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1.
The homogenization of inhomogeneities in the elemental composition of the interstellar medium due to stellar evolution and weak mixing are inevitably related to the action of shocks. This paper considers the influence of variations in the elemental composition on the thermal and ionizational evolution of a collisional gas with the solar metallicity that is cooled behind a shock front with a velocity of 50–120 km/s. The intensities of lines of heavy elements in plasma cooling behind a shock front depend not only on variations in the elemental composition, but also on the shock velocity, due to the different values of the critical density for the transition to the equilibrium level populations in atoms and ions of heavy elements. This circumstance can be used to determine the elemental composition of cool and warm gas of the interstellar medium, as well as the thermal history of the gas. 相似文献
2.
The properties of the relative abundances of rapid and slow neutron-capture elements are studied using a catalog containing spectroscopic abundance determinations for 14 elements produced in various nuclear-synthesis processes for 90 open clusters. The catalog also contains the positions, ages, velocities, and elements of the Galactic orbits of the clusters. The relative abundances of both r-elements (Eu) and s-elements (Y, Ba, La, and Ce) in clusters with high, elongated orbits and in field stars of the Galactic thin disk display different dependences on metallicity, age, Galactocentric distance, and the elements of the Galactic orbits, supporting the view that these objects have different natures. In young clusters, not only barium, but also the three other studied s-elements display significantly higher relative abundances than field stars of the same metallicity. The relative abundances of Eu are lower in highmetallicity clusters ([Fe/H] > -0.1) with high, elongated orbits than in field giants, on average, while the [Eu/Fe] ratios in lower-metallicity clusters are the same as those in field stars, on average, although with a large scatter. The metallicity dependence of the [O, Mg/Eu] ratios in clusters with high, elongated orbits and in field stars are substantially different. These and other described properties of the Eu abundances, together with the properties of the abundances of primary a-elements, can be understood in a natural way if clusters with high, elongated orbits with different metallicities formed as a result of interactions of two types of high-velocity clouds with the interstellar medium of the Galactic disk: low-metallicity highvelocity clouds that formed from “primordial” gas, and high-metallicity clouds with intermediate velocities that formed in “Galactic fountains.” 相似文献
3.
The influence of various chemical elements on radiative cooling of the gas flowing from a viscous jump is investigated in a model with a stationary shock in the atmosphere of a cool star. A closed system of equations is written for the thermal energy per heavy particle, the electron temperature, and the relative concentrations of elements in all ionization states. In addition to hydrogen and helium, atomic, singly ionized, and doubly ionized carbon, nitrogen, oxygen, sodium, magnesium, aluminum, silicon, sulfur, potassium, calcium, and iron are included, assuming they have their normal cosmic abundances. The high optical depth in Lyman-series lines leads to a return of the thermal energy to electrons via secondary collisions. As a result, the contribution of hydrogen to the cooling rate falls to the level of the contribution of metals, mainly carbon, magnesium, and iron. Thus, such shock models are able to explain the presence of bright metal lines in the spectra of cool and solar-type stars. 相似文献
4.
Andrew M Davis Tsuyoshi Tanaka Lawrence Grossman Typhoon Lee G.J Wasserburg 《Geochimica et cosmochimica acta》1982,46(9):1627-1651
Thirty-seven major, minor and trace elements were determined by INAA and RNAA in samples of hibonite, black rim and portions of friable rim from an unusual Allende inclusion, HAL. The peculiar isotopic, mineralogical and textural properties of HAL are accompanied by very unusual trace element abundances. The most striking feature of the chemistry is the virtual absence of Ce from an inclusion otherwise highly enriched in REE compared to Cl chondrites. HAL is also depleted in Sr, Ba, U, V, Ru, Os and Ir, relative to other refractory elements. Of the lithophile elements determined which are normally considered to be refractory in a gas of solar composition, Sr, Ba, Ce, U and V are the most volatile in oxidizing gases. The distribution of REE between hibonite and rims seems to have been established when hibonite and other refractory minerals were removed at slightly different temperatures from a hot, oxidizing gas in which they previously coexisted as separate grains. On the basis of HAL's chemical and isotopic composition, possible locations for the chemical and mass dependent isotopic fractionation are in ejecta from the low temperature helium-burning zone of a supernova and in the locally oxidizing environment generated by evaporation of interstellar grains of near-chondritic chemical composition. 相似文献
5.
