Type II Supernova Matter in a Silicon Carbide Grain from the Murchison Meteorite

The circumstellar silicon carbide (SiC) grain X57 from the Murchison meteorite contains large amounts of radiogenic calcium-44 (20 times its solar system abundance) and has an anomalous silicon isotopic composition, different from other circumstellar SiC grains. Its inferred initial 44Ti/Si and 44Ti/48Ti ratios are 1.6 x 10(-4) and 0.37. In addition, it contains radiogenic magnesium-26; the inferred initial 26Al/27Al ratio is 0.11. The isotopic and elemental data of X57 can be explained by selective mixing of matter from different zones of a typical type II supernova of 25 solar masses during its explosion. The high 44Ti/Si ratio requires contributions from the innermost nickel zone of the supernova to the SiC condensation site, as similarly suggested by astronomical observations.     

Isotopic compositions of cometary matter returned by Stardust

Hydrogen, carbon, nitrogen, and oxygen isotopic compositions are heterogeneous among comet 81P/Wild 2 particle fragments; however, extreme isotopic anomalies are rare, indicating that the comet is not a pristine aggregate of presolar materials. Nonterrestrial nitrogen and neon isotope ratios suggest that indigenous organic matter and highly volatile materials were successfully collected. Except for a single (17)O-enriched circumstellar stardust grain, silicate and oxide minerals have oxygen isotopic compositions consistent with solar system origin. One refractory grain is (16)O-enriched, like refractory inclusions in meteorites, suggesting that Wild 2 contains material formed at high temperature in the inner solar system and transported to the Kuiper belt before comet accretion.     

Existence of an 16O-rich gaseous reservoir in the solar nebula

Carbonaceous chondrite condensate olivine grains from two distinct petrographic settings, calcium-aluminum-rich inclusion (CAI) accretionary rims and amoeboid olivine aggregates (AOAs), are oxygen-16 (16O) enriched at the level previously observed inside CAIs. This requires that the gas in the nebular region where these grains condensed was 16O-rich. This contrasts with an 16O-poor gas present during the formation of chondrules, suggesting that CAIs and AOAs formed in a spatially restricted region of the solar nebula containing 16O-rich gas. The 16O-rich gas composition may have resulted either from mass-independent isotopic chemistry or from evaporation of regions with enhanced dust/gas ratios, possibly in an X-wind environment near the young Sun.     

18O/16O, 30Si/28Si, D/H, and 13C/12C Studies of Lunar Rocks and Minerals

The delta (18)O of minerals from lunar gabbros and basalts are: plgioclases +6.06 to +6.33), pyroxenes (+5.70 to +5.95), and ilmenites (+3.85 to +4.12). The uniformity of these results indicates isotopic equilibrium in the mineral assemblages. Estimated plagioclase-ilmenite temperautres range from 1150 degrees C to 1340 degrees C. The bulk (18)/ (16)O and (30)Si/ (28)Si ratios of these lunar rocks are identical with ratios of terrestrial basalts, but the lunar glass, breccia, and dust are slightly enriched in the heavier isotopes. The lunar hydrogen (formed from solar wind) has a delta D/H of less than-873 per mil and the value may be even lower, as it is probably contaminated with terrestrial hydrogen. The delta (13)C of lunar dust and breccia is unusually high relative to reduced carbon in meteorites or on earth.     

Formation of a black hole in the dark

We show that the black hole in the x-ray binary Cygnus X-1 was formed in situ and did not receive an energetic trigger from a nearby supernova. The progenitor of the black hole had an initial mass greater than 40 solar masses, and during the collapse to form the approximately 10-solar mass black hole of Cygnus X-1, the upper limit for the mass that could have been suddenly ejected is approximately 1 solar mass, much less than the mass ejected in a supernova. The observations suggest that high-mass stellar black holes may form promptly, when massive stars disappear silently.     

Olivine-garnet transformation in a meteorite

Garnet has been identified for the first time as a meteorite mineral in the Coorara chondrite from Western Australia. It replaces olivine grains in a 1-millimeter veinlet traversing the body of the meteorite. The associated olivine has abnormally low birefringence, which suggests a highly shocked condition. Microprobe analyses do not distinguish the garnet from the associated olivine, which has the composition (Mg(.75)Fe(.25))(2) SiO(4); the garnet may have the composition Mg(3)Fe(2)Si(3)O(12) but be unresolvable from the accompanying olivine, or alternatively is nonstoichiometric. Transformation of olivine to garnet under high pressure could have significant implications for the phase composition of the Earth's mantle.     

Comet 81P/Wild 2 under a microscope

The Stardust spacecraft collected thousands of particles from comet 81P/Wild 2 and returned them to Earth for laboratory study. The preliminary examination of these samples shows that the nonvolatile portion of the comet is an unequilibrated assortment of materials that have both presolar and solar system origin. The comet contains an abundance of silicate grains that are much larger than predictions of interstellar grain models, and many of these are high-temperature minerals that appear to have formed in the inner regions of the solar nebula. Their presence in a comet proves that the formation of the solar system included mixing on the grandest scales.     

