共查询到20条相似文献，搜索用时 31 毫秒
1.
S. G. Sichevskij 《Astronomy Reports》2017,61(3):193-205
A technique for determining a star’s radius from its atmospheric characteristics (effective temperature, surface gravity, and metallicity) is realized based on modernmodel computations of the stellar internal structure and evolution. The atmospheric characteristics can also be used to find the mass and luminosity of the star. The star’s rate of evolution and the initial mass function are taken into account when determining the stellar characteristics, increasing the correctness of the results. Computations of stellar evolution of with and without the stellar rotation taken into account make it possible to remove ambiguity due to missing data on the star’s rotational velocity. The results are checked and uncertainties estimated using stars occupying two heavily populated regions in the Hertzsprung–Russell diagram that have been well studied using various methods: the main sequence and red giant branch. Good agreement with the observations is achieved; there are almost no systematic deviations of the derived point estimates of the fundamental characteristics. The metallicities of the individual components of eclipsing variable stars are estimated using observational data on for such stars displaying lines of both components in their spectra. These metallicities were determined as a function of the stellar masses in a way that eliminates systematic deviations in the derived fundamental characteristics. 相似文献
2.
N. A. Tikhonov 《Astronomy Reports》2018,62(10):637-647
Stellar photometry of 53 low-mass spiral and irregular galaxies has been carried out using archival frames obtained by the Hubble Space Telescope. Young and old stars (blue supergiants and red giants) are distinguished on the resulting Hertzsprung–Russell diagrams, and the spatial sizes of the subsystems formed by these stars are determined. A correlation is found between the metallicities of red giants and the difference between the linear sizes of stellar systems of different ages. This dependence can be explained if the sizes of stellar subsystems expand over the lifetimes of galaxies, as well as the influence of the relationship between the mass of a galaxy and the metallicity of its stars. 相似文献
3.
V. V. Bruevich A. S. Gusev O. V. Ezhkova F. Kh. Sakhibov M. A. Smirnov 《Astronomy Reports》2007,51(3):222-233
We present the results of U BV RI CCD photometry of giant HII regions in the spiral galaxy NGC 628, acquired with the 1.5 m telescope of the Mt. Maidanak Observatory (Uzbekistan) with an angular resolution better than 1″. We estimate the ages and interstellar extinctions of these regions and identify the acting star-formation mode by comparing the observed color indices with a detailed grid of evolutionary models covering the entire range of parameters of the initial mass function and of ages of the young star-formation complexes, and taking into account two star-formation modes. We find a radial gradient of the interstellar extinction in NGC 628, which is consistent with the radial abundance gradient found earlier by other authors from independent spectrophotometry. Our age estimates agree with abundance estimates from independent observations. 相似文献
4.
Motivated by currently available direct observations of occulting tori in Seyfert galaxies and ongoing discussions about tori as important structural components in Active Galactic Nuclei, we discuss the possibility that the “spinning up” of a torus by radiation or winds could transform it into a dipolar toroidal vortex. The vortex motion balances the torus’ self-gravitation, and can explain the existence of cool, thick tori inferred from observations. In turn, the toroidal vortex can be a source of matter to feed the accretion disk. The resulting instability could result in flares accompanied by the ejection of matter. Our numerical estimates of the model parameters for luminosities close to the Eddington limit are consistent with the observational data. 相似文献
5.
Deep stellar photometry of one of the most distant Galactic globular clusters, Palomar 3, based on frames taken with the VLT in Johnson–Cousins broadband V and I filters is presented, together with medium-resolution stellar spectroscopy in the central region of the cluster obtained with the CARELEC spectrograph of the Observatoire de Haute Provence and measurements of the Lick spectral indices for the integrated spectrum. Computations of the orbital parameters of Palomar 3 and nine Galactic globular clusters with similar metallicities and ages are also presented. The orbital parameters, age, metallicity, and distance of Palomar 3 are estimated. The interstellar absorption is consistent with and supplements values fromthe literature. The need to obtainmore accurate data on the propermotions, ages, and chemical compositions of the cluster stars to elucidate the origin of this globular cluster is emphasized. 相似文献
6.
Results of a study of the kinematic and chemical properties of globular clusters of the Milky Way based on data from the Gaia DR2 catalog and meaurements with the Hubble Space Telescope are presented. A new method for dividing globular clusters into Galatic subsystems based on the elements of their Galactic orbits is proposed. Samples of globular clusters belonging to the bar/bulge, thick disk, and halo of the Milky Way are obtained. The mean metallicities of the globular clusters in various subsystems are calculated. The mean metallicities of globular clusters of the thick disk and halo display statistically significant differences. At the same time, no statistically significant differences are present between the mean metallicities of halo globular clusters moving in the direction of rotation of the Galactic disk and those moving in the retrograde direction. This argues against the suggestion that retrograde and prograde globular clusters have different origins.