A catalog compiling the parameters of 346 open clusters, including their metallicities, positions, ages, and velocities has been composed. The elements of the Galactic orbits for 272 of the clusters have been calculated. Spectroscopic determinations of the relative abundances, [el/Fe], for 14 elements synthesized in various nuclear processes averaged over data from 109 publications are presented for 90 clusters. The compiled data indicate that the relative abundances of primary α elements (oxygen and magnesium) exhibit different dependences on metallicity, age, Galactocentric distance, and the elements of the Galactic orbits in clusters with high, elongated orbits satisfying the criterion (Zmax2 + 4e2)1/2 > 0.40 and in field stars of the Galactic thin disk (Zmax is the maximum distance of the orbit from the Galactic plane in kiloparsec and e is the eccentricity of the Galactic orbit). Since no systematic effects distorting the relative abundances of the studied elements in these clusters have been found, these difference suggest real differences between clusters with high, elongated orbits and field stars. In particular, this supports the earlier conclusion, based on an analysis of the elements of the Galactic orbits, that some clusters formed as a result of interactions between high-velocity,metal-poor clouds and the interstellar mediumof theGalactic thin disk. On average, clusters with high, elongated orbits and metallicities [Fe/H] < -0.1 display lower relative abundances of the primary a elements than do field stars. The low [O, Mg/Fe] ratios of these clusters can be understood if the high-velocity clouds that gave rise to them were formed of interstellar material from regions where the star-formation rate and/or the masses of Type II supernovae were lower than near the Galactic plane. It is also shown that, on average, the relative abundances of the primary a elements are higher in relatively metal-rich clusters with high, elongated orbits than in field stars. This can be understood if clusters with [Fe/H] > -0.1 formed as a result of interactions between metal-rich clouds with intermediate velocities and the interstellar medium of the Galactic disk; such clouds could form from returning gas in a so-called “Galactic fountain.” 相似文献
6.
Extraterrestrial organic matter in meteorites potentially retains a unique record of synthesis and chemical/thermal modification by parent body, nebular and even presolar processes. In a survey of the elemental and isotopic compositions of insoluble organic matter (IOM) from 75 carbonaceous, ordinary and enstatite chondrites, we find dramatic variations within and between chondrite classes. There is no evidence that these variations correlate with the time and/or location of chondrite formation, or with any primary petrologic or bulk compositional features that are associated with nebular processes (e.g., chondrule and volatile trace element abundances). Nor is there evidence for the formation of the IOM by Fischer-Tropsch-Type synthesis in the nebula or in the parent bodies. The elemental variations are consistent with thermal maturation and/or oxidation of a common precursor. For reasons that are unclear, there are large variations in isotopic composition within and between chondrite classes that do not correlate in a simple way with elemental composition or petrologic type. Nevertheless, because of the pattern of elemental variations with petrologic type and the lack of any correlation with the primary features of the chondrite classes, at present the most likely explanation is that all IOM compositional variations are the result of parent body processing of a common precursor. If correct, the range of isotopic compositions within and between chondrite classes implies that the IOM is composed of several isotopically distinct components whose relative stability varied with parent body conditions. The most primitive IOM is found in the CR chondrites and Bells (CM2). Isotopically, the IOM from these meteorites resembles the IOM in interplanetary dust particles. Chemically, their IOM resembles the CHON particles of comet Halley. Despite the large isotopic anomalies in the IOM from these meteorites, it is uncertain whether the IOM formed in the interstellar medium or the outer Solar System, although the former is preferred here. 相似文献
7.
Detailed study of the petrographic and chemical properties of carbonaceous chondrites shows that the four distinct petrographic subtypes may be related to one of two distinct chemical subdivisions. These subdivisions are recognized primarily by the relative abundances of the nonvolatile elements Si, Ca, Al, Ti, Cu and Fe. C1, C2 and C3(O) chondrites form one subdivision. Vigarano subtype chondrites form the other subdivision and include chondrites previously referred to as C2, C3 and C4. Normalized to silicon, the abundances of Ca, Al and Ti are relatively enriched in Vigarano subtype chondrites, whereas Fe and Cu are relatively more abundant in C1, C2 and C3(O) chondrites. Volatile elements tend to correlate with petrographic subtypes rather than with chemical subdivisions. The available data suggest that nonvolatile element chemical fractionation of carbonaceous chondrites into the two chemical subdivisions occurred before chondrule formation and that present textural and mineralogic properties and volatile element abundances can be attributed to variations in chondrule-producing and accretion processes. 相似文献
8.
