analysis confirms that s‐process synthesis during the AGB phase only slightly modified the titanium compositions in the envelopes of the stars where mainstream silicon carbide grains formed, as it did for silicon. Our analysis suggests that the parent stars of the >1 μm silicon carbide grains that we measured were generally somewhat more massive than the Sun (2–3 M⊙) and had metallicities similar to or slightly higher than solar. Here we differ slightly from results of previous studies, which indicated masses at the lower end of the range 1.5–3 M⊙ and metallicities near solar. We also conclude that models using a standard 13C pocket, which produces a good match for the main component of s‐process elements in the solar system, overestimate the contribution of the 13C pocket to s‐process nucleosynthesis of titanium found in silicon carbide grains. Although previous studies have suggested that the solar system has a significantly different titanium isotopic composition than the parent stars of silicon carbide grains, we find no compelling evidence that the Sun falls off of the array defined by those stars. We also conclude that the Sun does lie on the low‐metallicity end of the silicon and titanium arrays defined by mainstream silicon carbide grains.  相似文献   

19.
Abstract— Low molecular weight monocarboxylic acids, including acetic acid, are some of the most abundant organic compounds in carbonaceous chondrites. So far, the 13C‐ and D‐enriched signature of water‐extractable carboxylic acids has implied an interstellar contribution to their origin. However, it also has been proposed that monocarboxylic acids could be formed by aqueous reaction on the meteorite parent body. In this study, we conducted hydrous pyrolysis of macromolecular organic matter purified from the Murchison meteorite (CM2) to examine the generation of monocarboxylic acids with their stable carbon isotope measurement. During hydrous pyrolysis of macromolecular organic matter at 270–330 °C, monocarboxylic acids with carbon numbers ranging from 2 (C2) to 5 (C5) were detected, acetic acid (CH3COOH; C2) being the most abundant. The concentration of the generated acetic acid increased with increasing reaction temperature; up to 0.48 mmol acetic acid/g macromolecular organic matter at 330 °C. This result indicates that the Murchison macromolecule has a potential to generate at least ?0.4 mg acetic acid/g meteorite, which is about four times higher than the amount of water‐extractable acetic acid reported from Murchison. The carbon isotopic composition of acetic acid generated by hydrous pyrolysis of macromolecular organic matter is ?‐27‰ (versus PDB), which is much more depleted in 13C than the water‐extractable acetic acid reported from Murchison. Intramolecular carbon isotope distribution shows that methyl (CH3‐)‐C is more enriched in 13C relative to carboxyl (‐COOH)‐C, indicating a kinetic process for this formation. Although the experimental condition of this study (i.e., 270–330 °C for 72 h) may not simulate a reaction condition on parent bodies of carbonaceous chondrite, it may be possible to generate monocarboxylic acids at lower temperatures for a longer period of time.  相似文献   

20.
Different stellar sources may have contributed to the 7Li enrichmentof the Galaxy: type-II supernovae, novae, and AGB stars. In the latter case, the interplay between the Hot Bottom Burning (HBB) process (via the Cameron-Fowler mechanism) and a very high mass-loss rate before the evolution off the AGB (the so-called ‘superwind’ phase), can lead to a significant production of 7Li from low- and intermediate-mass AGB stars (Travaglio et al., 2001). We have now undertaken an observational campaign aimed at constraining our stellar and Galactic models, with a twofold goal: (i) to assemble a compilation of high-resolution spectra of Galactic, unevolved (i.e. dwarfs), warm(spectral type F) stars, in a selected metallicity range (-1.0 ≤>[Fe/H] ≤ -0.3), using the ESO 1.5m telescope and the FEROS spectrograph; (ii) to carry out a Li survey among a sample of selected AGB stars, to investigate the possible correlation between7Li abundance (when detected) and mass-loss rate. This revised version was published online in September 2006 with corrections to the Cover Date.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号