相似文献7.
《Quaternary Science Reviews》2003,22(10-13):1035-1042
Single-grain OSL dating is applied to sediments from different depositional settings on the Southern High Plains of western Texas and eastern New Mexico. Criteria of acceptance are used to screen equivalent doses from individual grains and resulting distributions are evaluated in terms of normality. Wide variation is found in proportion of acceptable grains and in the distributions. While some of the latter are normal, many show broadening that may be the result of mixing of different-aged grains and skewness that may result from variant depositional and post-depositional modes. Geological modeling will be required to understand better these distributions, although for most samples means produced ages that agree with independent evidence. The resolution possible with single grains is necessary for best estimates of equivalent dose for other samples. A few samples do not agree with independent evidence, even with a normal distribution of equivalent dose. Sand dunes seem the most difficult to date accurately, probably because of mixing. 相似文献
8.
We have performed the first dynamical study of the relative motions of the components of the wide hierarchic triple star system
ADS 10288 (GL 649.1 = WDS 16579+4722), based on a 15-year series of photographic observations with the 26″ Pulkovo refractor,
supplemented with data from the WDS catalog, HIPPARCOS parallaxes, and radial velocities of the components. The radial velocities
were measured in Simeiz using the ILS stellar radial-velocity correlation spectrometer designed by A.A. Tokovinin. We applied
the parameters of the apparent motion technique, which makes it possible to derive a binary’s orbit and mass from observations
of a short arc, about 5°–10° in length. We derived preliminary orbits for the pairs AB and (AB)-C, whose orbital periods are
340 and 64 000 years, respectively. Mass estimates derived from the mass-luminosity relation are consistent with our dynamical
estimates. We determined the orientation of the orbits in Galactic coordinates. The orbital planes are not coplanar, and are
steeply inclined to the Galactic plane, as was the case in our earlier studies for most wide visual pairs we have studied. 相似文献
9.
Kingshuk Roy Choudhury 《Mathematical Geosciences》2009,41(5):535-553
Finite strain estimation is a widely used technique for the study of rock deformation in structural geology. One particular
algorithm proposed by Shimamoto and Ikeda uses the ‘average shape matrix’ of deformed markers. This paper provides a detailed
error analysis for resulting strain estimates in two dimensions. When the number of markers exceeds 100, estimators of components
of the strain tensor are shown to have an approximately Gaussian distribution with variances that increase with their mean.
Equal variance estimators are obtained by applying a log transform for the elongation and an arcsin transformation for the
orientation estimates. Confidence interval formulae for strain tensor components are proposed. Lithology specific constants
arising in these formulae are estimated from undeformed samples. The results are validated by application to simulated data
as well as observational data from thin sections of sandstone sampled from SE Ireland. 相似文献
10.
The relationship between the masses and metallicities of galaxies could be non-monotonic, due to the outflow of matter in
these systems. It is shown using a simple, one-zone, chemical-dynamicalmodel that the metallicity should be a non-monotonic
function of the mass for spheroidal dwarf galaxies with low masses of M ≤ 108
M
⊙, and a monotonically growing function for galaxies with higher masses. This is consistent with observations. 相似文献
11.
Spectra and radial-velocity curves for the precataclysmic variable EG UMa are analyzed. The system ephemeris has been improved and all system parameters determined. The parameters of the secondary are consistent with current evolutionary models for single main-sequence M stars with atmospheric metallicities exceeding the solar value by 0.5 dex. We have verified that the rotational velocity of the red dwarf exceeds the velocity corresponding to synchronous orbital motion by a factor of two to three. This suggests that the efficiency of tidal interactions between the components in the synchronization of their motion is low. The observed Ca II emission lines display reflection effects in a number of uniform spectra of EG UMa obtained during the quiescent state of the secondary. 相似文献
12.
The orbits of the visual binaries ADS 8814 and ADS 8065 are determined for the first time. The orbits were calculated using the parameters of the apparent motion, based on position observations along short arcs obtained on the 26-inch refrector of the Pulkovo Observatory, supplemented with radial-velocity observations for the stellar components in both pairs obtained on the 1-m telescope of the Simeiz Section of the Crimean Astrophysical Observatory. All previous visual and photographic observations of these stars after 1832 were also taken into account. The orbit of ADS 8814 was refined using the differential-correction method. The orbital periods of these two stars are about 800 and 6000 years, respectively. The mass estimates derived for the known parallaxes from the Hipparcos catalog correspond to the spectral types of these stars. The polar vectors of the obtained orbits in Galactic coordinates are also given. 相似文献
13.
