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1.
《Astronomische Nachrichten》1976,297(5):217-227
The published data on the abundance determinations for magnetic Ap-stars were considered critically. The traditional methods of curve of growth were shown to be unsufficient for quantitative analysis of the atmospheres of magnetic Ap-stars due to inhomogeneous structure of the surface of these stars. Only qualitative results can be obtained by these methods. Correlations between overabundances of different elements are considered. Very distinct correlations exist between overabundances of Mn and Zr, between Fe and rare earths and others. No correlation could be found between overabundance of elements and Teff. The methods of determination of local abundances and the results obtained — overabundances of some elements in spots and underabundances of them out of spots - are considered. The significanse of these data for the theory of chemical anomalies in magnetic Ap stars are mentioned.  相似文献   

2.
We present the results of our abundance analysis for the atmosphere of the chemically peculiar Ap star HD 8441. HD 8441 is interesting in that the lines of rare-earth elements in its spectrum are weak and the longitudinal magnetic field component is small, being only several hundred gauss. Our estimates of the evolutionary status for HD 8441 have confirmed that it belongs to the group of Ap stars with weak lines of rare-earth elements in their spectra that leave the main sequence. A stratification analysis of the atmosphere of HD 8441 assuming a stepwise distribution of elements with depth reveals a nonuniform distribution of Si, Ca, Cr,Mn, and Fe with a significant increase in elemental abundances in deeper layers. The derived distribution qualitatively agrees with predictions of the theory of diffusive separation of elements under the action of radiation pressure forces and gravity. Comparison of the chemical composition and evolutionary status of HD 8441 with those of the stars HD 66318 and HD 144897 with strong magnetic fields shows that their atmospheres differ mainly by the abundances of rare-earth elements. The iron-peak elements exhibit large overabundances irrespective of the magnetic field strength.  相似文献   

3.
We have derived the abundances of the rare-earth elements (REEs) Ce, Pr, Nd, and Eu in the atmospheres of 26 magnetic peculiar (Ap) stars in the range of effective temperatures 7000–10 000 K from spectra with resolutions R = 48 000 and 80 000 and investigated the dependence of the CePrNdEu anomalies (the difference in the element abundances determined separately from lines of the first and second ionization stages) on the effective temperature. The REE anomaly is shown to decrease with increasing effective temperature virtually to the point of disappearance for all of the investigated elements, except Eu. For the best-studied element Nd the Nd anomaly has also been found to decrease with increasing magnetic field strength for cool stars. For hot stars there is no Nd anomaly in a wide range of magnetic field strengths. Since the presence of anomalies in cool Ap stars is associated with the REE concentration in the upper atmospheric layers, the lower boundary of the REE layer apparently sinks into deeper layers with increasing effective temperature and magnetic field, causing the anomalies to disappear. We have detected an anticorrelation between the abundances of iron-peak elements and rare-earth elements, which serves as additional evidence for different stratification of these elements in the atmospheres of Ap stars.  相似文献   

4.
We analyze the chemical composition of the atmospheres of a single K-type star HD 115404 and the secondary component of the V471 Tau variable. We use the technique of modeling of synthetic spectra to analyze the high-resolution spectra of these stars, taken with the RTT 150 Russian-Turkish telescope and find the abundances of 23 and 17 elements in the atmospheres of HD 115404 and V471 Tau, respectively. We demonstrate the lack of composition anomalies in the HD 115404 and show it to be consistent with the published data, inferred from equivalent widths of spectral lines. We find the abundances of 15 elements from Na to Ba to be consistent with the metallicity of the atmosphere of V471 Tau ([Fe/H] = −0.22 ± 0.12dex), which differs significantly from the average metallicity of the Hyades cluster. We show the existence of strong carbon and oxygen overabundances (by more than 1dex) due to the enrichment of the secondary by the nucleosynthesis products during the common-envelope stage of the system. On the whole, we demonstrate that V471 Tau and the other precataclysmic variables share similar composition anomalies.  相似文献   

5.
We obtain the chemical abundances of six barium stars and two CH subgiant stars based on the high signal-to-noise ratio and high resolution Echelle spectra. The neu- tron capture process elements Y, Zr, Ba, La and Eu show obvious overabundances relative to the Sun, for example, their [Ba/Fe] values are from 0.45 to 1.27. Other elements, in- cluding Na, Mg, A1, Si, Ca, Sc, Ti, V, Cr, Mn and Ni, show comparable abundances to the Solar ones, and their [Fe/H] covers a range from -0.40 to 0.21, which means they belong to the Galactic disk. The predictions of the theoretical model of wind accretion for bi- nary systems can explain the observed abundance patterns of the neutron capture process elements in these stars, which means that their overabundant heavy-elements could be caused by accreting the ejecta of AGB stars, the progenitors of present-day white dwarf companions in binary systems.  相似文献   

