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1.
Evolutionary calculations are presented for spherically symmetric protoplanetary configurations with a homogeneous solar composition and with masses of 10?3, 1.5 × 10?3, 2.85 × 10?4, and 4.2 × 10?4M⊙. Recent improvements in equation-of-state and opacity calculations are incorporated. Sequences start as subcondensations in the solar nebula with densities of ～10?10 to 10?11 g cm?3, evolve through a hydrostatic phase lasting 105 to 107 years, undergo dynamic collapse due to dissociation of molecular hydrogen, and regain hydrostatic equilibrium with densities ～1 g cm?3. The nature of the objects at the onset of the final phase of cooling and contraction is discussed and compared with previous calculations.  相似文献   

2.
Abstract— Neodymium, strontium, and chromium isotopic studies of the LEW86010 angrite established its absolute age and the formation interval between its crystallization and condensation of Allende CAIs from the solar nebula. Pyroxene and phosphate were found to contain ～98% of its Sm and Nd inventory. A conventional 147Sm-143Nd isochron yielded an age of 4.53 ± 0.04 Ga (2 σ) and ?143 Nd = 0.45 ± 1.1. An 146Sm-142Nd isochron gives initial 146Sm/144Sm = 0.0076 ± 0.0009 and ?143 Nd = ?2.5 ± 0.4. The Rb-Sr analyses give initial 87Sr/86Sr (I87Sr) = 0.698972 ± 8 and 0.698970 ± 18 for LEW and ADOR, respectively, relative to 87Sr/86Sr = 0.71025 for NBS987. The difference, ΔI87Sr, between I87Sr for the angrites and literature values for Allende CAIs, corresponds to ～9 Ma of growth in a solar nebula with a CI chondrite value of 87Rb/86Sr = 0.91, or ～5 Ma in a nebula with solar photospheric 87Rb/86Sr = 1.51. Excess 53Cr from extinct 53Mn (t1/2 = 3.7 Ma) in LEW86010 corresponds to initial 53Mn/55Mn = 1.44 ± 0.07 × 10?6 and closure to Cr isotopic homogenization 18.2 ± 1.7 Ma after formation of Allende inclusions, assuming initial 53Mn/55Mn = 4.4 ± 1.0 × 10?5 for the inclusions as previously reported by the Paris group (Birck and Allegre, 1988). The 146Sm/144Sm value found for LEW86010 corresponds to solar system initial (146Sm/144Sm)o = 0.0080 ± 0.0009 for crystallization 8 Ma after Allende, the difference between Pb-Pb ages of angrites and Allende, or 0.0086 ± 0.0009 for crystallization 18 Ma after Allende, using the Mn-Cr formation interval. The isotopic data are discussed in the context of a model in which an undifferentiated “chondritic” parent body formed from the solar nebula ～2 Ma after Allende CAIs and subsequently underwent differentiation accompanied by loss of volatiles. Parent bodies with Rb/Sr similar to that of CI, CM, or CO chondrites could satisfy the Cr and Sr isotopic systematics. If the angrite parent body had Rb/Sr similar to that of CV meteorites, it would have to form slightly later, ～2.6 Ma after the CAIs, to satisfy the Sr and Cr isotopic systematics.  相似文献   

