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A. A. Boyarchuk B. M. Shustov I. S. Savanov M. E. Sachkov D. V. Bisikalo L. I. Mashonkina D. Z. Wiebe V. I. Shematovich Yu. A. Shchekinov T. A. Ryabchikova N. N. Chugai P. B. Ivanov N. V. Voshchinnikov A. I. Gomez de Castro S. A. Lamzin N. Piskunov T. Ayres K. G. Strassmeier S. Jeffrey S. K. Zwintz D. Shulyak J.-C. Gérard B. Hubert L. Fossati H. Lammer K. Werner A. G. Zhilkin P. V. Kaigorodov S. G. Sichevskii S. Ustamuich E. N. Kanev E. Yu. Kil’pio 《Astronomy Reports》2016,60(1):1-42
The article presents a review of scientific problems and methods of ultraviolet astronomy, focusing on perspective scientific problems (directions) whose solution requires UV space observatories. These include reionization and the history of star formation in the Universe, searches for dark baryonic matter, physical and chemical processes in the interstellar medium and protoplanetary disks, the physics of accretion and outflows in astrophysical objects, from Active Galactic Nuclei to close binary stars, stellar activity (for both low-mass and high-mass stars), and processes occurring in the atmospheres of both planets in the solar system and exoplanets. Technological progress in UV astronomy achieved in recent years is also considered. The well advanced, international, Russian-led Spektr-UV (World Space Observatory—Ultraviolet) project is described in more detail. This project is directed at creating a major space observatory operational in the ultraviolet (115–310 nm). This observatory will provide an effective, and possibly the only, powerful means of observing in this spectral range over the next ten years, and will be an powerful tool for resolving many topical scientific problems. 相似文献
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A helium model atom that includes 55 He I levels and the He II ground level in a detailed consideration has been constructed to investigate the departures from local thermodynamic equilibrium (LTE) in the formation of helium lines in stars with effective temperatures from 9300 to 20 000 K. For eight stars with effective temperatures from 9380 to 17 500 K the helium abundance has been determined from He I lines. The neutral helium lines in B stars cannot be described under LTE conditions using the common helium abundance. Furthermore, the profiles of several lines cannot be described in terms of the LTE approach at all. In contrast, a satisfactory coincidence of the theoretical and observed profiles for the entire set of helium lines observed in a wide spectral range can be achieved using virtually the same helium abundance by taking into account the departures from LTE. The LTE and non-LTE helium abundances can differ by up to a factor of 2–3, depending on the stellar parameters. The higher the stellar temperature, the stronger the departures from LTE. As a rule, the lines in the blue spectral region are less affected by non- LTE effects. In the atmospheres of six stars the helium abundance corresponds, within the error limits, to the present-day solar value. A helium underabundance is observed in the atmospheres of Sirius and HD 72660 classified as hot Am stars. 相似文献
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We present our calculations of the spectrum and oscillator strengths for the 4f7?(4f65d+4f66s) Eu III transitions. The calculations were performed with Cowan's RCN-RCG-RCE codes in the single-configuration approximation. A comparison of computed level lifetimes with experimental data for three levels shows that the scale of theoretical oscillator strengths could be overestimated by a factor of 3. The theoretical oscillator strengths of red Eu III lines are two orders of magnitude smaller than their astrophysical oscillator strengths derived by Ryabchikova et al. (1999) from the condition of ionization balance. The new oscillator strengths were tested by analyzing the Eu abundance using Eu II and Eu III lines in the spectra of hot peculiar stars (α2 CVn is a typical representative) and cool peculiar stars (β CrB is a typical representative). First, we computed non-LTE corrections, which proved to be significant for α2 CVn. We also analyzed the Eu II λ6645.11-Å line as well as ultraviolet and optical Eu III lines. We show that the new oscillator strengths together with the non-LTE corrections allow the contradiction between the Eu abundances derived by Ryabchikova et al. (1999) separately from optical Eu II and Eu III lines in α2 CVn to be resolved. The new Eu abundance, log(Eu/N tot)=?6.5, also faithfully describes the blended near-ultraviolet resonance Eu III lines. Using the new Eu III oscillator strengths to analyze the spectrum of the cool Ap star β CrB, we found a significant deviation of the n(Eu II)/n(Eu III) ratio from its equilibrium value. For a chemically homogeneous model atmosphere, to obtain the observed intensity of the Eu III λ 6666.35-Å line, the Eu abundance must be increased by two orders of magnitude compared to that required to describe the Eu II λ 6645.11-Å line. We discuss the possibility of explaining the observed intensities of Eu II and Eu III lines in the spectrum of β CrB by the presence of an inhomogeneous atmosphere with Eu concentrated in its uppermost layers. In such atmospheres, the role of non-LTE effects becomes dominant. 相似文献
