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恒星尘埃的实验室研究--实验天体物理学 总被引：1，自引：0，他引：1
原始球粒陨石含有来自恒星的微小固体颗粒(微米级),这些尘埃的同位素组成与太阳系物质截然不同,它们是目前唯一能直接获得的恒星固体样品．已发现的恒星尘埃有金刚石、石墨、碳化硅、刚玉、尖晶石、氮化物、和硅酸盐等,它们的母体恒星包括红巨星,AGB恒星、新星和超新星．对恒星尘埃的研究,使得更深入地了解星系的化学演化历史、恒星内部的核反应和湍流机制、恒星大气中尘埃的形成、星际介质物理现象等．恒星尘埃把天体物理领域延伸到了微观世界,它有机地结合了地球化学实验技术和天体物理理论,开辟了一门崭新的天文学分支实验天体物理学． 相似文献
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Abstract— Carbon stars are an important source of presolar TiC, SiC, and graphite grains found in meteorites. The elemental abundances in the stellar sources of the SiC grains are inferred by using condensation calculations. These elemental abundances, together with C isotopic compositions, are used to identify possible groups of carbon stars that may have contributed SiC grains to the presolar dust cloud. The most likely parent stars of meteoritic SiC mainstream grains are N-type carbon stars and evolved subgiant CH stars. Both have s-process element abundances higher than solar and 10 < 12C/13C < 100 ratios. The J stars and giant CH stars, with solar and greater than solar abundances of s-process elements, respectively, are good candidate parents for the ‘A’ and ‘B’ SiC grains with low 12C/13C ratios. A special subgroup of CH giant stars with very large 12C/13C ratios could have parented the ‘Y’ SiC grains with 12C/13C ratios > 100. The carbon star population (e.g., N, R, J, CH groups) needed to provide the observed SiC grains is compared to the current population of carbon stars. This comparison suggests that low-metallicity CH stars may have been more abundant in the past (>4.5 Ga ago) than at present. This suggestion is also supported by condensation-chemistry modeling of the trace element patterns in the SiC grains that shows that subsolar Fe abundances may be required in the stellar sources for many SiC grains. The results of this study suggest that presolar SiC grains in meteorites can provide information about carbon stars during galactic evolution. 相似文献
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Abstract— We have measured the titanium isotopic compositions of 23 silicon carbide grains from the Orgueil (CI) carbonaceous chondrites for which isotopic compositions of silicon, carbon, and nitrogen and aluminum‐magnesium systematics had been measured previously. Using the 16 most‐precise measurements, we estimate the relative contributions of stellar nucleosynthesis during the asymptotic giant branch (AGB) phase and the initial compositions of the parent stars to the compositions of the grains. To do this, we compare our data to the results of several published stellar models that employ different values for some important parameters. Our analysis confirms that s‐process synthesis during the AGB phase only slightly modified the titanium compositions in the envelopes of the stars where mainstream silicon carbide grains formed, as it did for silicon. Our analysis suggests that the parent stars of the >1 μm silicon carbide grains that we measured were generally somewhat more massive than the Sun (2–3 M⊙) and had metallicities similar to or slightly higher than solar. Here we differ slightly from results of previous studies, which indicated masses at the lower end of the range 1.5–3 M⊙ and metallicities near solar. We also conclude that models using a standard 13C pocket, which produces a good match for the main component of s‐process elements in the solar system, overestimate the contribution of the 13C pocket to s‐process nucleosynthesis of titanium found in silicon carbide grains. Although previous studies have suggested that the solar system has a significantly different titanium isotopic composition than the parent stars of silicon carbide grains, we find no compelling evidence that the Sun falls off of the array defined by those stars. We also conclude that the Sun does lie on the low‐metallicity end of the silicon and titanium arrays defined by mainstream silicon carbide grains. 相似文献
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Laura B. Seifert Pierre Haenecour Tarunika Ramprasad Adrian J. Brearley Thomas J. Zega 《Meteoritics & planetary science》2023,58(3):360-382
Dust grains that formed around ancient stars and in stellar explosions seeded the early solar protoplanetary disk. While most of such presolar grains were destroyed during solar system formation, a fraction of such grains were preserved in primitive materials such as meteorites. These grains can provide constraints on stellar origins and secondary processing such as aqueous alteration and thermal metamorphism on their parent asteroids. Here, we report on the nature of aqueous alteration in the Miller Range (MIL) 07687 chondrite through the analysis of four presolar silicates and their surrounding material. The grains occur in the Fe-rich and Fe-poor lithologies, reflecting relatively altered and unaltered material, respectively. The O-isotopic compositions of two grains, one each from the Fe-rich and Fe-poor matrix, are consistent with formation in the circumstellar envelopes of low-mass Asymptotic Giant Branch (AGB)/Red Giant Branch (RGB) stars. The other two grains, also one each from the Fe-rich and Fe-poor matrix, have O-isotopic compositions consistent with formation in the ejecta of type-II supernovae (SNe). The grains derived from AGB/RGB stars include two polycrystalline pyroxene grains that contain Fe-rich rims. The SNe grains include a polycrystalline Ca-bearing pyroxene and a polycrystalline assemblage consistent with a mixture of olivine and pyroxene. Ferrihydrite is observed in all focused ion beam sections, consistent with parent-body aqueous alteration of the fine-grained matrix under oxidizing conditions. The Fe-rich rims around presolar silicates in this study are consistent with Fe-diffusion into the grains resulting from early-stage hydrothermal alteration, but such alteration was not extensive enough to lead to isotopic equilibration with the surrounding matrix. 相似文献
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We studied 14 presolar SiC mainstream grains for C‐, Si‐, and S‐isotopic compositions and S elemental abundances. Ten grains have low levels of S contamination and CI chondrite‐normalized S/Si ratios between 2 × 10?5 and 2 × 10?4. All grains have S‐isotopic compositions compatible within 2σ of solar values. Their mean S isotope composition deviates from solar by at most a few percent, and is consistent with values observed for the carbon star IRC+10216, believed to be a representative source star of the grains, and the interstellar medium. The isotopic data are also consistent with stellar model predictions of low‐mass asymptotic giant branch (AGB) stars. In a δ33S versus δ34S plot the data fit along a line with a slope of 1.8 ± 0.7, suggesting imprints from galactic chemical evolution. The observed S abundances are lower than expected from equilibrium condensation of CaS in solid solution with SiC under pressure and temperature conditions inferred from the abundances of more refractory elements in SiC. Calcium to S abundance ratios are generally above unity, contrary to expectations for stoichiometric CaS solution in the grains, possibly due to condensation of CaC2 into SiC. We observed a correlation between Mg and S abundances suggesting solid solution of MgS in SiC. The low abundances of S in mainstream grains support the view that the significantly higher abundances of excess 32S found in some Type AB SiC grains are the result of in situ decay of radioactive 32Si from born‐again AGB stars that condensed into AB grains. 相似文献
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Gwendolyn Meeus 《Earth, Moon, and Planets》2011,108(1):45-50
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
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J. G. Barzyk M. R. Savina A. M. Davis R. Gallino F. Gyngard S. Amari E. Zinner M. J. Pellin R. S. Lewis R. N. Clayton 《Meteoritics & planetary science》2007,42(7-8):1103-1119
Abstract— Analyses of the isotopic compositions of multiple elements (Mo, Zr, and Ba) in individual mainstream presolar SiC grains were done by resonant ionization mass spectrometry (RIMS). While most heavy element compositions were consistent with model predictions for the slow neutron capture process (s‐process) in low‐mass (1.5–3 M⊙) asymptotic giant branch stars of solar metallicity when viewed on single‐element three‐isotope plots, grains with compositions deviating from model predictions were identified on multi‐element plots. These grains have compositions that cannot result from any neutron capture process but can be explained by contamination in some elements with solar system material. Previous work in which only one heavy element per grain was examined has been unable to identify contaminated grains. The multi‐element analyses of this study detected contaminated grains which were subsequently eliminated from consideration. The uncontaminated grains form a data set with a greatly reduced spread on the three‐isotope plots of each element measured, corresponding to a smaller range of 13C pocket efficiencies in parent AGB stars. Furthermore, due to this reduced spread, the nature of the stellar starting material, previously interpreted as having solar isotopic composition, is uncertain. The constraint on 13C pocket efficiencies in parent stars of these grains may help uncover the mechanism responsible for formation of 13C, the primary neutron source for s‐process nucleosynthesis in low‐mass stars. 相似文献