Discovery of a transient absorption edge in the X-ray spectrum of GRB 990705

We report the discovery of a transient equivalent hydrogen column density with an absorption edge at approximately 3.8 kiloelectron volts in the spectrum of the prompt x-ray emission of gamma-ray burst (GRB) 990705. This feature can be satisfactorily modeled with a photoelectric absorption by a medium located at a redshift of approximately 0.86 and with an iron abundance of approximately 75 times the solar one. The transient behavior is attributed to the strong ionization produced in the circumburst medium by the GRB photons. The high iron abundance points to the existence of a burst environment enriched by a supernova along the line of sight. The supernova explosion is estimated to have occurred about 10 years before the burst. Our results agree with models in which GRBs originate from the collapse of very massive stars and are preceded by a supernova event.     

A general synthetic procedure for heteropolyniobates

The heteropolyanions of W, Mo, and V, which have found numerous applications, are formed simply by acidification of solutions of their oxoanions. Under similar conditions, these oxoanion precursors are not available for Nb, and Nb-oxo chemistry is dominated by formation of the Lindquist ion [Nb6O19]8- only. However, heteropolyniobate formation is favored in hydrothermal reactions of aqueous, alkaline precursor mixtures. Here we give two examples of heteropolyniobates formed by this general reaction type: K12[Ti2O2][SiNb12O40].16H2O [1], which contains chains of silicododecaniobate Keggin ions, and Na14[H2Si4Nb16O56].45.5H2O [2], a new heteropolyanion structure type.     

Exsolution in ferromagnesian olivine of the divnoe meteorite

Ferromagnesian olivine, one of the most common minerals in the solar system, has been widely regarded as a continuous solid solution, although several thermodynamic analyses have suggested the possibility of a miscibility gap at low temperatures. Natural ferromagnesian olivine from the Divnoe meteorite contains compositionally different exsolution lamellae, providing direct evidence for the existence of a miscibility gap in iron-magnesium olivine solid solutions.     

Binary Pulsar PSR 1913 + 16: Model for Its Origin

The existing observational data for the binary pulsar PSR 1913 + 16 are sufficient to give a rather well-defined model for the system. On the basis of evolutionary considerations, the pulsar must be a neutron star near the upper mass limit of 1.2 solar masses (M.). The orbital inclination is probably high, i>/= 700, and the mass of the unseen companion probably lies close to the upper limit of the range 0.25 M. to 1.0 M.. The secondary cannot be a main sequence star and is probably a degenerate helium dwarf. At the 5.6-kiloparsec distance indicated by the dispersion measure, the magnetic dipole model gives an age of approximately 4 x 104 years, a rate of change of the pulsar period of P approximately 2 nanoseconds per day, and a surface magnetic field strength approximately (1/3) that of the Crab pulsar. The pulsar is fainter than an apparent magnitude V approximately + 26.5 and is at least approximately 80 times fainter than the Crab pulsar in the x-ray band. The companion star should be fainter than V approximately + 30, and a radio supernova remnant may be detectable near the position of the pulsar at a flux level of 相似文献   

The first chemical enrichment in the universe and the formation of hyper metal-poor stars

The recent discovery of a hyper-metal-poor (HMP) star, with a metallicity Fe/H smaller than 1/100,000 of the solar ratio, together with one earlier HMP star, has raised a challenging question whether these HMP stars are the actual first-generation, low-mass stars of the universe. We argue that these HMP stars are second-generation stars formed from gases that were chemically enriched by the first-generation supernovae. The key to this solution is the very unusual abundance patterns of these HMP stars and the similarities and differences between them. We can reproduce these abundance features with core-collapse "faint" supernova models that include extensive matter mixing and fallback during explosions.     

Cordierite-spinel troctolite, a new magnesium-rich lithology from the lunar highlands

A clast of spinel troctolite containing 8 percent cordierite (Mg(2)Al(4)Si(5)O(18)) has been identified among the constituents of Apollo 15 regolith breccia 15295. The cordierite and associated anorthite, forsteritic olivine, and pleonaste spinel represent a new, Mg-rich lunar highlands lithology that formed by metamorphism of an igneous spinel cumulate. The cordierite-forsterite pair in the assemblage is stable at a maximum pressure of 2.5 kilobars, equivalent to a depth of 50 kilometers, or 10 kilometers above the lunar crust-mantle boundary. The occurrence of the clast indicates that spinel cumulates are a more important constituent of the lower lunar crust than has been recognized. The rarity of cordierite-spinel troctolite among lunar rock samples suggests that it is excavated only by large impact events, such as the one that formed the adjacent Imbrium Basin.     

