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1.
We present mid-infrared spectra from individual enstatite silicate grains separated from primitive type 3 chondritic meteorites. The 2–16 μm transmission spectra were taken with microspectroscopic Fourier-transform infrared (FT-IR) techniques as part of a project to produce a data base of infrared spectra from minerals of primitive meteorites for comparison with astronomical spectra. In general, the wavelength of enstatite bands increases with the proportion of Fe. However, the wavelengths of the strong En100 bands at 10.67 and 11.67 decrease with increasing Fe content. The 11.67-μm band exhibits the largest compositional wavelength shift (twice as large as any other). Our fits of the linear dependence of the pyroxene peaks indicate that crystalline silicate peaks in the 10-μm spectra of Herbig AeBe stars, HD 179218 and 104237, are matched by pyroxenes of En90−92 and En78−80, respectively. If these simplistic comparisons with the astronomical grains are correct, then the enstatite pyroxenes seen in these environments are more Fe-rich than are the forsterite (Fo100) grains identified in the far-infrared which are found to be Mg end-member grains. This differs from the general composition of type 3 chondritic meteoritic grains in which the pyroxenes are more Mg-rich than are the olivines from the same meteorite.  相似文献   

2.
Abstract— We have investigated the 7.5–13.5 μm spectra of 30 definite or candidate carbon stars. We discuss the discrepancies between properties of SiC grains found in meteorites and the spectral properties of dust emitting in red giant winds, where most of the meteoritic grains are believed to have formed. We have investigated the nature of carbon star SiC and its relationship to meteoritic SiC dust, by using a X2-minimisation routine to fit the observed SiC features with laboratory optical constants that have been published for a variety of SiC samples. All but one of the observed astronomical SiC features are best fitted by α-SiC grains. All but one of the sources with 8–13 μm colour temperatures >1200 K (corresponding to mass-loss rates at the bottom end of the range) are best fitted by α-SiC in pure emission; whereas, all but one of the sources with 8–13 μm colour temperatures <1200 K (corresponding to higher mass-loss rates) are best fitted using self-absorbed α-SiC emission. The four sources whose SiC features are in net absorption (and which have the lowest 8–13 μm colour temperatures and, therefore, presumably the highest mass-loss rates) are also well fitted by self-absorbed α-SiC emission but with higher optical depths. Given that β-SiC is the form most commonly found in meteorites, we have searched for evidence of β-SiC in the circumstellar shells of all these stars. However, our observations provide no unequivocal evidence for the presence of β-SiC around these stars. Other discrepancies between meteoritic SiC grains and astronomical spectra are discussed. The self-absorption that we find in the observed SiC emission features has not previously been taken into account in radiative transfer modeling and so the amount of SiC present in the outflows may have been underestimated in the past.  相似文献   

3.
We have compiled infrared photometric data from the literature of practically all T Tauri stars found up to date including 444 classical T Tauri stars (CTTSs), 1698 weak-line T Tauri stars (WTTSs) and 1258 not classified T Tauri stars (3400 in total) in addition to 196 post-T Tauri stars (PTTSs). From this data bank we extract the infrared characteristics of the different groups and discuss different origins of the infrared radiation. The observational data are taken from the AKARI, IRAS, WISE and 2MASS missions. We show that in the wavelength range 1–140 μm, all T Tauri stars have infrared excesses. CTTSs have more infrared excess than WTTSs, while PTTSs have little or no infrared excess. We found that in the 1–3 μm wavelength range the infrared emission of T Tauri stars is mainly due to thermal radiation from the photosphere and hot dust grains from circumstellar envelopes. In the 3–140 μm wavelength range the infrared emission of T Tauri stars is mainly due to radiation from dusty/gaseous disks surrounding the stars. In addition, we also make a comparison between T Tauri stars and Herbig AeBe stars (HAeBe). There are some differences between these two kinds of objects in that for HAeBe stars the infrared radiation as a rule originates in dusty/gaseous disks in the 1–3 μm wavelength range, while in the range 3–12 μm it is possibly due to PAH emission for about half of HAeBe stars. In other wavelength ranges both kinds of stars have similar infrared characteristics indicating emission from dusty/gaseous disks.  相似文献   

4.
Discs around young stars are the sites of planet formation. The first step in this process is the growth of submicron grains to larger sized grains. I will review evidence for dust growth in CS discs, based on ISO and Spitzer infrared spectroscopic observations. Intermediate-mass stars, solar-type stars and even brown dwarfs will be discussed in the context of dust evolution. Furthermore, I will compare objects of several star-forming regions of different ages, and discuss the influence of the stellar parameters and environment on dust evolution, as witnessed by the observed dust characteristics. The main focus lies on what one can learn from 10 micron spectroscopy, the region where most astronomical dust species have important spectral features.  相似文献   

