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1.
We have analysed whole rock L chondrites and separate minerals from the L3 chondrite Mezö Madaras for 87Rb-87Sr dating. Contrary to other groups of chondrites, whole rocks do not define a straight line in the (87Rb/86Sr, 87Sr/86Sr) diagram. Whereas unshocked meteorites of old K-Ar age plot on the isochron defined by the other chondrites, shocked objects, with younger K-Ar ages, plot above it. Exceptions to this correlation are Bjürbole for the first type and Homestead and Peace River for the second type. Our preferred interpretation is that this displacement in the (87Rb/86Sr, 87Sr/86Sr) diagram corresponds to Rb volatilization induced by reheating. Hand-picked and heavy liquid separates from Mezö Madaras scatter in the same diagram. In addition to possible Rb loss from feldspar, evidences are found for migration of Rb and Sr between olivine and feldspar, glass or clinopyroxene. We interpret this as the consequence of a short though important heating at the time of brecciation.  相似文献   

2.
The origin of the elements from Cu to As in the UH (ultra-heavy) cosmic rays is investigated and related to current concepts of the nucleosynthesis of solar system material. The charge spectrum of the UH cosmic rays in the interval 29Z60 is studied via a fully developed propagation calculation for source abundances given by solar system material, ther-process, the massive-star core helium-burnings-process, and explosive carbon burning. None of these sources considered individually can explain the cosmic ray observations. However a combination of material produced in ther-process, the core helium-burnings-process and in explosive carbon burning provides a good representation of the experimental data. The cosmic-rayr-process is found to differ from solar systemr-process events by an underproduction of the low-massr-process peaks relative to theA195 peak. The large cosmicray abundance forZ=40–44 may be due to anr-process fission component, but this explanation is by no means certain. Improved cosmic-ray data, especially for Zn–Sr, can provide limits to the various source contributions. The model described here gives a consistent picture for the origin of both the cosmic rays and the solar system elements just beyond iron, and adds additional evidence for the importance of massive stars as a site of nucleosynthesis and the birthplace of the cosmic rays.Enrico Fermi Institute.  相似文献   

3.
The supernova yields of r-process elements are obtained as a function of the mass of their progenitor stars from the abundance patterns of extremely metal-poor stars on the left-side [{Ba/Mg}]--[{Mg/H}] boundary with a procedure proposed by Tsujimoto and Shigeyama. The ejected masses of r-process elements associated with stars of progenitor mass M ms ≤ 18 M ⊙ are infertile sources and the SNe II with 20 M ⊙ ≤ M ms ≤ 40 M ⊙ are the dominant source of r-process nucleosynthesis in the Galaxy. The ratio of these stars 20 M ⊙ ≤ M ms ≤ 40 M ⊙ with compared to the all massive stars is about∼ 18%. In this paper, we present a simple model that describes a star's [r/Fe] in terms of the nucleosynthesis yields of r-process elements and the number of SN II explosions. Combined the r-process yields obtained by our procedure with the scatter model of the Galactic halo, the observed abundance patterns of the metal-poor stars can be well reproduced.  相似文献   

4.
The abundances of 22 heavy elements from Sr to Pb have been determined for the halo star HD 29907 (T eff = 5500 K, log g = 4.64) with [Fe/H] = −1.55 using high-quality VLT/UVES spectra (ESO, Chile). The star has a moderate enhancement of r-process elements (Eu-Tm) with [r/Fe] = 0.63. In the range from Ba to Yb, the derived abundance pattern agrees well with those for strongly r-process enhanced stars (r-II stars with [Eu/Fe] > 1 and [Ba/Eu] < 0), such as CS 22892-052 and CS 31082-001, as well as with the scaled solar r-process curve and the r-process model HEW. Thus, Ba-Yb in HD 29907 originate in the r-process. Just as other moderately r-process enhanced stars studied in the literature, HD 29907 exhibits higher Sr, Y, and Zr abundances than those for r-II stars. These results confirm the assumption by other authors about the existence of an additional Sr-Zr synthesis mechanism in the early Galaxy before the onset of nucleosynthesis in asymptotic giant branch (AGB) stars. The same mechanism can be responsible for the enhancement of Mo-Ag in the star being investigated compared to r-II stars. There are no grounds to suggest the presence of s-nuclei of lead in the material of the star being investigated, because its measured abundance ratio log ɛ(Pb/Eu) = 1.20 lies within the range for the comparison stars: from log ɛ(Pb/Eu) = 0.17 (CS 31082-001) to < 1.55 (HE 1219-0312). Thus, even if there was a contribution of AGB stars to the heavy-element enrichment of the interstellar medium at the epoch with [Fe/H] = −1.55, it was small, at the level of the abundance error.  相似文献   