3.
We review recent results on O‐ and Mg‐isotope compositions of refractory grains (corundum, hibonite) and calcium, aluminum‐rich inclusions (CAIs) from unequilibrated ordinary and carbonaceous chondrites. We show that these refractory objects originated in the presence of nebular gas enriched in 16O to varying degrees relative to the standard mean ocean water value: the Δ17OSMOW value ranges from approximately ?16‰ to ?35‰, and recorded heterogeneous distribution of 26Al in their formation region: the inferred (26Al/27Al)0 ranges from approximately 6.5 × 10?5 to <2 × 10?6. There is no correlation between O‐ and Mg‐isotope compositions of the refractory objects: 26Al‐rich and 26Al‐poor refractory objects have similar O‐isotope compositions. We suggest that 26Al was injected into the 26Al‐poor collapsing protosolar molecular cloud core, possibly by a wind from a neighboring massive star, and was later homogenized in the protoplanetary disk by radial mixing, possibly at the canonical value of 26Al/27Al ratio (approximately 5 × 10?5). The 26Al‐rich and 26Al‐poor refractory grains and inclusions represent different generations of refractory objects, which formed prior to and during the injection and homogenization of 26Al. Thus, the duration of formation of refractory grains and CAIs cannot be inferred from their 26Al‐26Mg systematics, and the canonical (26Al/27Al)0 does not represent the initial abundance of 26Al in the solar system; instead, it may or may not represent the average abundance of 26Al in the fully formed disk. The latter depends on the formation time of CAIs with the canonical 26Al/27Al ratio relative to the timing of complete delivery of stellar 26Al to the solar system, and the degree of its subsequent homogenization in the disk. The injection of material containing 26Al resulted in no observable changes in O‐isotope composition of the solar system. Instead, the variations in O‐isotope compositions between individual CAIs indicate that O‐isotope composition of the CAI‐forming region varied, because of coexisting of 16O‐rich and 16O‐poor nebular reservoirs (gaseous and/or solid) at the birth of the solar system, or because of rapid changes in the O‐isotope compositions of these reservoirs with time, e.g., due to CO self‐shielding in the disk.  相似文献   

4.
Heterogeneities in terrestrial samples for 182W/183W and 142Nd/144Nd are only preserved in Hadean and Archean rocks while heterogeneities in 129Xe/130Xe and 136Xe/130Xe persist to very young mantle‐derived rocks. In contrast, meteorites from Mars show that the Martian mantle preserves heterogeneities in 182W/183W and 142Nd/144Nd up to the present. As a consequence of the probable “deep magma ocean” core formation process, we assume that the Earth and Mars both had a very early two‐mantle‐reservoir structure with different initial extinct nuclide isotopic compositions (different 182W/183W, 142Nd/144Nd, 129Xe/130Xe, 136Xe/130Xe ratios). Based on this assumption, we developed a simple stochastic model to trace the evolution of a mantle with two initially distinct layers for the extinct isotopes and its development into a heterogeneous mantle by convective mixing and stretching of these two layers. Using the extinct isotope system 182Hf‐182W, we find that the mantles of Earth and Mars exhibit substantially different mixing or stirring rates. This is consistent with Mars having cooled faster than the Earth due to its smaller size, resulting in less efficient mantle mixing for Mars. Moreover, the mantle stirring rate obtained for Earth using 182Hf‐182W is consistent with the mantle stirring rate of ~500 Myr constrained by the long‐lived isotope system, 87Rb‐87Sr and 147Sm‐143Nd. The apparent absence of 182W/183W isotopic heterogeneity in modern terrestrial rocks is attributed to very active mantle stirring which reduced the 182W/183W isotopic heterogeneity to a relatively small scale (~83 m for a mantle stirring rate of 500 Myr) compared to the common sampling scale of terrestrial basalts (~30 or 100 km). Our results also support the “deep magma ocean” core formation model as being applicable to both Mars and Earth.  相似文献   