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Via a study of the evolutionary tracks of 3∼10 M stars on the Hertzsprung-Russell diagram, the variations of the energy, density, temperature at the peak of helium-shell burning, ratio of surface luminosity of helium shell to stellar surface luminosity as well as the stellar radius are analyzed. Then the demarcation point of medium-mass stars in the evolution from early AGB stars to thermally pulsing AGB stars on the HR diagram is determined, and for 119 carbon stars our analysis agrees rather well with observation. At the same time the following is suggested. After arriving at this demarcation point in stellar evolution, in the formula of the loss of stellar wind material it is probably needed to introduce a quantity which is not concerned with the surface luminosity, but it dominates the formation of super stellar wind. On this basis and via the analysis of the structure and evolution of 5 M stars as well as the rate of mass loss of stellar wind, it is found that the effect of turbulent pressure on the mass loss of stellar wind in the stage of thermally pulsing AGB stars is rather great, hence the turbulent pressure of thermally pulsing AGB stars cannot be overlooked. Furthermore, the physical factors which possibly affect the matter loss of the stellar winds of thermally pulsing AGB stars are suggested. 相似文献
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Summary Cosmic dust grains play an important role for the thermal, dynamical, and chemical structure of the interstellar medium. This is especially true for the star formation process and the late stages of stellar evolution. Dust grains determine the spectral appearance of protostars, very young stellar objects with disk-like structures as well as of evolved stars with circumstellar envelopes.In this review, we will demonstrate that solid particles in interstellar space are both agent and subject of galactic evolution. We will especially discuss the different dust populations in circumstellar envelopes, the diffuse interstellar medium, and the molecular clouds with strong emphasis on the evolutionary aspects and the metamorphosis of these populations. 相似文献
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ERNST ZINNER 《Meteoritics & planetary science》1998,33(4):549-564
Abstract— A series of trends can be discerned in the study of presolar dust grains from primitive meteorites, and these trends might give us hints in which direction this new field of astronomy is developing. They include: (1) a focus on ever smaller components of meteorites; (2) a shift from the study of the elemental abundances in the solar system to the study of isotopic abundances; (3) a shift of emphasis from averages of the isotopic abundances as represented by the whole solar system to individual isotopic components preserved in circumstellar dust grains; (4) the preferential study of rare types of presolar dust grains; (5) the emergence of new technical capabilities for the study of individual presolar dust grains; examples include isotopic imaging and resonance ionization mass spectrometry (RIMS); and (6) a shift from a situation in which grain data confirm previously held theoretical ideas to a situation in which the experimental data impose new constraints on theoretical models of nucleosynthesis, stellar mixing and grain formation in stellar outflows. In other words, the data do not confirm but drive the theory. An example is the distribution of Si isotopic ratios in individual mainstream SiC grains for which many different theoretical explanations have been offered. There are still many unsolved problems posed by the grain data, the most difficult being the interpretation of the isotopic ratios of grains with a supernova signature (evidence for 44Ti and excesses in 28Si) in terms of theoretical models of nucleosynthesis and the mixing of supernova ejecta. Future progress is expected to come from the analysis of larger numbers of grains, the search for new types of presolar grains, the analysis of smaller grains and of more elements in a given grain, both made possible by the increase in sensitivity of ion microprobes and the extended application of RIMS, from multi-dimensional models of stellar evolution with enlarged nuclear networks, and from new measurements of nuclear cross sections. 相似文献
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As low- and intermediate-mass stars reach the asymptotic giant branch (AGB), they have developed into intriguing and complex objects that are major players in the cosmic gas/dust cycle. At this stage, their appearance and evolution are strongly affected by a range of dynamical processes. Large-scale convective flows bring newly-formed chemical elements to the stellar surface and, together with pulsations, they trigger shock waves in the extended stellar atmosphere. There, massive outflows of gas and dust have their origin, which enrich the interstellar medium and, eventually, lead to a transformation of the cool luminous giants into white dwarfs. Dust grains forming in the upper atmospheric layers play a critical role in the wind acceleration process, by scattering and absorbing stellar photons and transferring their outward-directed momentum to the surrounding gas through collisions. Recent progress in high-angular-resolution instrumentation, from the visual to the radio regime, is leading to valuable new insights into the complex dynamical atmospheres of AGB stars and their wind-forming regions. Observations are revealing asymmetries and inhomogeneities in the photospheric and dust-forming layers which vary on time-scales of months, as well as more long-lived large-scale structures in the circumstellar