Optimization photoionization models are computed for nine planetary nebulae with various metallicities with the aim of determining their chemical compositions. The algorithm used to compute the models is based on identifying best-fit values of the physical parameters of the nebulae by minimizing the χ2 residual for fits to the observed spectra of these objects. The reliability of the results is investigated. The abundances of He, C, N, O, Ne, Si, S, and Ar are derived, as well as the physical characteristics of the nebulae. The Y -Z dependences (where Y and Z are the mass abundances of helium and heavy elements, respectively) are analyzed for the planetary nebulae together with HII regions in blue compact dwarf galaxies. The primordial helium abundance, Y p = 0.2479 ± 0.0014, and dY/dZ = 2.99 ± 0.32 are determined, and found to be close to the corresponding values obtained using ionization correction factors. 相似文献
14.
Glauconite from eight stratigraphic horizons (Cambrian to Pennsylvanian) in the Llano Uplift, central Texas and two Cretaceous glauconites were analyzed by the Rb-Sr method. Only two untreated samples provide ages in agreement with those anticipated from current best estimates of the geologic time scale. With one exception all the other apparent ages fall short of the estimated age of deposition by as much as 22%. Low ages, the pattern customarily observed, are attributed to postdepositional loss of radiogenic 87Sr from expandable layers by weathering or during diagenesis.Detailed leaching experiments using a variety of reagents were performed to characterize the behavior of glauconite. The most promising treatment, which we recommend as standard procedure in all future studies, is with ammonium acetate which is able to purge the mineral of loosely-bound Rb and Sr while leaving tightly-bound components intact. After appropriate leach, three other Rb-Sr ages were brought into coincidence with their estimated ages of deposition.In contrast an Upper Cambrian glauconite was found to be extremely resistant to further alteration by chemical attack′, providing an age of 429 ± 17 M yr. Although 17% short of the age of deposition, this age is interpreted as the time of a real event: diagenetic recrystallization induced by burial. Comparison of data from four samples indicates that for Paleozoic glauconite, conditions exist in which the Rb-Sr system is less susceptible to mild disturbance than is the K-Ar system. 相似文献
15.
Ajay Jasra David A. Stephens Kerry Gallagher Christopher C. Holmes 《Mathematical Geology》2006,38(3):269-300
In this paper we develop a generalized statistical methodology for characterizing geochronological data, represented by a distribution of single mineral ages. The main characteristics of such data are the heterogeneity and error associated with its collection. The former property means that mixture models are often appropriate for their analysis, in order to identify discrete age components in the overall distribution. We demonstrate that current methods (e.g., Sambridge and Compston, 1994) for analyzing such problems are not always suitable due to the restriction of the class of component densities that may be fitted to the data. This is of importance, when modelling geochronological data, as it is often the case that skewed and heavy tailed distributions will fit the data well. We concentrate on developing (Bayesian) mixture models with flexibility in the class of component densities, using Markov chain Monte Carlo (MCMC) methods to fit the models. Our method allows us to use any component density to fit the data, as well as returning a probability distribution for the number of components. Furthermore, rather than dealing with the observed ages, as in previous approaches, we make the inferences of components from the “true” ages, i.e., the ages had we been able to observe them without measurement error. We demonstrate our approach on two data sets: uranium-lead (U-Pb) zircon ages from the Khorat basin of northern Thailand and the Carrickalinga Head formation of southern Australia. 相似文献
16.
Pierre Le Hir Andre Ficht Ricardo Silva Jacinto Patrick Lesueur Jean-Paul Dupont Robert Lafite Isabelle Brenon Benedicte Thouvenin Philippe Cugier 《Estuaries and Coasts》2001,24(6):950-963
A comprehensive study of fine sediment transport in the macrotidal Seine estuary has been conducted, including observations of suspended particulate matter (SPM), surficial sediment, and bathymetric data, as well as use of a three dimensional mathematical model. Tide, river regime, wind, and wave forcings are accounted. The simulated turbidity maximum (TM) is described in terms of concentration and location according to tidal amplitude and the discharge of the Seine River. The TM is mainly generated by tidal pumping, but can be concentrated or stretched by the salinity front. The computed deposition patterns depend on the TM location and are seasonally dependent. The agreement with observations is reasonable, although resuspension by waves may be overestimated. Although wave resuspension is likely to increase the TM mass, it generally occurs simultaneously with westerly winds that induce a transverse circulation at the mouth of the estuary and then disperse the suspended material. The resulting effect is an output of material related to wind and wave events, more than to high river discharge. The mass of the computed TM remains stable over 6 months and independent of the river regime, depending mainly on the spring tide amplitude. Computed fluxes at different cross-sections of the lower estuary show the shift to the TM according to the river flow and point out the rapidity of the TM adjustment to any change of river discharge. The time for renewing the TM by riverine particles has been estimated to be one year. 相似文献
17.