Yu. N. Mishurov 《Astronomy Reports》2006,50(1):12-22
The effects on the formation of Galactic shocks and the vertical structure of the Galactic disk due to thermal processes in a cloudy interstellar medium as it flows through a spiral density wave in the plane of the Galactic disk are considered. The evolution of the gas is fundamentally different, depending on the thermal properties of the medium. For example, if it is compressed in the horizontal direction (parallel to the Galactic plane) by the gravitational forces of the spiral density waves responsible for the formation of spiral arms, an isothermal and adiabatic medium is swept out in the vertical direction. However, on the contrary, a medium whose volume loss function increases fairly rapidly with density and temperature is further compressed under the action of the overall gravitational field of the galaxy. This effect is referred to as “self-focusing,” and may serve as an additional mechanism to explain the recently discovered anticorrelation between the width of the atomic hydrogen layer in the Galaxy and the gas density. The difference in the vertical behavior of media with different thermal properties can be used as an indicator of the thermal properties of a particular component of the interstellar gas (atomic or molecular). Attention is drawn to the fact that Galactic shocks themselves represent a mechanism that can heat the ensemble of clouds, i.e., increase the dispersion of cloud velocities. The vertical structure of a Galactic shock front is constructed, which is in qualitative agreement with the “bow shock” inferred from radio data. 相似文献
9.
R Alexander R.I Kagi S.J Rowland P.N Sheppard T.V Chirila 《Geochimica et cosmochimica acta》1985,49(2):385-395
The distributions of di- and trimethylnaphthalenes in two sedimentary sequences from Western Australia have been examined by capillary gas chromatography and combined gas chromatography-mass spectrometry. A general decrease was observed in the relative proportions of αα-dimethylnaphthalenes and ααβ-trimethylnaphthalenes with increasing thermal maturity. Similar trends were also observed for six crude oils which have very different ratios of ethylcholestane epimers indicating very different thermal histories. These results suggest that changes in the relative abundances of certain methyl substituted naphthalenes may be useful indicators of thermal maturity of sedimentary organic matter, and the use of a number of isomer ratios is illustrated. 相似文献
10.
The elemental abundances in the atmosphere of the red dwarf HD 32147, which belongs to the HR 1614 moving groups, are analyzed. The atmospheric parameters determined from spectroscopic data (the condition of equal abundances for neutral and ionized atoms of a given element) differ considerably from those derived from photometry and parallax data. The abundances of several elements are also anomalous, with the anomaly increasing with decreasing ionization potential. It is concluded that this star is a red dwarf displaying solar-like activity; i.e., having dark (cool) spots on its surface, which may sometimes be considerable in size. Modeling synthetic spectra of stars with cool spots on their surfaces, with the spectral lines consisting of two components formed in media with different temperatures, indicate that the spectroscopic atmospheric parameters derived in such cases are incorrect; this can also explain the observed dependence of the elemental abundances on the corresponding ionization potentials. This leads to the conclusion thatHD32147 is indeed a star with solar-like activity. Several other such stars considered as examples display the same anomalies as those of HD 32147. These modeling results are also valid for Ap and Am stars, and are able to explain short-wavelength observations of the Sun and some stars (the FIP effect). 相似文献
11.