6.
The chemical enrichment in the galactic halo is studied, on the basis of the numerical model developed in Paper I, with paricular attention to the overabundances of O and light elements with respect to Fe shown by metal poor stars. Some representative nucleosynthesis pictures for stars of both Population I and Population II are considered and their yields are compared with observations of relative abundances in the Sun and in the halo, to identify the possible reasons of the observed compositional differences. It is found that solar elemental ratios can be reproduced if intermediate mass stars are allowed to give some contribution to the production of Fe by type-I supernovae, while the ratios of abundances observed in the halo are more similar to the relative yields produced by massive stars. These features are shared by all the nucleosynthesis schemes which have been considered. Using the best model of Paper I, we show that the steep star formation induced by the collapse has a decisive effect in maintaining the overabundances of light elements during the whole evolution of the halo. The relevance of this conclusion is discussed also in the light of a possible interpretation of the differences between the two abundance scales for globular clusters.  相似文献   

7.
The atmospheric parameters and elemental abundances for ten thick-disk red giants have been determined from high-resolution spectra by the method of model stellar atmospheres. The results of a comparative analysis of the [Na/Fe] abundances in the atmospheres of the investigated stars and thin-disk red giants are presented. Sodium in the atmospheres of thick-disk red giants is shown to have no overabundances typical of thin-disk red giants.  相似文献   

8.
Hot cluster horizontal branch (HB) stars and field subdwarf B (sdB) stars are core helium burning stars that exhibit abundance anomalies that are believed to be due to atomic diffusion. Diffusion can be effective in these stars because they are slowly rotating. In particular, the slow rotation of the hot HB stars (Teff > 11000 K), which show abundance anomalies, contrasts with the fast rotation of the cool HB stars, where the observed abundances are consistent with those of red giants belonging to the same cluster. The reason why sdB stars and hot HB stars are rotating slowly is unknown. In order to assess the possible role of magnetic fields on abundances and rotation, we investigated the occurrence of such fields in sdB stars with Teff < 30 000 K, whose temperatures overlap with those of the hot HB stars. We conclude that large‐scale organised magnetic fields of kG order are not generally present in these stars but at the achieved accuracy, the possibility that they have fields of a few hundred Gauss remains open. We report the marginal detection of such a field in SB 290; further observations are needed to confirm it (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

9.
High-resolution, high signal-to-noise ratio, blue-violet spectra of three red giant branch tip stars in M15 have been obtained with the Keck I High-Resolution Echelle Spectrograph. These spectra have been analyzed to determine the abundances of several neutron-capture elements, including the radioactive chronometer element thorium. There are two principal results of this study. First, the abundances of the heavier (Z>/=56) elements for each of the three stars is well matched by a scaled solar system r-process abundance distribution. Second, a weighted mean-observed Th/Eu ratio for the stars implies an age for the neutron-capture material in M15 stars of 14+/-3 Gyr, in reasonable agreement with other recent age estimates for Galactic globular clusters.  相似文献   

10.
The abundances of long-lived radioactive elements Th and U observed in metal-poor halo stars can be used as chronometers to determine the age of individual stars, and hence set a lower limit on the age of the Galaxy and hence of the universe. This radioactive dating requires the zero-decay productions of Th and U, which involves complicated r-process nucleosynthesis calculations. Several parametric r-process models have been used to calculate the initial abundance ratios of Th/Eu and U/Th, but, due to the sharp sensitivity of these models to nuclear physics inputs, the calculations have relatively large uncertainties which lead to large uncertainties in the age determinations. In order to reduce these uncertainties, we present a simple method to estimate the initial productions of Th and U, which only depends on the solar system abundances and the stellar abundances of stable r-process elements. From our calculations of the initial abundance ratios of Th/Eu and U/Th, we re-estimate the ages of those ver  相似文献   

11.
Absorption features in the UV spectra in about 10–15% of quasars show very broad profiles, indicating outflow velocities from the quasars of one or even two tenths the speed of light. Determinations of the abundances of some elements, particularly phosphorus, have yielded apparent large overabundances relative to solar. However, it has recently been shown that the absorbing gas which does not completely cover the continuum source of the emission-line clouds may have large optical depth so that the absorption lines are heavily saturated, leading to spurious overabundances of some elements, including phosphorus. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

12.
The abundances of light elements (deuterium, helium and lithium) are calculated in unconventional cosmologies in which these elements are produced by interaction of fast -particles ejected by massive pregalactic stars with a purely hydrogenous cosmological substrate. The number density and energetics of such stars needed for suitable production of light elements are estimated. It is shown that the models always lead to overproduction of lithium. The intensity of ultraviolet, X-ray, and -ray emission caused by explosions of pregalactic massive stars is calculated. The distortions of cosmic background radiation by Compton losses of electrons heated by explosive stars are also calculated.  相似文献   