5.
《New Astronomy》2002,7(2):55-84
In this paper we compute the time evolution of the elements (4He, 12C, 14N, 16O, 20Ne, 24Mg, 28Si, 32S, 40Ca and 56Fe) and of the supernova rates in the solar neighbourhood by means of a galactic chemical evolutionary code that includes in detail the evolution of both single and binary stars. Special attention is payed to the formation of black holes.Our main conclusions:
  • •in order to predict the galactic time evolution of the different types of supernovae, it is essential to compute in detail the evolution of the binary population,
  • •the observed time evolution of carbon is better reproduced by a galactic model where the effect is included of a significant fraction of intermediate mass binaries,
  • •massive binary mass exchange provides a possible solution for the production of primary nitrogen during the very early phases of galactic evolution,
  • •chemical evolutionary models with binaries or without binaries but with a detailed treatment of the SN Ia progenitors predict very similar age–metallicity relations and very similar G-dwarf distributions whereas the evolution of the yields as function of time of the elements 4He, 16O, 20Ne, 24Mg, 28Si, 32S and 40Ca differ by no more than a factor of two or three,
  • •the observed time evolution of oxygen is best reproduced when most of the oxygen produced during core helium burning in ALL massive stars serves to enrich the interstellar medium. This can be used as indirect evidence that (massive) black hole formation in single stars and binary components is always preceded by a supernova explosion.
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6.
On 14 December 1971, during the maximum of the Geminid Meteor Shower, the positive ion composition was measured in the D- and E-regions above Sardinia. The payload was launched at 12:11 UT, and measurements were made between 68.5 and 152 km altitude. A magnetic sector type mass spectrometer with dual collector and a liquid helium cryopump was used. The instrument covered the mass range from 11 to 73 AMU and had a resolution at the 1 % level of M/ΔM = 60.In the E-region two distinct metal ion layers were observed, centred at 95 and 119 km, respectively. In the lower layer Fe+ and Mg+ were the most abundant metal ions, and in the upper layer Si+ was dominant. Si+ ions were conspicuously absent in the lower layer (Si+/Mg+ < 2 × 10−3). This particular behaviour of Si could be due to the inability of atomic oxygen to reduce SiO, whereas in the upper layer Si+ions might be formed directly by the charge rearrangement reaction SiO + O+ → Si++ O2. In addition, Na+, Al+, K+, Ca+, Ti+, Cr+, Ni+ and Co+ were also identified. The metal oxide ions AlO+ and SiO+ were detected, and probably also MgO+ and SiOH+. The concentrations of NO+ and O2+ show a deep minimum at the maximum of the lower metal ion layer. A very high neutral metal density of 6 × 107 cm−3 would be required to explain this feature as resulting from charge transfer reactions between the molecular and metal ions Such a high metal density is in contradiction to direct measurements and to cosmic dust influx rates. The isotopic ratios of Mg+, Si+, and of the major isotopes of Fe+ and Ni+ were measured, some of them with an accuracy of a few per cent (25Mg+/24Mg+ = 0.124 ± 0.006; 26Mg+/24Mg+ = 0.139 ± 0.008; 29Si+/28Si+ = 0.050 ± 0.004; 54Fe+/56Fe+ = 0.069 ± 0.005; 57Fe+/56Fe+ = 0.029 ± 0.004; 60Ni+/58Ni+ = 0.31 ± 0.12). The isotopic ratios agree within the experimental errors with the corresponding terrestrial ratios, thus giving evidence that these elements have the same isotopic composition in the Geminid meteors as in the Earth's crust, in chrondrites, and in lunar material.In the D-region the ions Na+H2O, Na+(H2O)2, NaO+ and NaOH+ were tentatively identified. Below 95 km altitude the relative abundances of the ions 32+, 33+ and 34+ deviate from the values expected for molecular oxygen isotopes. Their abundances can not be explained by the presence of S-ions only, and we conclude that HO2+ and H2O2+ are present.The ion density profiles of the major D-region constituents show some remarkable deviations from typical D-region conditions. These deviations are related to the winter anomaly in ionospheric absorption observed over Spain during the launch day, and our data represent the first ion composition measurements during such conditions. In particular, H+(H2O)2 is the major ion only up to 77 km, and at 80 km altitude the NO+ concentration exceeds the total water cluster ion density by almost two orders of magnitude. An increase of the mesospheric NO, O3 and O concentrations as well as of the O/H2O ratio could explain the observed ion profiles. The low NO+/O2+ ratios of approximately unity measured in the E-region are in agreement with a strong downward transport of NO and/or O into the mesosphere during the launch day. A simple four-ion model was used to interpret our D-region data. The calculated neutral NO concentration increases from about 2 × 107 cm−3 at 85 km to 5 × 107 cm−3 at 80 km. In addition, evidence for an increased O2+ production rate above 83 km was found, probably due to an enhanced O3 concentration. We conclude that our data strongly support vertical transport of minor neutral consituents as cause of the winter anomaly.  相似文献   