From their birth as condensates in the outflows of oxygen-rich evolved stars, processing in interstellar space, and incorporation into disks around new stars, amorphous silicates predominate in most astrophysical environments. Amorphous silicates were a major building block of our Solar System and are prominent in infrared spectra of comets. Anhydrous interplanetary dust particles (IDPs) thought to derive from comets contain abundant amorphous silicates known as GEMS (glass with embedded metal and sulfides) grains. GEMS grains have been proposed to be isotopically and chemically homogenized interstellar amorphous silicate dust. We evaluated this hypothesis through coordinated chemical and isotopic analyses of GEMS grains in a suite of IDPs to constrain their origins. GEMS grains show order of magnitude variations in Mg, Fe, Ca, and S abundances. GEMS grains do not match the average element abundances inferred for ISM dust containing on average, too little Mg, Fe, and Ca, and too much S. GEMS grains have complementary compositions to the crystalline components in IDPs suggesting that they formed from the same reservoir. We did not observe any unequivocal microstructural or chemical evidence that GEMS grains experienced prolonged exposure to radiation.We identified four GEMS grains having O isotopic compositions that point to origins in red giant branch or asymptotic giant branch stars and supernovae. Based on their O isotopic compositions, we estimate that 1-6% of GEMS grains are surviving circumstellar grains. The remaining 94-99% of GEMS grains have O isotopic compositions that are indistinguishable from terrestrial materials and carbonaceous chondrites. These isotopically solar GEMS grains either formed in the Solar System or were completely homogenized in the interstellar medium (ISM). However, the chemical compositions of GEMS grains are extremely heterogeneous and seem to rule out this possibility. Based on their solar isotopic compositions and their non-solar elemental compositions we propose that most GEMS grains formed in the nebula as late-stage non-equilibrium condensates. 相似文献
12.
We propose a model explaining the presence of vast regions of partially ionized gas in the interstellar medium. The circumstellar envelope of a hot star absorbs soft ionizing radiation, but transmits an appreciable fraction of the hard photons, which are absorbed much more weakly than photons with energies close to the ionization limit. For this reason, the radiation attenuated by the envelope becomes harder, and can penetrate to larger distances. For stars hotter than 50 000 K, the transition zone between the ionized and neutral gas can extend to tens or hundreds of parsecs. Thus, a region of partially ionized hydrogen, with a small gradient of the degree of ionization without a well-defined inner HII zone, can form in the interstellar medium. 相似文献
13.
E. R. Kasimova 《Astronomy Reports》2004,48(11):934-940
The impact of variations in the fraction of binary stars producing type Ia supernovae, β, on the chemical evolution of spiral galaxies is analyzed numerically. Even modest variations in β appreciably affect the evolution of the relative abundances of iron-group and alpha-process elements. If a substantial number of the damped Lα systems manifest in the spectra of quasars are due to spiral galaxies, the large scatter of the abundances of various elements displayed by these systems can be accounted for by variations in β. 相似文献
14.
Bongor盆地位于乍得境内的中非剪切带西北部,是中新生代重要的含油气盆地之一。笔者从Bongor盆地天然气组分组成及组分碳、氢同位素分析入手,综合运用常规有机地球化学分析方法,探讨了Bongor盆地不同构造单元天然气的成因及成熟度差异。分析认为,Bongor盆地天然气化学组成主要由烃类气体、非烃类气体和微量稀有气体组成,其中烃类气体属于生物热催化过渡带气、原油伴生气和凝析油伴生气等有机成因气,非烃气体主要包括无机成因的氮气、二氧化碳和氢气等。在天然气成因研究的基础上,笔者还开展了Bongor盆地烃类气体的组成及来源分析,包括气气对比和气源对比,研究认为,不同构造单元的烃类气体虽然组成相同,但相对含量略有差异,表明不同构造单元的天然气具有相似的母质来源：M组和P组的暗色泥页岩;但也不排除K组暗色泥页岩的生烃贡献。不同构造单元天然气烃类气体的成熟度差异较大,这是由于天然气源自不同层位烃源岩或同一层位烃源岩在不同演化阶段的生烃产物。 相似文献
15.