5.
Abstract— We present laboratory mid‐infrared absorption spectra (2.5 urn to 16.0 μm) of powdered calcium‐aluminum‐rich inclusions (CAIs) and matrix separated from the carbonaceous chondrites Allende (CV3.2), Vigarano (CV3.3), and Ornans (C03.3). Two groups of spectra with different features were found for the CAI: in the first group spectra are dominated by spinel, pyroxene, and sodalite ± nepheline, where main features occur at 9.3 μm, 10.3 μm, and 11.3 μm. In the second group, characteristic minerals are spinel and melilite with typical band maxima at 11.0 μm and 12.3 μm, and a broad feature between 14.0 μn and 15.0 μn. The position of the broad spinel feature probably depends on its iron content. Comparison of band positions in spectra from the CAI components to observed circumstellar emission spectra indicates the potential occurrence of CAI‐like material. Pyroxene‐ and spinel‐rich features could occur in spectra of dust around the Herbig Ae star HD104237, the T Tauri star Hen3‐600 and the post‐AGB star R Sge. Melilite‐ and spinel‐rich components possibly appear in the spectrum of HD 104237, Hen3‐600, 04187_1927, R Sge, and the planetary nebula Hb 12. There is also indication for a spinel component in dust from the Herbig Ae/Be star HD 179218. The spectra of the AGB stars R Cas and θ Aps show no features of CAl‐type spinel.  相似文献   

6.
Abstract— Understanding the fundamental crystal chemical controls on visible and near‐infrared reflectance spectra of pyroxenes is critical to quantitatively assessing the mineral chemistry of pyroxenes viewed by remote sensing. This study focuses on the analysis of spectroscopic measurements of a comprehensive set of synthetic Mg‐Fe pyroxenes from the visible through the near‐infrared (0.3–2.6 μm) to address the constraints of crystal structure and Fe2+ content on spin‐forbidden and spin‐allowed crystal field absorptions in Ca‐free orthopyroxenes. The chemistry and oxidation state of the synthetic pyroxenes are characterized. Coordinated Mössbauer spectroscopy is used to determine site occupancy of Fe2+ in the M1 and M2 crystallographic sites. Properties of visible and near‐infrared absorption bands of the synthetic pyroxenes are quantified using the modified Gaussian model. The 1 and 2 μm spin‐allowed crystal field absorption bands move regularly with increasing iron content, defining a much tighter trend than observed previously. A spin‐allowed crystal field absorption band at 1.2 μm is explicitly verified, even at low total iron contents, indicating that some portion of Fe2+ resides in the M1 site. The 1.2 μm band intensifies and shifts to longer wavelengths with increasing iron content. At visible wavelengths, spin‐forbidden crystal field absorptions are observed in all iron‐bearing samples. The most prominent absorption near 506 nm, attributed to iron in the M2 site, shifts to slightly longer wavelengths with iron content. The purity and extent of this pyroxene series allows visible wavelength absorption bands to be directly assigned to specific transitions of Fe2+ in the M1 and M2 sites.  相似文献   

7.
Abstract— We review the results of our recent experimental studies of astrophysical dust analogs. We discuss the condensation of amorphous silicates from mixed metal vapors, including evidence that such condensates form with metastable eutectic compositions. We consider the spectral evolution of amorphous magnesium silicate condensates as a function of time and temperature. Magnesium silicate smokes anneal readily at temperatures of about 1000–1100 K. In contrast we find that iron silicates require much higher temperatures (?1300 K) to bring about similar changes on the same timescale (days to months). We first apply these results to infrared space observatory observations of crystalline magnesium silicate grains around high‐mass‐outflow asymptotic giant branch stars in order to demonstrate their general utility in a rather simple environment. Finally, we apply these experimental results to infrared observations of comets and protostars in order to derive some interesting conclusions regarding large‐scale nebular dynamics, the natural production of organic molecules in protostellar nebulae, and the use of crystalline magnesium silicates as a relative indicator of a comet's formation age.  相似文献   