5.
Abstract— We report Rb-Sr analyses of phosphates from nine ordinary chondrites, more than doubling the number of meteorites for which such data are available. Ordinary chondrite phosphates characteristically have Rb/Sr ratios sufficiently low to permit accurate identification of initial 87Sr/86Sr, which is generally (but not in all cases) found to be significantly higher than the more primitive initial 87Sr/86Sr ratios inferred for carbonaceous chondrite refractory inclusions (ALL), basaltic achondrites (BABI), or bulk ordinary chondrites (in the ALL-BABI range). Such elevation of initial 87Sr/86Sr is generally considered to reflect isotopic redistribution during metamorphism, and with a model for Rb/Sr in this environment can lead to an inferred metamorphic timescale. For whole rock Rb/Sr the inferred formation intervals are typically tens of Ma (range nil to > 100 Ma). There is no evident relation between initial Sr elevation and metamorphic grade. There is not a clear difference in initial Sr effects between H and L chondrites; LL chondrites show much less (if any) elevation of initial Sr, but data are available for only two meteorites. For the first time it is possible to make a detailed comparison of initial Sr and I-Xe chronologies for several meteorites: these two potential metamorphic chronometers conspicuously fail to agree, in terms of both age and sequence of ages. A comparably definitive assessment of the comparison between initial Sr and Pb-Pb chronologies is not yet possible, but presently available data suggest that these two approaches to chondrite chronology also fail to agree. Without a correlation with metamorphic grade, or detailed agreement with an independent chronometer, it remains unclear whether initial 87Sr/86Sr in phosphates can be translated into a reliable chronometer for ordinary chondrite metamorphism, at least within simple interpretational frameworks.  相似文献   

6.
Abstract– Rb‐Sr and Sm‐Nd isotopic analyses of the lherzolitic shergottite Grove Mountains (GRV) 99027 are reported. GRV 99027 yields a Rb‐Sr mineral isochron age of 177 ± 5 (2σ) Ma and an initial 87Sr/86Sr ratio (ISr) of 0.710364 ± 11 (2σ). Due to larger uncertainties of the Sm‐Nd isotopic data, no Sm‐Nd isochron age was obtained for GRV 99027. The ε143Nd value is estimated approximately +12.2, assuming an age of 177 Ma. The ISr of GRV 99027 is distinguishable from other lherzolitic shergottites, confirming our previous conclusion that it is not paired with them ( Lin et al. 2005 ). The new data of GRV 99027 support the same age of approximately 180 Ma for most lherzolitic shergottites, and fill the small gap of ISr between Allan Hills A77005 and Lewis Cliff 88516 ( Borg et al. 2002 ). All available data are consistent with a single igneous source for the intermediate subgroup of lherzolitic shergottites.  相似文献   