envelopes. High-angular-resolution observations indicate at what distances from the stars dust condensation occurs, and they give information on the chemical composition and sizes of dust grains in the close vicinity of cool giants. These are essential constraints for building realistic models of wind acceleration and developing a predictive theory of mass loss for AGB stars, which is a crucial ingredient of stellar and galactic chemical evolution models. At present, it is still not fully possible to model all these phenomena from first principles, and to predict the mass-loss rate based on fundamental stellar parameters only. However, much progress has been made in recent years, which is described in this review. We complement this by discussing how observations of emission from circumstellar molecules and dust can be used to estimate the characteristics of the mass loss along the AGB, and in different environments. We also briefly touch upon the issue of binarity. 相似文献
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P. M. Williams 《Astrophysics and Space Science》1995,224(1-2):267-270
The presence of heated circumstellar dust around WC type Wolf-Rayet stars requires the episodic or persistent condensation of carbon grains in their stellar winds. In order to survive in the stars' strong ultraviolet radiation fields, the grains must be located at least 100AU from the stellar surfaces. The densities in isotropic winds at such large distances are too low to allow grain growth and anisotropies such as clumps, disks or wind-collision wakes in colliding-wind binary systems are required to provide grain nurseries. Observational evidence for such features in grain-forming W-R stars is examined. 相似文献
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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
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Abstract— A wide range of stellar nucleosynthetic sources has been analyzed to derive their contributions of short‐lived and stable nuclei to the presolar cloud. This detailed study is required to infer the most plausible source(s) of short‐lived nuclei through a critical comparison among the various stellar sources that include AGB stars, novae, supernovae II, Ia, and Wolf‐Rayet stars that evolved to supernovae Ib/c. In order to produce the canonical value of 26Al/27Al in the early solar system, almost all stellar sources except low‐mass AGB stars imply large isotopic anomalies in Ca‐Al‐rich inclusions (CAIs). This is contrary to the observed isotopic compositions of CAIs. The discrepancy could impose stringent constraints on the formation and thermal evolution of CAIs from different chondrites. Among the various stellar scenarios, the injection of short‐lived nuclei into the previously formed solar protoplanetary disc by a massive star of an ad hoc chosen high‐injection mass cut is a possible scenario. There is a possibility of the contribution of short‐lived nuclides by a 1.5–3 M⊙ AGB star as it implies the smallest shift in stable isotopes. A low‐mass AGB star of relatively low metallicity would be even a better source of short‐lived nuclei. However, this scenario would require independent gravitational collapse of the presolar cloud coupled with ambipolar diffusion of magnetic flux. Alternatively, numerous scenarios can be postulated that involve distant (≥10 pc) massive stars can contribute 60Fe to the presolar cloud and can trigger its gravitational collapse. These scenarios would require production of 26Al and 41Ca by irradiation in the early solar system. Significant production of 26Al and 60Fe can be explained if massive, rotating Wolf‐Rayet stars that evolved to supernovae Ib/c were involved. 相似文献
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Thomas J. Zega Pierre Haenecour Christine Floss 《Meteoritics & planetary science》2020,55(6):1207-1227
We report on the isotopic, chemical, and structural properties of four O‐rich presolar grains identified in situ in the Adelaide ungrouped C2, LaPaZ Icefield (LAP) 031117 CO3.0, and Dominion Range (DOM) 08006 CO3.0 chondrites. All four grains have oxygen‐isotopic compositions consistent with origins in the circumstellar envelopes (CSE) of low‐mass O‐rich stars evolved along the red‐giant and asymptotic‐giant branch (RGB, AGB, respectively) of stellar evolution. Transmission electron microscope (TEM) analyses, enabled by focused‐ion‐beam scanning electron microscope extraction, show that the grain from Adelaide is a single‐crystal Mg‐Al spinel, and comparison with equilibrium thermodynamic predictions constrains its condensation to 1500 K assuming a total pressure ≤10?3 atm in its host CSE. In comparison, TEM analysis of two grains identified in the LAP 031117 chondrite exhibits different microstructures. Grain LAP‐81 is composed of olivine containing a Ca‐rich and a Ca‐poor domain, both of which show distinct orientations, suggesting changing thermodynamic conditions in the host CSE that cannot be precisely constrained. LAP‐104 contains a polycrystalline assemblage of ferromagnesian silicates similar to previous reports of nanocrystalline presolar Fe‐rich silicates that formed under nonequilibrium conditions. Lastly, TEM shows that the grain extracted from DOM 08006 is a polycrystalline assemblage of Cr‐bearing spinel. The grains occur in different orientations, likely reflecting mechanical assembly in their host CSE. The O‐isotopic and Cr‐rich compositions appear to point toward nonequilibrium condensation. The spinel is surrounded by an isotopically solar pyroxene lacking long‐range atomic order and could have served as a nucleation site for its condensation in the interstellar medium or the inner solar protoplanetary disk. 相似文献