8.
We have investigated the optical properties of the carbon dust grains in the envelopes around carbon-rich asymptotic giant branch stars, paying close attention to the infrared observations of the stars and the laboratory-measured optical data of the candidate dust grain materials. We have compared the radiative transfer model results with the observed spectral energy distributions of the stars including IRAS Point Source Catalog and IRAS Low Resolution Spectrograph data. We have deduced an opacity function of amorphous carbon dust grains from model fitting with infrared carbon stars. From the opacity function, we have derived the optical constants of the AMC grains. The optical constants satisfy the Kramers–Kronig relation and produce the opacity function that fits the observations of infrared carbon stars better than previous works in the wide wavelength range 1–1000 μm. We have used simple mixtures of the AMC and silicon carbide grains for modelling. We have compared the contributions that AMC and SiC grains make to the opacity for the cases of simple mixtures of them and spherical core–mantle type grains consisting of a SiC core and an AMC mantle .  相似文献   

9.
Ceres’ surface has commonly been linked with carbonaceous chondrites (CCs) by ground‐based telescopic observations, because of its low albedo, flat to red‐sloped spectra in the visible and near‐infrared (VIS/NIR) wavelength region, and the absence of distinct absorption bands, though no currently known meteorites provide complete spectral matches to Ceres. Spatially resolved data of the Dawn Framing Camera (FC) reveal a generally dark surface covered with bright spots exhibiting reflectance values several times higher than Ceres’ background. In this work, we investigated FC data from High Altitude Mapping Orbit (HAMO) and Ceres eXtended Juling (CXJ) orbit (~140 m/pixel) for global spectral variations. We found that the cerean surface mainly differs by spectral slope over the whole FC wavelength region (0.4–1.0 μm). Areas exhibiting slopes ?1 constitute only ~3% of the cerean surface and mainly occur in the bright material in and around young craters, whereas slopes ≥?10% μm?1 occur on more than 90% of the cerean surface; the latter being denoted as Ceres’ background material in this work. FC and Visible and Infrared Spectrometer (VIR) spectra of this background material were compared to the suite of CCs spectrally investigated so far regarding their VIS/NIR region and 2.7 μm absorption, as well as their reflectance at 0.653 μm. This resulted in a good match to heated CI Ivuna (heated to 200–300 °C) and a better match for CM1 meteorites, especially Moapa Valley. This possibly indicates that the alteration of CM2 to CM1 took place on Ceres.  相似文献   

10.
Identification of astronomical dust composition rests on comparison of Infrared (IR) spectra with standard laboratory spectra; frequently, however, a single mineralogical composition is assumed for spectral matching. Advances in laboratory instrumentation have enabled very precise IR spectra to be measured on single grains and zones within grains; with a more complete set of spectral data for planetary dust, better compositional matches will be achieved for astronomical dust. We have compared several FT-IR spectroscopy techniques (open path transmission spectroscopy and diffuse reflectance spectroscopy of powders; microspectroscopy of single grains and powders and ATR spectroscopy of thin sections) to determine their utility for the direct measurement of the mid-IR spectra of small amounts of extraterrestrial grains. We have focussed our investigation on the spectra of the olivine series of silicates, (Mg,Fe)2SiO4, a species frequently identified as one of the major constituents of interstellar dust. The positions of three characteristic SiO4 stretching bands at ∼10.4, 11.3 and 12 μm were measured for comparison of the techniques. All methods gave satisfactory results, although care must be taken to guard against artefacts from sample thickness and orientation effects. Single grains hand-picked from meteorites can be analysed, but results are inaccurate if the grain size is too large (>1-10 μm). Spectra for single grains also show variations that arise from sample orientation effects. Once the analytical artefacts are taken into account, we found that measurement of powder with a diamond compression cell is best suited for the analysis of small amounts of materials.  相似文献   

11.
Abstract– Low‐iron, manganese‐enriched (LIME) olivine grains are found in cometary samples returned by the Stardust mission from comet 81P/Wild 2. Similar grains are found in primitive meteoritic clasts and unequilibrated meteorite matrix. LIME olivine is thermodynamically stable in a vapor of solar composition at high temperature at total pressures of a millibar to a microbar, but enrichment of solar composition vapor in a dust of chondritic composition causes the FeO/MnO ratio of olivine to increase. The compositions of LIME olivines in primitive materials indicate oxygen fugacities close to those of a very reducing vapor of solar composition. The compositional zoning of LIME olivines in amoeboid olivine aggregates is consistent with equilibration with nebular vapor in the stability field of olivine, without re‐equilibration at lower temperatures. A similar history is likely for LIME olivines found in comet samples and in interplanetary dust particles. LIME olivine is not likely to persist in nebular conditions in which silicate liquids are stable.  相似文献   