7.
Abstract— The classical model of s-process nucleosynthesis, based on the concept of a steady neutron flux under astrophysical conditions pertaining to the He-burning phase of red giant stars, has successfully described observed isotopic abundances and provided information on the physical conditions of the s-process environment. Because most of the isotopes on the s-process path are stable, their relevant nuclear parameters can be measured in the laboratory so that as more accurate elemental abundance and neutron capture cross-section data have become available, the classical model has been tested under increasingly stringent conditions. Accurate determinations of the neutron capture cross sections at appropriate astrophysical conditions for the Ba isotopes have shown that the abundance of the s-only isotope 136Ba is under-produced by ～20% according to the classical model. This paper describes the accurate assessment of the meteoritic abundance of Ba by the stable isotope dilution mass spectrometric technique, based on the Cl carbonaceous chondrites Orgueil and Ivuna. Repeated analyses of these two Cl chondrites give an abundance that is identical to the presently accepted solar system value for Ba within experimental errors, which indicates a deficiency in the classical model. When combined with similar data for the s-only nuclides 116Sn and 142Nd, it is apparent that the classical model, having served a valuable function for many years, must be replaced by stellar models that more accurately reflect the dynamic nature of the He-burning phase in red giant stars, in particular, during the thermal pulses of low-mass asymptotic giant branch (AGB) stars.  相似文献   

8.
Following our hypothesis that each supernova (SN) event triggers star formation in the swept-up gas, so that newly formed stars inherit the elemental abundance pattern of individual SNe, we deduce the production sites and yields for r-process elements. We further show that a strong evidence for the origin of r-process nucleosynthesis products was just there in our backyard - supernova SN1987A -, and conclude that 20 M ⊙ SNe are the predominant production sites for r-process elements. This revised version was published online in September 2006 with corrections to the Cover Date.  相似文献   

9.
Ten spectrograms of T Vul at a dispersion of 8.47 Å mm–1 have been used for an analysis of the parameters of stellar atmosphere during its pulsation cycle. A differential curve of growth method relative to the Sun has been used to evaluate atmospheric parameters. The range in exc is from 0.88 to 1.00. Mean abundances for twenty elements have been estimated relative to [N Fe]. A deficiency by a factor of 2.4 has been found ins-process elements with respect to those formed by thee-process.  相似文献   

10.
Abstract— Video observations of the Leonid shower aboard two aircraft in the 1998 Leonid multi-instrument aircraft campaign and from ground locations in China are presented. Observing at altitude proved particularly effective, with four times higher rates due to low extinction and low angular velocity at the horizon. The rates, derived from a total of 2500 Leonid meteors, trace at least two distinct dust components. One dominated the night of 1998 November 16/17. This two-day wide component was rich in bright meteors with r = N (m + 1)/N (m) ≈ 1.5 (s = 1.4) and peaked at an influx of 3.1 ± 0.4 × 10?12 m?2 s?1 (for particles of mass <7 × 10?5 g) at solar longitude Λ0 ≈ 234.52 (Eq. J2000). The other more narrow component peaked on 1998 November 17/18 at Λ0 = 235.31 ± 0.01. Rates were elevated above the broad component between Λ0 = 235.15 and 235.40, symmetric around the current node of the parent comet 55P/Tempel-Tuttle, peaking at 5.1 ± 0.2 × 10?12 m?2 s?1. The population index was higher, r = 1.8 ± 0.1 (s = 1.7), but not as high as in past Leonid storms (r = 3.0). The flux profile of this component has an unusual asymmetric shape, which implies a blend of contributions from at least two different but relatively recent epochs of ejection. The variation of r across the profile might be due to mass-dependent ejection velocities of the narrowest component. High rates of faint meteors occurred only in an isolated five-minute interval at Λ0 = 235.198, which is likely the result of a single meteoroid breakup in space.  相似文献   

11.
Recently synthesized (t109 yr)r-process material has actinide elemental and isotopic abundance characteristics which can be used to distinguish it from ancientr-process material. The time-dependence of relative actinide abundances provides detailed chronometric information. Also it is shown that, ifN=184 is a neutron magic number as predicted by nuclear thery, the resultingr-process production peak atA281 will in turn yield, due to symmetric fission, a broad abundance peak aroundA135–140. The present results can be used to help verify or refute some current hypotheses concerning the origin of UH cosmic rays and some of the abundance anomalies in peculiar stars.  相似文献   