7.
Abstract— Anti‐correlation of initial iodine ratios with trapped 129Xe/132Xe ratios has been interpreted as resulting from 129I decay to 129Xe in a closed system. However, many of the 129Xe/132Xe ratios that contribute to the anti‐correlations are lower than 1.04, the value characteristic of major solar system reservoirs; 129I decay cannot lead to a decrease in this ratio. We offer an alternative explanation for the anti‐correlations, based on trapped iodine and xenon components similar to those observed in Nakhla, that does not require the existence of components with 129Xe/132Xe lower than solar.  相似文献   

8.
A high-resolution sunspot umbra spectrum recorded in National Solar Observatory, Kitt Peak in the visible and infrared wave number range 13, 600 – 25, 000cm−1 was taken in the present study for identifying the rotational lines of barium hydride (BaH) molecule. Number of chance coincidences was evaluated for the A 2Π1/2 – X 2Σ ((0,0), (1,1), (2,2), (1,0), (2,1)), A 2Π3/2 – X 2Σ ((0,0), (1,1), (2,2)), B 2Π1/2 – X 2Σ (0,0), B 2Π3/2 – X 2Σ (0,0), C 2Σ – X 2Σ ((1,1), (1,0), (2,2), (2,1), (3,2)) and D 2Σ – X 2Σ ((1,0), (2,0), (3,0), (4,0), (5,0), (8,0), (9,0)) band systems of BaH using line identification procedure. The obtained number of chance of coincidences was compared with I- parameter values. The highly resolved rotational lines were chosen to evaluate equivalent widths using triangle approximation method. The effective rotational temperatures were calculated for the bands (0,0), (1,1), (2,2) and (2,1) of A 2Π1/2 – X 2Σ, (0,0) (1,1) and (2,2) of A 2Π3/2 – X 2Σ, B 2Π1/2 – X 2Σ (0,0) and B 2Π3/2 – X 2Σ (0,0) of BaH molecule. The rotational temperature values calculated for these bands were found to be in the range 1185 – 3514 K. They were also compared with the already reported sunspot temperatures.  相似文献   

9.
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10.
Solar abundances of light nuclei and mixing of the Sun   总被引：1，自引：0，他引：1  
Radial profiles of the light nuclei (A 15) are calculated in the non-mixing Sun, taking into account the changes of solar structure with time. The results are discussed in relation to models of solar mixing and compared with abundance determinations at the solar surface or in the solar wind. B cannot be depleted in the outer convective zone without producing a large increase in the He3/He4 ratio. A decrease in He3/He4 would be accompanied by changes in C13/C12 and N15/N14 of a magnitude which is not observed.It is shown that boron could be depleted in the pre-main sequence period of the Sun, if mixing was on a time-scale of 106 yr. The simultaneous small increase in He3/He4 does not contradict observation. However, Be would be depleted more strongly than B.A He3/He4 decrease is always accompanied by large changes in N15/N14 and C13/C12. Since such changes are not observed, it is concluded that the He3/He4 ratio in the outer convective zone is a reliable upper limit for (He3 + D)/He4 in the solar nebula. Thus the D/H ratio in the protosolar material was much lower than it is in sea water or in carbonaceous chondrites.  相似文献   

11.
The problem of the ionospheric formation in the Jovian upper atmosphere is examined. By adopting two plausible atmospheric models, we solve coupled time-dependent continuity equations for ions H2+, H5+, H+, H3+ and HeH+ simultaneously. It is shown that both radiative and three body association of H+ to H2 are important for the determination of the structure of the Jovian ionosphere. The maximum electron density in the daytime is found to be about 105 cm?3. It is also shown that diurnal variation with large-amplitude can exist in the Jovian ionosphere.  相似文献   