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E. A. Semenko M. E. Sachkov T. A. Ryabchikova D. O. Kudryavtsev N. E. Piskunov 《Astronomy Letters》2008,34(6):413-422
We present the results of our abundance analysis for the magnetic chemically peculiar star HD 115708 based on high-resolution spectra. The atmospheric chemical composition of HD 115708 (T eff = 7550 K) is shown to be typical of cool Ap stars with a significant ionization disequilibrium for the first and second rare earth ions, which is commonly observed in the atmospheres of pulsating Ap (roAp) stars. Our study of the vertical distribution of elements has shown that Mg, Ca, Cr, and Fe concentrate in deeper atmospheric layers, with their abundances decreasing sharply in the upper layers. The jumps in abundance are 1.5–3 orders of magnitude. Silicon is distributed in depth almost uniformly in the atmosphere of HD 115708. The derived empirical Cr and Fe distributions agree qualitatively with the results of diffusion calculations. Since the atmospheric chemical peculiarities in HD 115708 correspond to roAp stars, we have performed a spectroscopic monitoring to find nonradial pulsations. We have been able to determine only an upper limit for the amplitude of the possible radial velocity pulsations, ～100 m s?1, due to the insufficient temporal resolution and instability of the main stellar spectrograph (MSS) of the 6-m telescope. 相似文献
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In quiet stellar atmospheres that are stabilized, for example, by the presence of strong magnetic fields, a non-uniform distribution
of chemical elements (abundance stratification) is predicted by the diffusion theory. Spectroscopic observations of magnetic
Ap stars provide a strong support for the diffusion theory. Stratification predictions in the upper atmosphere, in particular,
strongly depend on the magnetic field geometry. Optical lines used in stratification analysis of Ap stars do not probe the
upper atmospheric layers and, hence, cannot provide reliable information on abundance gradients there. We show the expected
effect of stratification on the profiles of strong lines in the UV region and how these lines may be used for constraining
diffusion theory. 相似文献
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We have performed non-LTE calculations for O I with a multilevel model atom using currently available atomic data for a set of parameters corresponding to stars of spectral types from A to K. Departures from LTE lead to a strengthening of O I lines, and the difference between the non-LTE and LTE abundances (non-LTE correction) is negative. The non-LTE correction does not exceed 0.05 dex in absolute value for visible O I lines for main-sequence stars in the entire temperature range. For the infrared O I 7771 Å line, the non-LTE correction can reach ?1.9 dex. The departures from LTE are enhanced with increasing temperature and decreasing surface gravity. We have derived the oxygen abundance for three A-type mainsequence stars with reliably determined parameters (Vega, Sirius, HD 32115). For each of the stars, allowance for the departures from LTE leads to a decrease in the difference between the abundances from infrared and visible lines, for example, for Vega from 1.17 dex in LTE to 0.14 dex when abandoning LTE. In the case of Procyon and the Sun, inelastic collisions with HI affect the statistical equilibrium of OI, and agreement between the abundances from different lines is achieved when using Drawin’s classical formalism. Based on the O I 6300, 6158, 7771-5, and 8446 Å lines of the solar spectrum, we have derived the mean oxygen abundance log ? = 8.74 ± 0.05 using a classical plane-parallel model solar atmosphere and log ? +3D = 8.78 ± 0.03 by applying the 3D corrections taken from the literature. 相似文献
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A. P. Titarenko T. A. Ryabchikova O. P. Kochukhov V. V. Tsymbal 《Astronomy Letters》2013,39(5):347-356
We present the results of our study of the atmospheric chemical composition and evolutionary status for the chemically peculiar Ap star HD 138633. In contrast to ordinary Ap stars that exhibit strong lines of rare-earth elements in their spectra, these elements are represented very poorly in the spectrum of HD 138633. The magnetic field is estimated to be less than 700 G, which is also atypical of peculiar Ap stars. We have detected a stratification of such elements as Fe, Si, Ca, and Y in the atmosphere of HD 138633 whose pattern agrees qualitatively with the predictions of the theory of diffusive separation of elements under the joint action of gravity and radiation pressure forces. The nonuniform distribution obtained by assuming a stepwise distribution of elements in depth agrees qualitatively with the stratification distribution for β CrB. The search for pulsations points to the possible existence of lowamplitude pulsations with a period of about 17 min. 相似文献
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