12.
During the formation of heavy elements in the neutron star merger (NSM) scenario with a fairly long duration of the r-process, most of the seed nuclei rapidly burn out at the initial stage. The nucleosynthesis wave rapidly reaches the region of actinoids, where beta-delayed, neutron-induced, and spontaneous fission are the main reaction channels. The fission products of transuranium elements are again drawn into the r-process as new seed nuclei to form the yields of elements with mass numbers A > 100. The contribution from the various types of fission to the formation of heavy and superheavy nuclei is investigated. The proposed r-process model applied to the NSM scenario describes well the observed abundances of chemical elements, which confirms the formation of the main r-process component in the NSM scenario. Simple extrapolations of the spontaneous fission half-lives are shown to be inapplicable for the region of nuclei with N ∼ 184, because the formulas do not reflect the increase in half-life when the shell structure changes as the number of neutrons approaches 184. The formation of superheavy elements in the r-process is possible, but their survival depends to a large extent on how reliable the predictions of nuclear parameters, including the half-lives of the forming nuclei from the island of long-lived isotopes, are. The contributions from various types of fission—neutron-induced, beta-delayed, and spontaneous one—to the formation of heavy elements in the main r-process have been determined.  相似文献   

13.
《New Astronomy Reviews》2002,46(2-7):239-245
The nearby E0 galaxy M87 in the Virgo Cluster contains one of the nearest examples of a powerful non-thermal jet source. In addition, HST spectroscopy of its nucleus give strong evidence for the presence of a ≈3×109 M⊙ central black hole (corresponding to Schwarzschild radius Rs∼0.0004 pc). These two facts together make M87 the best possible target for studying the initial jet formation and collimation process. Herein we report new 43 GHz VLBI observations of the nucleus of this galaxy, which clearly show the jet opening angle expanding rapidly as one approaches the core on scales ∼0.01 pc. We believe we have, for the first time, imaged the initial collimation region of a powerful extragalactic jet. We find significant collimation occurring on scales of 30–100 Rs, and argue this is consistent with expectations for poloidal collimation by a rotating accretion disk.  相似文献   

14.
Based on our compiled catalogue of positions, velocities, ages, and abundances of nine chemical elements for 221 classical Cepheids, we analyze the dependences of the relative abundances of α-elements as well as rapid and slow neutron capture elements on metallicity, space velocity components, and Galactocentric distance. We have found that the relative abundances of all elements in Cepheids do not depend on velocity but increase with Galactocentric distance and decrease with increasing metallicity, just as in thin-disk dwarfs and giants. In Cepheids, however, the [α/Fe]-[Fe/H] relation lies below, while the [r/Fe]-[Fe/H] and [s/Fe]-[Fe/H] relations lie above the analogous sequences for dwarfs and giants. We hypothesize that upon reaching a nearly solar metallicity in the interstellar medium of the thin disk, the most massive stars ceased to explode as type II supernovae, which mostly enriched the interstellar medium with α-elements. As a result, an underabundance of α-elements and a slight overabundance of r-process elements, which are ejected into the interstellar medium by less massive (8–10 M ⊙) type II supernovae, were formed in the next generations of stars. The overabundance of s-process elements in Cepheids can be explained by the fact that some of the s-elements were produced in the weak s-process in the interiors of massive stars, which may be able to eject the upper parts of their envelopes even without any explosion like asymptotic giant branch stars. And since such massive stars, exploding as type II supernovae, also enriched the interstellar medium with a considerable amount of iron atoms, the [s/Fe] ratios (along with [r/Fe]) in the next generations of stars must be higher in their absence.  相似文献   

15.
Recent determinations of precise abundance ratios for nearby halo and thick disk stars in the metallicity range −1.3 < [Fe/H] < −0.5 have revealed a significant cosmic spread in the abundances of oxygen, magnesium, sodium, nickel, s-process and r-process elements relative to iron. Possible explanations of these variations are reviewed. In particular, it is discussed if the differences in abundance ratios are correlated with the kinematics of the stars, and hence can be used to identify stellar populations in the Galaxy. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