12.
Abstract— Thick spherical targets, one made of gabbro (R = 25 cm) and one made of iron (R = 10 cm), were irradiated isotropically with 1.6 GeV protons at Laboratoire National Saturne (LNS)/Saclay to simulate the interactions of galactic cosmic ray protons with meteoroids in space. At various depths, both artificial meteoroids contained a large number of high‐purity, single‐element target foils and chemical compounds of up to 28 target elements. In these individual target foils, the elemental production rates of radionuclides and noble gas isotopes were measured. Here, we report the results for the light noble gas isotopes 3,4He, 20,21,22Ne, and 36,38,39Ar for the most cosmochemically relevant target elements as well as for some meteoritic material from Jilin, Farmington, and Cape York. From 3He analyses done several years apart, 3H diffusive losses during sample storage have been obtained, and direct as well as cumulative 3He production rates for O, Mg, Al, Si, Fe, Ni, and the meteoritic material are given. Losses by diffusion of tritium from metallic Mg and Fe are found to occur on time scales of months, while metallic Al, Si, and stone meteorites are much more retentive. The production rate ratios P(3H)/P(3He)d obtained in the simulation experiments are 0.73, 1.28, and 1.16 for O, Al, and Si, respectively. These rates are based on our best knowledge about the 3H and 3He production rates and should, therefore, replace data published earlier (Leya et al. 2000a). The earlier calculations for 4He, 20,21,22Ne, and 36,38,39Ar remain valid. The new modeled correlation 3Hecum/21Ne versus 22Ne/21Ne for chondrites exposed to cosmic rays with an energy spectrum characterized by a modulation parameter of φ = 650 MeV is in fair agreement with the empirical relationship (“Berne plot”). However, for small meteorites and little shielding in larger ones, there are systematic differences that most likely are due to an underestimation of the spallogenic 22Ne/21Ne ratio by ?2%.  相似文献   

13.
Abstract– We review current observational and theoretical constraints on the galactic chemical evolution (GCE) of oxygen isotopes to explore whether GCE plays a role in explaining the lower 17O/18O ratio of the Sun, relative to the present‐day interstellar medium, or the existence of distinct 16O‐rich and 16O‐poor reservoirs in the solar system. Although the production of both 17O and 18O are related to the metallicity of progenitor stars, 17O is most likely produced in stars that evolve on longer timescales than those that produce 18O. Therefore, the 17O/18O ratio need not have remained constant over time, contrary to preconceptions and the simplest models of GCE. An apparent linear, slope‐one correlation between δ17O and δ18O in the ISM need not necessarily reflect an O isotopic gradient, and any slope‐one galactocentric gradient need not correspond to evolution in time. Instead, increasing 17O/18O is consistent both with observational data from molecular clouds and with modeling of the compositions of presolar grains. Models in which the rate of star formation has decelerated over the past few Gyr or in which an enhanced period of star formation occurred shortly before solar birth (“starburst”) can explain the solar‐ISM O‐isotopic difference without requiring a local input of supernova ejecta into the protosolar cloud. “Cosmic chemical memory” models in which interstellar dust is on average older than interstellar gas predict that primordial solar system solids should be 16O‐rich, relative to the Sun, in conflict with observations. However, scenarios can be constructed in which the 16O‐rich contribution of very massive stars could lead to 16O‐poor solids and a 16O‐rich bulk Sun, if the solar system formed shortly after a starburst, independent of the popular scenario of photochemical self‐shielding of CO.  相似文献   

14.
Abstract— The abundances and isotopic compositions of N and Ar have been measured by stepped combustion of the Allan Hills 84001 (ALH 84001) Martian orthopyroxenite. Material described as shocked is N-poor ([N] ～ 0.34 ppm; δ15N ～ +23%); although during stepped combustion, 15N-enriched N (δ15N ～ +143%) is released in a narrow temperature interval between 700 °C and 800 °C (along with 13C-enriched C (δ13C ～ +19%) and 40Ar). Cosmogenic species are found to be negligible at this temperature; thus, the iso-topically heavy component is identified, in part, as Martian atmospheric gas trapped relatively recently in the history of ALH 84001. The N and Ar data show that ALH 84001 contains species from the Martian lithosphere, a component interpreted as ancient trapped atmosphere (in addition to the modern atmospheric species), and excess 40Ar from K decay. Deconvolution of radiogenic 40Ar from other Ar components, on the basis of end-member 36Ar/14N and 40Ar/36Ar ratios, has enabled calculation of a K-Ar age for ALH 84001 as 3.5–4.6 Ga, depending on assumed K abundance. If the component believed to be Martian palaeoatmos-phere was introduced to ALH 84001 at the time the K-Ar age was set, then the composition of the atmosphere at this time is constrained to: δ15N ≥ +200%, 40Ar/36Ar ≤ 300 and 36Ar/14N ≥ 17 × 10?5. In terms of the petrogenetic history of the meteorite, ALH 84001 crystallised soon after differentiation of the planet, may have been shocked and thermally metamorphosed in an early period of bombardment, and then subjected to a second event. This later process did not reset the K-Ar system but perhaps was responsible for introducing (recent) atmospheric gases into ALH 84001; and it might mark the time at which ALH 84001 suffered fluid alteration resulting in the formation of the plagioclase and carbonate mineral assemblages.  相似文献   