Composition of the Earth

New estimates of solar composition, compared to earlier measurements, are enriched in Fe and Ca relative to Mg, Al, and Si. The Fe/Si and Ca/Al atomic ratios are 30 to 40 percent higher than chondritic values. These changes necessitate a revision in the cosmic abundances and in the composition of the nebula from which the planets accreted (which have been based on chondritic values). These new values imply that the mantle could contain about 15 weight percent FeO and more CaMgSi(2)O(6) than has been supposed. Geophysical data are consistent with a dense, FeO-rich lower mantle and a CaMgSi(2)O(6) (diopside)-rich transition region. FeO contents of 13 to 18 weight percent appear to be typical of the mantles of bodies in the inner solar system. The oldest komatiites (high-temperature MgO-rich magmas) have a similar chemistry to the derived mantle. These results favor a chemically zoned mantle.     

Neutron activation analysis of a particle returned from asteroid Itokawa

A single grain (~3 micrograms) returned by the Hayabusa spacecraft was analyzed by neutron activation analysis. This grain is mainly composed of olivine with minor amounts of plagioclase, troilite, and metal. Our results establish that the Itokawa sample has similar chemical characteristics (iron/scandium and nickel/cobalt ratios) to chondrites, confirming that this grain is extraterrestrial in origin and has primitive chemical compositions. Estimated iridium/nickel and iridium/cobalt ratios for metal in the Itokawa samples are about five times lower than CI carbonaceous chondrite values. A similar depletion of iridium was observed in chondrule metals of ordinary chondrites. These metals must have condensed from the nebular where refractory siderophile elements already condensed and were segregated.     

Remnants of the early solar system water enriched in heavy oxygen isotopes

Oxygen isotopic composition of our solar system is believed to have resulted from mixing of two isotopically distinct nebular reservoirs, 16O-rich and (17,18)O-rich relative to Earth. The nature and composition of the (17,18)O-rich reservoir are poorly constrained. We report an in situ discovery of a chemically and isotopically unique material distributed ubiquitously in fine-grained matrix of a primitive carbonaceous chondrite Acfer 094. This material formed by oxidation of Fe,Ni-metal and sulfides by water either in the solar nebula or on a planetesimal. Oxygen isotopic composition of this material indicates that the water was highly enriched in 17O and 18O (delta(17,18)O(SMOW) = +180 per thousand per mil), providing the first evidence for an extremely (17,18)O-rich reservoir in the early solar system.     

Detection of a red supergiant progenitor star of a type II-plateau supernova

We present the discovery of a red supergiant star that exploded as supernova 2003gd in the nearby spiral galaxy M74. The Hubble Space Telescope (HST) and the Gemini Telescope imaged this galaxy 6 to 9 months before the supernova explosion, and subsequent HST images confirm the positional coincidence of the supernova with a single resolved star that is a red supergiant of 8(+4)(-2) solar masses. This confirms both stellar evolution models and supernova theories predicting that cool red supergiants are the immediate progenitor stars of type II-plateau supernovae.     

Oxygen isotope variations at the margin of a CAI records circulation within the solar nebula

Micrometer-scale analyses of a calcium-, aluminum-rich inclusion (CAI) and the characteristic mineral bands mantling the CAI reveal that the outer parts of this primitive object have a large range of oxygen isotope compositions. The variations are systematic; the relative abundance of (16)O first decreases toward the CAI margin, approaching a planetary-like isotopic composition, then shifts to extremely (16)O-rich compositions through the surrounding rim. The variability implies that CAIs probably formed from several oxygen reservoirs. The observations support early and short-lived fluctuations of the environment in which CAIs formed, either because of transport of the CAIs themselves to distinct regions of the solar nebula or because of varying gas composition near the proto-Sun.     

Eight-membered cyclosilicate rings in muirite

The determination of the crystal structure of muirite, Ba(10)(Ca,Mn,Ti)(4)Si(8)O(24)(Cl,OH,O)(12) . 4H(2)O, revealed the presence of discrete cyclic silicate anions, (Si(8)O(24))(16-), formed by the condensation of eight silicate tetrahedra. This first reported occurrence of eight-membered rings is of particular interest, because rings with eight tetrahedra are reported to be energetically less stable than rings with six tetrahedra, which have been found in many minerals.     

In situ discovery of graphite with interstellar isotopic signatures in a chondrule-free clast in an L3 chondrite

Optical and scanning electron microscopy of a chondrule-free clast in the unequilibrated L3 chondrite Khohar revealed a spherical object consisting of an aggregate of small ( approximately 2- micrometer diameter), Ni-poor (0.5 to 2.89 weight percent) metal particles and fine-grained graphite (<1-micrometer diameter). The graphite has large D and 15N excesses (deltaD approximately 1500 per mil and delta15N approximately 1300 per mil) with two isotopically distinct signatures: N rich with a high D/H ratio and N poor with a high 15N/14N ratio. These excesses are the largest D and 15N excesses observed in situ in a well-characterized phase in a meteorite. The isotopic characteristics are suggestive of an interstellar origin, probably by ion-molecule reactions at low temperature in the interstellar molecular cloud from which the solar system formed. The structure and nonchondritic composition of the metal particles suggest they did not form under equilibrium conditions in the solar nebula.