16.
Abstract— We report Sr-Nd isotope parameters, rare earth element (REE), and major element data for isolated findings of tektite-like objects from western Siberia (urengoites, South-Ural glass), as well as for two indochinites. The latter were recovered in Vietnam and their overall geochemical characteristics equal those of other tektites from the indochinite subgroup of the Australasian strewn field. The three urengoites (～24 Ma) are extremely silica-rich (89 to 96 wt% SiC2), and their REE abundances vary between 45 and 76 ppm. With LaN/YbN ranging from 7.6 to 10.4 and EuN/EU* between 0.69 and 0.75, their REE distribution patterns match that of average upper crust. The urengoites have present-day ?Sr of +155 to +174 and ?Nd ranging from ?18 to ?23. Their model ages in million years are: TSruR = 1200 up to 4060 and TNdcHUR = 1570 up to 2070. Data points for the urengoites plot colinearly in the Rb-Sr evolution diagram. The age corresponding to the slope is 183 ± 30 Ma (2s?), which is indistinguishable from the intercept age of 211 Ma in the TSrUR vs. l/fRb diagram. Rubidium-strontium and Sm-Nd systematics of the urengoites indicate a heterogeneous precursor material, derived from Paleoproterozoic continental crust, which underwent Rb/Sr fractionation and partial Sr isotope homogenization in Jurassic times. Any relation between the urengoites and the Haughton impact crater, having within 2s? errors an identical age, can be excluded on the basis of isotope relationships and geochemical data. The only known South-Ural glass (～6.2 Ma) is characterized by intermediate SiO2 (65 wt%), high Al2O3 (14 wt%) and CaO (12 wt%), and low FeOTOT (0.4 wt%) contents. This unique tektite-like object contains 110 ppm REE displaying a steeply negative C1 normalized distribution with LaN/YbN of 17, and EuN/Eu1 of 0.71. The Rb abundance (10 ppm) and Rb/Sr ratio are low, and combined with a “crustal” 87Sr/86Sr ratio of 0.722, yielding an unrealistic TSruR age of 2.5 Ga. The Rb-Sr systematics imply a rather recent parent/daughter element decoupling. The TNdCHUR age of the South-Ural glass is ～1690 Ma. Geochemical data suggest that urengoites and the South-Ural glass belong to two discrete groups of tektites, whose source craters remain to be discovered.  相似文献   

17.
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   

18.
In this paper temperature enhanced -decay at high stellar temperatures is investigated in conjunction withs-process nucleosynthesis. The temperature dependence of -decay is the result of more energetic, hence faster, decay from thermally excited nuclear states. A formula for the effective halflife is derived as a function of the atomic number and weight, temperature, and the -decayQ-value of the isotope. The effect of such enhanced -decay upons-process abundances is discussed, and information abouts-process neutron flux intensities is deduced based on the calculated -decay halflives of the isotopes Nd 144 and Sm 150. The neutron flux-temperature relationships are compared to specific constant temperature carbon-burning models of massive stars which are considered to be possible sources of solar systems-process abundances. Finally, information regarding the time betweens-process termination and final interstellar injection ofs-process material is derived using the decay calculations.  相似文献   

19.
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   

20.
In Sections 1–6, we determine an approximate analytical model for the density and temperature distribution in the protoplanetary could. The rotation of the planets is discussed in Section 7 and we conclude that it cannot be determined from simple energy conservation laws.The velocity of the gas of the protoplanetary cloud is found to be smaller by about 5×103 cm s–1 in comparison to the Keplerian circular velocity. If the radius of the planetesimals is smaller than a certain limitr 1, they move together with the gas. Their vertical and horizontal motion for this case is studied in Sections 8 and 9.As the planetesimals grow by accretion their radius becomes larger thanr 1 and they move in Keplerian orbits. As long as their radius is betweenr 1 and a certain limitr 2 their gravitational interaction is negligible. In Section 10, we study the accretion for this case.In Section 11, we determine the change of the relative velocities due to close gravitational encounters. The principal equations governing the late stages of accretion are deduced in Section 12, In Section 13 there are obtained approximate analytical solutions.The effect of gas drag and of collisions is studied in Sections 14 and 15, respectively. Numerical results and conclusions concerning the last and principal stage of accretion are drawn in Section 16.  相似文献   

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