12.
《New Astronomy》2004,9(1):33-42
We obtained low resolution (R=100) mid-infrared (8–13 μm wavelengths) spectra of 8 nearby young main sequence stars with the Keck 1 telescope and Long-Wavelength Spectrometer (LWS) to search for 10 μm silicate (Si–O stretch) emission from circumstellar dust. No stars exhibited readily apparent emission: Spectra were then analyzed by least-squares fitting of a template based on a spectrum of Comet Hale-Bopp. Using this technique, we were able to constrain the level of silicate emission to a threshold 10 times below what was previously possible from space. We found one star, HD 17925, with a spectrum statistically different from its calibrator and consistent with a silicate emission peak of 7% of the photosphere at a wavelength of 10 μm. Excess emission at 60 μm from this star has already been reported.  相似文献   

13.
Abstract Reflectance spectra were collected from chondritic interplanetary dust particles (IDPs), a polar micrometeorite, Allende (CV3) meteorite matrix, and mineral standards using a microscope spectrophotometer. Data were acquired over the 380–1100 nm wavelength range in darkfield mode using a halogen light source, particle aperturing diaphrams, and photomultiplier tube (PMT) detectors. Spectra collected from titanium oxide (Ti4O7), magnetite (Fe3O4), and Allende matrix establish that it is possible to measure indigenous reflectivities of micrometer-sized (>5 μm in diameter) particles over the visible (VIS) wavelength range 450–800 nm. Below 450 nm, small particle effects cause a fall-off in signal into the ultraviolet (UV). Near-infrared (IR) spectra collected from olivine and pyroxene standards suggest that the ～1 μm absorption features of Fe-bearing silicates in IDPs can be detected using microscope spectrophotometry. Chondritic IDPs are dark objects (<15% reflectivity) over the VIS 450–800 nm range. Large (>1 μm in diameter) embedded and adhering single mineral grains make IDPs significantly brighter, while surficial magnetite formed by frictional heating during atmospheric entry makes them darker. Most chondritic smooth (CS) IDPs, dominated by hydrated layer silicates, exhibit generally flat spectra with slight fall-off towards 800 nm, which is similar to type CI and CM meteorites and main-belt C-type asteroids. Most chondritic porous (CP) IDPs, dominated by anhydrous silicates (pyroxene and olivine), exhibit generally flat spectra with a slight rise towards 800 nm, which is similar to outer P and D asteroids. The most C-rich CP IDPs rise steeply towards 800 nm with a redness comparable to that of the outer asteroid object Pholus (Binzel, 1992). Chondritic porous IDPs are the first identified class of meteoritic materials exhibiting spectral reflectivities (between 450 and 800 nm) similar to those of P and D asteroids. Although large mineral grains, secondary magnetite, and small particle effects complicate interpretation of IDP reflectance spectra, microscope spectrophotometry appears to offer a rapid, nondestructive technique for probing the mineralogy of IDPs, comparing them with meteorites, investigating their parent body origins, and identifying IDPs that may have been strongly heated during atmospheric entry.  相似文献   

14.
Abstract— New data in the wavelength region of approximately 0.4–2.5 μm have been obtained for asteroid 434 Hungaria. This is the most complete visible to near‐infrared spectrum to date for this object. The near‐infrared portion of the spectrum (about 0.8–2.5 μm) is smooth, featureless, and agrees well in the overlap region with new visible region data. However, visible region (about 0.45–0.9 μm) data appear to exhibit weak, broad spectral absorption features near 0.5, 0.6–0.7, and 1 μm. If real, the presence of such features would strongly constrain the compositional determination of Hungaria since it has a relatively high albedo of 46%. Most minerals that exhibit similar absorption features, and are commonly found in meteorites, have a much lower albedo. Asteroid 434 Hungaria has been observed more than six times in these overlapping spectral regions, and it is now possible to assess its mineral composition with some confidence. The dominant phase on this asteroid is an iron‐free mineral, probably enstatite. Hungaria may contain secondary phases causing subtle, visible‐region absorption features. Alternatively, the surface layer(s) of the asteroid may be contaminated by an absorbing species from an external source.  相似文献   

15.
《Planetary and Space Science》1999,47(6-7):781-785
Amorphous silicate dust grains have been produced in the laboratory by means of laser ablation of solid targets in different ambient atmospheres. In this work we show that, if the condensation occurs in the presence of hydrogen, the spectra of silicate grains, together with the characteristic 10 and 20 μm features, exhibit an absorption band around 4.6 μm. Such features, absent in the spectra of the same silicate grains produced in an oxygen atmosphere, may be attributed to a fundamental stretching vibration of –SiH functional groups bound into the grains or on their surface.Based on the cosmic abundance of the elements, silicates are expected to condense in the atmospheres of oxygen-rich stars where hydrogen is also abundant. This means that –SiH functional groups may be present also in the circumstellar and interstellar silicate dust grains. An absorption feature at 4.6 μm has already been observed in the absorbing dust of several protostellar embedded sources. The observation of a similar feature in comets can give important information on the origin and evolution of cometary material. © 1999 Elsevier Science Ltd. All rights reserved.  相似文献   