We present a compiled catalog of effective temperatures, surface gravities, iron and magnesium abundances, distances, velocity components, and orbital elements for stars in the solar neighborhood. The atmospheric parameters and iron abundances are averages of published values derived from model synthetic spectra for a total of about 2000 values in 80 publications. Our relative magnesium abundances were found from 1412 values in 31 publications for 876 dwarfs and subgiants using a three-step iterative averaging procedure, with weights assigned to each source of data as well as to each individual determination, and taking into account systematic deviations of each scale relative to the reduced mean scale. The estimated completeness for data sources containing more than five stars up to late December 2003 exceeds 90%. For the vast majority of stars in the catalog, the spatial-velocity components were derived from modern high-precision astrometric observations and their Galactic orbital elements were computed using a three-component model of the Galaxy, consisting of a disk, a bulge, and a massive extended halo. 相似文献
18.
We have obtained the first U BV R photoelectric light curves for a recently discovered eclipsing binary with a period of 9.33 d and an appreciable eccentricity (e = 0.08). We have used these data to determine the photometric elements of the system for a model with two spherical stars with linear limb darkening. The high accuracy of the observations enabled determination of the absolute parameters of the components using available calibrations. The masses of the components are 1.28 and 1.08 M ⊙, and their ages are two billion years. The present orientation of the orbital ellipse is unfavorable for studies of the apsidal rotation, which is essentially due to relativistic effects. 相似文献
19.
《Quaternary Science Reviews》2003,22(10-13):1139-1143
As part of a systematic palaeohydrological reconstruction of lake level fluctuations during the Last Glacial Maximum, a transect of cores from ancient Lake Xinias in central Greece has already been studied with respect to pollen, sediment and mineral magnetic analyses. The chronology was based on 14C AMS dating of terrestrial plant macrofossil remains from peat and clayey peat (Palaeogeog. Palaeoclimatol. Palaeoecol. 158 (2000) 65). This site thus provides an opportunity for the comparison of fine grain optically stimulated luminescence (OSL) ages of water–lain sediments with an independent chronology. We present here infrared (IR), post-IR blue and blue OSL characteristics of the fine grain sediments from Lake Xinias and a preliminary comparison with independent ages. The equivalent doses based on the IR results are about 40% of those based on post-IR blue stimulation, which in turn are 10–15% below those based on quartz OSL. We discuss the ages derived from the 3 signals in terms of the independent chronology, and draw conclusions about initial bleaching of the quartz and feldspar components, and the reliability of the post-IR blue signal as a chronometer. 相似文献
20.
We report an electron probe investigation of the eucrite Moore County. Moore County consists predominantly of four pyroxene phases and plagioclase, with minor tridymite, ilmenite, chromite, troilite, iron metal and possibly apatite. Temperatures have been computed for Moore County and other eucrites using three independent techniques for coexisting pairs of pyroxenes. These computed temperatures are all subsolidus and approach those calculated for slowly cooled terrestrial igneous and metamorphic rocks. These temperatures conflict with the conclusions of earlier workers that Moore County was catastrophically removed from its cumulate environment at high temperatures. The eucrites cooled to temperatures substantially below the solidus on their parent body.Our results are consistent with the conclusion that Moore County resided for an extended period of time on the eucrite parent body, perhaps from the time of its crystallization from a basaltic melt (～-4.5AE) until a few tens of millions of years ago. This extended residence time is consistent with the conclusion that the eucrite parent body is still intact, but raises dynamical objections to the tentative identification of the eucrite parent body as asteroid 4 Vesta.An alternative scenario involves ejection from the eucrite parent body, at times significantly older than those indicated by cosmic ray exposure ages, of meteoroids of sufficient size that their interiors were shielded from cosmic rays. These meteoroids were removed by low-probability mechanisms into Earth-crossing orbits where they were disrupted and ultimately sampled by the Earth. This proposal appears to remove the dynamical objections to the tentative identification of the eucrite parent body as Vesta. 相似文献