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Peter HOPPE Roger STREBEL Peter EBERHARDT Sachiko AMARI Roy S. LEWIS 《Meteoritics & planetary science》2000,35(6):1157-1176
Abstract— We report isotopic abundances for C, N, Mg‐Al, Si, Ca‐Ti, and Fe in 99 presolar silicon carbide (SiC) grains of type X (84 grains from this work and 15 grains from previous studies) from the Murchison CM2 meteorite, ranging in size from 0.5 to 1.5 μm. Carbon was measured in 41 X grains, n in 37 grains, Mg‐Al in 18 grains, Si in 87 grains, Ca‐Ti in 25 grains, and Fe in 8 grains. These X grains have 12C/13C ratios between 18 and 6800, 14N/15n ratios from 13 to 200, δ29Si/28Si between ?750 and +60%0, δ30Si/28Si from ?770 to ?10%0, and 54Fe/56Fe ratios that are compatible with solar within the analytical uncertainties of several tens of percent. Many X grains carry large amounts of radiogenic 26Mg (from the radioactive decay of 26Al, half‐life ? 7 times 105 years) and radiogenic 44Ca (from the radioactive decay of 44Ti, half‐life = 60 years). While all X grains but one have radiogenic 26Mg, only ～20% of them have detectable amounts of radiogenic 44Ca. Initial 26Al/27Al ratios of up to 0.36 and initial 44Ti/48Ti ratios of up to 0.56 can be inferred. The isotopic data are compared with those expected from the potential stellar sources of SiC dust. Carbon stars, Wolf‐Rayet stars, and novae are ruled out as stellar sources of the X grains. The isotopic compositions of C and Fe and abundances of extinct 44Ti are well explained both by type Ia and type II supernova (SN) models. The same holds for 26Al/27Al ratios, except for the highest 26Al/27Al ratios of >0.2 in some X grains. Silicon agrees qualitatively with SN model predictions, but the observed 29Si/30Si ratios in the X grains are in most cases too high, pointing to deficiencies in the current understanding of the production of Si in SN environments. The measured 14n/15n ratios are lower than those expected from SN mixing models. This problem can be overcome in a 15 Modot; type II SN if rotational mixing, preferential trapping of N, or both from 15n‐rich regions in the ejecta are considered. The isotopic characteristics of C, N, Si, and initial 26Al/27Al ratios in small X grains are remarkably similar to those of large X grains (2–10 μm). Titanium‐44 concentrations are generally much higher in smaller grains, indicative of the presence of Ti‐bearing subgrains that might have served as condensation nuclei for SiC. The fraction of X grains among presolar SiC is largely independent of grain size. This implies similar grain‐size distributions for SiC from carbon stars (mainstream grains) and supernovae (X grains), a surprising conclusion in view of the different conditions for dust formation in these two types of stellar sources. 相似文献
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V. G. Karetnikov 《Astrophysics and Space Science》1996,246(2):309-314
It is shown that during contact eclipsing binaries evolution under the influence of stellar wind, magnetic stellar wind and with matter transfer by gas flow, in binary stellar systems there may take place a process of star merger (low mass stars) within 105–107 yr and a fast increase of distance between stars of massive binaries. W UMa-type stars are a finite evolutionary stage of very close and low mass binary pairs. As for contact systems of early spectral types (CE-systems), they are more varied in evolution. 相似文献
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Qian-Sheng Zhang 《天文和天体物理学研究(英文版)》2015,(4):549-568
We have developed a new stellar evolution and oscillation code YNEV,which calculates the structures and evolutions of stars,taking into account hydrogen and helium burning.A nonlocal turbulent convection theory and an updated overshoot mixing model are optional in this code.The YNEV code can evolve low-and intermediate-mass stars from the pre-main sequence to a thermally pulsing asymptotic branch giant or white dwarf.The YNEV oscillation code calculates the eigenfrequencies and eigenfunctions of the adiabatic oscillations for a given stellar structure.The input physics and numerical scheme adopted in the code are introduced.Examples of solar models,stellar evolutionary tracks of low-and intermediate-mass stars with different convection theories(i.e.mixing-length theory and nonlocal turbulent convection theory),and stellar oscillations are shown. 相似文献