新型常压热解离化学电离装置的研制及应用 总被引：1，自引：1，他引：0
为了克服常压热解离化学电离装置(TDCI)热源控制不稳定、背景干扰较大、实验重现性相对较差等问题。文章在前期研究的基础上,分别对仪器各组成部件进行优化和改进,研制出能够根据待测物的热稳定性、极性、挥发性、熔点等性质,能够对仪器的温度、加热时间、加速电压等仪器参数连续、精确可调的新型常压热解电离源实验装置,并进行了优化和表征。与原研制的TDCI实验装置相比,新型TDCI电离装置的高压调节和温度控制的连续性和精确度以及热源的稳定性得到了极大的提高。高压调节的最小刻度从原来的1 V减小到0.1 V,温度控制系统的调控精确由原来的±5℃减小到±1℃。采用新型TDCI-MS技术,在常压和无需样品预处理的条件下,实现了实际样品可乐中痕量兴奋剂可卡因和牛奶中弱极性物质三聚氰胺的快速检测,单个样品检测时间都小于30 s,检出限低,可卡因和三聚氰胺的最低检出限分别为1.12×10-12 g/mL和2.62×10-11 g/mL,重现性好,相对标准偏差(RSD)为4.61%~9.80%,优于原TDCI电离装置的检测结果,完全满足实际样品的检测需要。由于其具有独立稳定的加热装置,与表面解吸常压电离源相比,本仪器装置更适于难挥发性化合物的快速分析。 相似文献
16.
The effect of variations in the rate of ionization of neutral chemical species by cosmic rays, ζ, on the abundances of some observed molecules in the dense cores of dark molecular clouds is studied. Changes in molecular abundances accompanying an increased (decreased) ionization rate have a single origin: the acceleration (deceleration) of processes that are affected directly or indirectlybychemical reactions with charged species. In addition to affecting the gas-phase chemistry, an increased cosmic-ray flux leads to the more efficient destruction of dust-grain mantles and also accelerates the freezing of some components onto dust. In particular, in a model with an increased ζ, the destruction of the volatile N2 molecule by ionized helium leads to the rapid accumulation of nitrogen atoms in dust-phase ammonia, which has a higher desorption energythan N2. As a result, the gas-phase abundance of NH3 and N2H+ decreases significantly. This mechanism can explain the unusual chemical structures of some dense globules, such as B68, where surprisingly low abundances of nitrogen-bearing molecules are observed together with a central drop in the NH3 and N2H+ column densities. Observations of clouds in HCN and HNC lines can discriminate between the two possible origins of the reduced NH3 and N2H+ abundances: an increased cosmic-ray flux or N2 freezing due to the higher desorption energy of this molecule. 相似文献
17.
A set of empirical relationships for the ionization correction factors used by various authors to determine the chemical compositions of the gas in nebular objects is tested. New expressions for the ionization correction factors are used to find the nebular-gas compositions in HII regions in blue compact dwarf galaxies. The abundances of He, N, O, Ne, S, and Ar in 41 HII regions are determined. The derived elemental abundances are compared with the results of other studies. The Y-O/H, Y-N/H, and Y-Z dependences are analyzed in detail. The primordial helium abundance Y p and its enrichment dY/dZ are also determined. 相似文献
18.
The results of laboratory mass-spectrometer studies of the laser-induced dissociation of molecules of simple aromatic hydrocarbons adsorbed on a quartz substrate under the conditions of deep vacuum and low temperatures are adapted to the physical and chemical conditions in regions of active star formation in molecular clouds. The main properties of the photolysis of physically adsorbed molecules compared to the photodissociation of isolated molecules in the gas phase are identified. The relevance of molecular photolytic desorption to the real conditions in the interstellar medium is analyzed, in particular, to the conditions in photodissociation regions. It is shown that the photodissociation of adsorbed benzene occurs along other channels and with appreciably lower efficiency than does the corresponding process in the gas phase. The photodissociation of aromatic hydrocarbons adsorbed on the surfaces of interstellar grains cannot make a large contribution to the abundance of hydrocarbons with small numbers of atoms observed in the interstellar medium.
相似文献19.
The results of a study of the chemical compositions of Galactic planetary nebulae taking into account two types of inhomogeneity in the nebular gas density in their envelopes are reported. New analytical expressions for the ionization correction factors have been derived and are used to determine the chemical compositions of the nebular gas in Galactic planetary nebulae. The abundances of He, N, O, Ne, S, and Ar have been found for 193 objects. The Y–Z diagrams for various He abundances are analyzed for type II planetary nebulae separately and jointly with HII regions. The primordial helium abundance Y p and enrichment ratio dY/dZ are determined, and the resulting values are compared with the data of other authors. Radial abundance gradients in the Galactic disk are studied using type II planetary nebulae. 相似文献