15.
The evolution of the charged particles are followed during contraction of a model of an interstellar cloud, with initial density number of n = 10 cm–3. The contraction is followed up to density increase by five orders of magnitude. Special care is given to the details of the negative ions. In addition, we have tested the ambipolar diffusion according to the results of the ion density.The results predict the importance of atomic ions in the diffuse regions. H+ and C+ are distinctly enhanced in the beginning of contraction but decrease as contraction proceeds. Molecular ions enhance as contraction proceeds and becomes important in dense regions. The most enhanced molecular ions are HCO+, O2 +, C2H3 +, H3O+ and SO+, H3 + is less abundant. The atomic ions (except metalic ions) decrease noticeably as density increases. In general the negative ions are of negligible fractional abundances. It has also been found that the time of ambipolar diffusion is shorter than the dynamical time, hence the magnetic field should be weakened in the central core as the central density increases to n = 104 cm–3.  相似文献   

16.
Abstract– We report measurements of cosmogenic nuclides in up to 11 bulk samples from various depths in Norton County. The activities of 36Cl, 41Ca, 26Al, and 10Be were measured by accelerator mass spectrometry; the concentrations of the stable isotopes of He, Ne, Ar, and Sm were measured by electron and thermal ionization mass spectrometry, respectively. Production rates for the nuclides were modeled using the LAHET and the Monte Carlo N‐Particle codes. Assuming a one‐stage irradiation of a meteoroid with a pre‐atmospheric radius of approximately 50 cm, the model satisfactorily reproduces the depth profiles of 10Be, 26Al, and 53Mn (<6%) but overestimates the 41Ca concentrations by about 20%. 3He, 21Ne, and 26Al data give a one‐stage cosmic‐ray exposure (CRE) age of 115 Ma. Argon‐36 released at intermediate temperatures, 36Arn, is attributed to production by thermal neutrons. From the values of 36Arn, an assumed average Cl concentration of 4 ppm, and a CRE age of 115 Ma, we estimate thermal neutron fluences of 1–4 × 1016 neutrons cm?2. We infer comparable values from ε149Sm and ε150Sm. Values calculated from 41Ca and a CRE age of 115 Ma, 0.2–1.4 × 1016 neutrons cm?2, are lower by a factor of approximately 2.5, indicating that nearly half of the 149Sm captures occurred earlier. One possible irradiation history places the center of proto‐Norton County at a depth of 88 cm in a large body for 140 Ma prior to its liberation as a meteoroid with a radius of 50 cm and further CRE for 100 Ma.  相似文献   

17.
S.K. Atreya  T.M. Donahue 《Icarus》1975,25(2):335-338
The role of hydrocarbons as a possible sink for H+ and H3+ ions in the lower ionosphere of the outer planets is examined. Calculations indicate that H+ and H3+ are efficiently converted to hydrocarbon ions on reaction with methane. The terminal ions, CH5+ and C2H5+ are rapidly neutralized in dissociative recombination with electrons. Extreme ultraviolet photolysis of hydrocarbons as a potential additional source of lower elevation ions in investigated.  相似文献   