13.
Efficient spectrographs at large telescopes have made it possible to obtain high-resolution spectra of stars with high signal-to-noise ratio and advances in model atmosphere analyses have enabled estimates of high-precision differential abundances of the elements from these spectra, i.e. with errors in the range 0.01–0.03 dex for F, G, and K stars. Methods to determine such high-precision abundances together with precise values of effective temperatures and surface gravities from equivalent widths of spectral lines or by spectrum synthesis techniques are outlined, and effects on abundance determinations from using a 3D non-LTE analysis instead of a classical 1D LTE analysis are considered. The determination of high-precision stellar abundances of the elements has led to the discovery of unexpected phenomena and relations with important bearings on the astrophysics of galaxies, stars, and planets, i.e. (i) Existence of discrete stellar populations within each of the main Galactic components (disk, halo, and bulge) providing new constraints on models for the formation of the Milky Way. (ii) Differences in the relation between abundances and elemental condensation temperature for the Sun and solar twins suggesting dust-cleansing effects in proto-planetary disks and/or engulfment of planets by stars; (iii) Differences in chemical composition between binary star components and between members of open or globular clusters showing that star- and cluster-formation processes are more complicated than previously thought; (iv) Tight relations between some abundance ratios and age for solar-like stars providing new constraints on nucleosynthesis and Galactic chemical evolution models as well as the composition of terrestrial exoplanets. We conclude that if stellar abundances with precisions of 0.01–0.03 dex can be achieved in studies of more distant stars and stars on the giant and supergiant branches, many more interesting future applications, of great relevance to stellar and galaxy evolution, are probable. Hence, in planning abundance surveys, it is important to carefully balance the need for large samples of stars against the spectral resolution and signal-to-noise ratio needed to obtain high-precision abundances. Furthermore, it is an advantage to work differentially on stars with similar atmospheric parameters, because then a simple 1D LTE analysis of stellar spectra may be sufficient. However, when determining high-precision absolute abundances or differential abundance between stars having more widely different parameters, e.g. metal-poor stars compared to the Sun or giants to dwarfs, then 3D non-LTE effects must be taken into account.  相似文献   

14.
We consider the possible observational consequences of galactic civilizations which utilize their local star as a repository for radioactive fissile waste material. If a relatively small fraction of the nuclear resources present in the crust of a terrestrial-type planet were processed via breeder reactors, the resulting stellar spectrum would be selectively modified over geological time periods provided the star has a sufficiently shallow outer convective zone. Consideration of surface convective mixing and stellar lifetimes restricts the possible candidate stars to the approximate spectra range A5-F2. The abundance anomalies resulting from the slow neutron fission of plutonium-239 and uranium-233 are presented and it is argued that these anomalous distributions are unlikely to be duplicated by natural nucleosynthesis processes. Relative to solar system abundances, the elements praseodymium and neodymium are found to be the most overabundant. These elements, along with the radioactive elements technetium and plutonium, could be used to identify A5-F2 candidate stars in a preliminary spectral survey.  相似文献   

15.
The abundances of the light (Na to Ca) elements in disc and halo stars are reviewed. New analyses are emphasized. Elements considered are the α-nuclei (Mg, Si, and Ca), and the odd-even nuclei (Na and Al, also25Mg and26Mg). The α-nuclei are overabundant (relative to Fe) in the old disc and halo stars. Halo stars ([Fe/H] < —1.2) have [α/Fe] ∼0.3 with extreme halo ([Fe/H] ≲ −2.0) stars showing possibly higher overabundances. The scatter in [α/Fe] at a given [Fe/H] is small. To within the observational errors, the abundance patterns for Mg, Si, and Ca are identical. For disc stars, the Na and Al abundances relative to Mg are almost independent of the [Fe/H]. Halo stars ([Fe/H] < −1) show [Na/Mg] < 0 and [AI/Mg] < 0, but the form of the mean relation and the scatter about the relation between [odd-even/Mg] and [Fe/H] remains uncertain.  相似文献   

16.
Based on spectra taken with a 6-m telescope, we analyzed the abundances of chemical elements in the He-weak stars HD 21699 and HD 217833, estimated their surface magnetic fields (Bs = 4000 and 4500 G, respectively) from the magnetic intensification of spectral lines, and determined their microturbulences (V t = 0.80 and 0.75 km s−1, respectively). The low values of V t show that the stellar atmospheres are stabilized by a magnetic field, which explains the presence of diffusion processes that lead to chemical anomalies. Helium is strongly underabundant, and its deficiency is −1.50 and −1.81 dex in HD 21699 and HD 217833, respectively. We used model atmospheres to determine the effective temperatures, T eff = 16 000 and 15 450 K, and surface gravities, log g = 4.15 and 3.88, for the stars from the Hδ line, implying that they lie on the main sequence near the stars of luminosity class V.  相似文献   