16.
The optical properties of circumstellar silicate dust grains around oxygen-rich giant stars are investigated with close attention to infrared observations of OH/IR stars. The optical constants are deduced from available astronomical and laboratory data. The deduced opacities at longer wavelengths (12 m) for OH/IR stars are higher than the one for M-type Miras possibly because of the change of optical constants depending on temperature of dust grains. Absorption and scattering efficiencies are evaluated for various grain size distributions and shapes. The results of detailed radiative transfer model calculations based on our dust parameters are compared with observational data. The Planck mean values incorporating substantial far-infrared absorption are also calculated.Yonsei University Observatory Contribution No. 91.Department of Astronomy and Space Science, Chungbuk National University Contribution No. 4.  相似文献   

17.
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   

18.
Abstract– Diagnostic mineral absorption features for pyroxene(s), olivine, phyllosilicates, and hydroxides have been detected in the near‐infrared (NIR: approximately 0.75–2.50 μm) spectra for 60% of the Tholen‐classified ( Tholen 1984, 1989 ) M‐/X‐asteroids observed in this study. Nineteen asteroids (42%) exhibit weak Band I (approximately 0.9 μm) ± Band II (approximately 1.9 μm) absorptions, three asteroids (7%) exhibit a weak Band I (approximately 1.05–1.08 μm) olivine absorption, four asteroids (9%) display multiple absorptions suggesting phyllosilicate ± oxide/hydroxide minerals, one (1) asteroid exhibits an S‐asteroid type NIR spectrum, and 18 asteroids (40%) are spectrally featureless in the NIR, but have widely varying slopes. Tholen M‐asteroids are defined as asteroids exhibiting featureless visible‐wavelength (λ) spectra with moderate albedos ( Tholen 1989 ). Tholen X‐asteroids are also defined using the same spectral criterion, but without albedo information. Previous work has suggested spectral and mineralogical diversity in the M‐asteroid population ( Rivkin et al. 1995, 2000 ; Busarev 2002 ; Clark et al. 2004 ; Hardersen et al. 2005 ; Birlan et al. 2007 ; Ockert‐Bell et al. 2008, 2010 ; Shepard et al. 2008, 2010 ; Fornasier et al. 2010 ). The pyroxene‐bearing asteroids are dominated by orthopyroxene with several likely to include higher‐Ca clinopyroxene components. Potential meteorite analogs include mesosiderites, CB/CH chondrites, and silicate‐bearing NiFe meteorites. The Eos family, olivine‐bearing asteroids are most consistent with a CO chondrite analog. The aqueously altered asteroids display multiple, weak absorptions (0.85, 0.92, 0.97, 1.10, 1.40, and 2.30–2.50 μm) indicative of phyllosilicate ± hydroxide minerals. The spectrally featureless asteroids range from metal‐rich to metal‐poor with meteorite analogs including NiFe meteorites, enstatite chondrites, and stony‐iron meteorites.  相似文献   

19.
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   

20.
Abstract— The 1.2 μm band in near‐infrared spectra of pyroxenes results from Fe2+in the M1 crystallographic site. The distribution of Fe and Mg between the M1 and M2 sites is in part a function of the cooling rate and thermal history of a pyroxene. Combining near‐infrared and Mössbauer spectra for a series of compositionally controlled synthetic Mg, Fe, Ca pyroxenes, we quantify the strength of the 1.2 μm band as a function of Fe2+in the M1 site. Near‐infrared spectra are deconvolved into component absorptions that can be assigned to the M1 and M2 sites using the modified Gaussian model. The relative strength of the 1.2 μm band is shown to be directly related to the amount of Fe2+in the M1 site measured by Mössbauer spectroscopy. The strength of the 1.2 μm band relative to the combined strengths of the 1.2 and 2 μm bands, or the M1 intensity ratio, is calculated for 51 howardite, eucrite, and diogenite (HED) meteorites. Diogenites and cumulate eucrites exhibit the lowest M1 intensity ratios, consistent with their formation as slowly cooled cumulates. Basaltic eucrites exhibit a large range of M1 intensity ratios, all of which are consistently higher than the diogenites and cumulate eucrites. This example illustrates how the M1 intensity ratio can be a used as a tool for characterizing the cooling history of remotely detected pyroxene‐dominated rocks.  相似文献   

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