18.
Data from the Ion Mass Analyzer (IMA) sensor of the ASPERA-3 instrument suite on Mars Express have been analyzed to determine the mass composition of the escaping ion species at Mars. We have examined 77 different ion-beam events and we present the results in terms of flux ratios between the following ion species: CO+2/O+ and O+2/O+. The following ratios averaged over all events and energies were identified: CO+2/O+ = 0.2 and O+2/O+ = 0.9. The values measured are significantly higher, by a factor of 10 for O+2/O+, than a contemporary modeled ratio for the maximum fluxes which the martian ionosphere can supply. The most abundant ion species was found to be O+, followed by O+2 and CO+2. We estimate the loss of CO+2 to be by using the previous measurements of Phobos-2 in our calculations. The dependence of the ion ratios in relation to their energy ranges we studied, 0.3-3.0 keV, indicated that no clear correlation was found.  相似文献   

19.
Over the past 10 years the isotopic ratios of carbon (12C/13C) and nitrogen (14N/15N) have been determined for a dozen comets, bright enough to allow obtaining the required measurements from the ground. The ratios were derived from high-resolution spectra of the CN coma measured in the B2∑+−X2∑+ (0, 0) emission band around 387 nm. The observed comets belong to different dynamical classes, including dynamically new as well as long- and short-period comets from the Halley- and Jupiter-family. In some cases the comets could be observed at various heliocentric distances. All values determined for the carbon and nitrogen isotopic ratios were consistent within the error margin irrespective of the type of comet or the heliocentric distance at which it was observed. Our investigations resulted in average ratios of 12C/13C=91±21 and nitrogen 14N/15N=141±29. Whilst the value for the carbon isotopic ratio is in good agreement with the solar and terrestrial value of 89, the nitrogen isotopic ratio is very different from the telluric value of 272.  相似文献   

20.
Abstract— The presence of several short-lived (now extinct) radionuclides in the early solar system demands that they were synthesized and added to preexisting solar system materials shortly (on a time scale on the order of the relevant radionuclide lifetime) before formation of solar system solids. For diverse reasons, it is often suggested that the solar system distributions of these radionuclides were radically heterogeneous, perhaps because of the late addition. Much attention has been given to the astrophysical circumstances that might govern the synthesis and distribution of these short-lived radionuclides, but comparatively little attention has been devoted to the distribution of cosynthesized isotopes. The focus of this paper is a systematic, quantitative evaluation of the collateral consequences in stable and long-lived isotopes that might be expected if short-lived radionuclides, in particular 26Al or 53Mn, were injected at their canonical levels and inhomogeneously distributed in the early solar system. We mix model massive star yields of Meyer et al. (1995) and Woosley and Weaver (1995) into a reservoir of cosmic composition, as tabulated by Anders and Grevesse (1989). To mitigate the effects of systematic deviations that may be present in these mixtures due to uncertainties in model stellar yields, we follow Timmes and Clayton (1996) and also mix into a “renormalized” proxy solar system composition computed from a galactic chemical evolution model based primarily on the stellar yields of Woosley and Weaver (1995). The results are very unfavorable to the likelihood of heterogeneously distributed 26Al derived from supernova ejecta. If a massive star is invoked to account for 26Al, its ejecta must have been rather uniformly distributed, as inferred from the lack of measured collateral anomalies in several elements, notably Ca, Cr, and Ni. Conversely, if 26Al were indeed radically heterogeneously distributed, some other nucleosynthetic source more efficient at producing 26Al is required. In principle, a similar statement applies to 53Mn, but the situation is more complicated. The inferred anomalies at 53Cr will depend not only on how much 53Mn is added by a heterogeneous component, but also more sensitively on the contributions to the associated stable nuclides, 53Cr, 52Cr, and 50Cr. Consideration of predicted collateral anomalies provides no direct support for heterogeneously distributed supernova-derived 53Mn, but the required quantity of supernova contribution, and thus also the collateral anomalies, are much less for 53Mn than for 26Al. With allowance for model calculation uncertainties, it could be argued that anomalies collateral to heterogeneous 53Mn might be small enough to have evaded detection.  相似文献   

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