17.
We analyze the temperature dependence of the abundances of the chemical elements Si, Ca, Cr, and Fe in the atmospheres of normal, metallic-line (Am), magnetic peculiar (Ap), and pulsating magnetic peculiar (roAp) stars in the range 6000–15000 K. The Cr and Fe abundances in the atmospheres of Ap stars increase rapidly as the temperature rises from 6000 to 9000–10000 K. Subsequently, the Cr abundance decreases to values that exceed the solar abundance by an order of magnitude, while the Fe abundance remains enhanced by approximately +1.0 dex compared to the solar value. The temperature dependence of the abundances of these elements in the atmospheres of normal and Am stars is similar in shape, but its maximum is several orders of magnitude lower than that observed for Ap stars. In the range 6000–9500 K, the observed temperature dependences for Ap stars are satisfactorily described in terms of element diffusion under the combined action of gravitational settling and radiative acceleration. It may well be that diffusion also takes place in the atmospheres of normal stars, but its efficiency is very low due to the presence of microturbulence. We show that the magnetic field has virtually no effect on the Cr and Fe diffusion in Ap stars in the range of effective temperatures 6000–9500 K. The Ca abundance and its variation in the atmospheres of Ap stars can also be explained in terms of the diffusion model if we assume the existence of a stellar wind with a variable moderate rate of ～(2–4) × 10? 15M⊙ yr?1.  相似文献   

18.
We present the initial results of an abundance analysis of echelle UV spectra of five hot subdwarf B (sdB) stars. These stars have been identified as core helium burning objects on the extreme Horizontal Branch. Around 5% of sdBs show short-period acoustic-mode oscillations. Models predict that these oscillations are due to an opacity bump caused by the ionisation of iron group elements. The necessary metal abundance has to be maintained by diffusive equilibrium between gravitational settling and radiative levitation. However, analyses of high-resolution optical spectra has revealed that we cannot discriminate between pulsating and non-pulsating sdBs on the basis of the surface iron abundance. We have therefore obtained HST/STIS observations of three pulsators and two non-pulsators in the near- and far-UV to measure the surface abundance of elements that are unobservable from the ground. The overall aim of our study is to test diffusion and pulsation calculations by searching for significant differences between these surface abundances.  相似文献   

19.
The eclipsing and double-lined spectroscopic binary system V453 Cygni consists of two early B-type stars, one of which is nearing the terminal age main sequence and one which is roughly halfway through its main-sequence lifetime. Accurate measurements of the masses and radii of the two stars are available, which makes a detailed abundance analysis both more interesting and more precise than for isolated stars. We have reconstructed the spectra of the individual components of V453 Cyg from the observed composite spectra using the technique of spectral disentangling. From these disentangled spectra, we have obtained improved effective temperature measurements of  27 900 ± 400  and  26 200 ± 500 K  , for the primary and secondary stars, respectively, by fitting non local thermodynamic equilibrium theoretical line profiles to the hydrogen Balmer lines. Armed with these high-precision effective temperatures and the accurately known surface gravities of the stars we have obtained the abundances of helium and metallic elements. A detailed abundance analysis of the primary star shows a normal (solar) helium abundance if the microturbulence velocity derived from metallic lines is used. The elemental abundances show no indication that CNO-processed material is present in the photosphere of this high-mass terminal age main-sequence star. The elemental abundances of the secondary star were derived by a differential study against a template spectrum of a star with similar characteristics. Both the primary and secondary components display elemental abundances which are in the ranges observed in the Galactic OB stars.  相似文献   

20.
We review recent observational and theoretical results concerning the presence of actinide nuclei on the surfaces of old halo stars and their use as an age determinant. We present model calculations which show that the observed universality of abundances for 56<Z<75 elements in these stars does not necessarily imply a unique astrophysical site for the r-process. Neither does it imply a universality of abundances of nuclei outside of this range. In particular, we show that a variety of astrophysical r-process models can be constructed which reproduce the same observed universal r-process curve for 56<Z<75 nuclei, yet have vastly different abundances for Z≥75 and possibly Z<56 as well. This introduces an uncertainty into the use of the Th/Eu chronometer as a means to estimate the ages of the metal deficient stars. We do find, however, that the U/Th ratio is a robust chronometer. This is because the initial production ratio of U to Th is almost independent of the astrophysical nucleosynthesis environment. The largest remaining uncertainties in the U/Th initial production ratio are due to the input nuclear physics models.  相似文献   

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