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This review summarises recent studies of O-stars, Luminous Blue Variables (LBVs) and Wolf-Rayet (WR) stars, emphasising observations and analyses of their atmospheres and stellar winds yielding determinations of their physical and chemical properties. Studies of these stellar groups provide important tests of both stellar wind theory and stellar evolution models incorporating mass-loss effects. Quantitative analyses of O-star spectra reveal enhanced helium abundances in Of and many luminous O-supergiants, together with CNO anomalies in OBN and Ofpe/WN9 stars, indicative of evolved objects. Enhanced helium, and CNO-cycle products are observed in several LBVs, implying a highly evolved status, whilst for the WR stars there is strong evidence for the exposition of CNO-cycle products in WN stars, and helium-burning products in WC and WO stars. The observed wind properties and mass-loss rates derived for O-stars show, in general terms, good agreement with predictions from the latest radiation-driven wind models, although some discrepancies are apparent. Several LBVs show similar mass-loss rates at maximum and minimum states, contrary to previous expectations, with the mechanism responsible for the variability and outbursts remaining unclear. WR stars exhibit the most extreme levels of mass-loss and stellar wind momenta. Whilst alternative mass-loss mechanisms have been proposed, recent calculations indicate that radiation pressure alone may be sufficient, given the strong ionization stratification present in their winds. 相似文献
53.
We report on our recent progress in modeling non-LTE atmospheres of O-stars including blanketing by lines from the iron group elements. The numerical method to account for the huge number of atomic levels and line transitions is presented. Results of exploratory model calculations examining the effects on the temperature structure, the hydrogen and helium line profiles and UV/EUV fluxes are discussed. 相似文献
54.
Karel A. Van Der Hucht 《Space Science Reviews》1993,66(1-4):21-35
The numbers and distribution of Population I O-type stars and Wolf-Rayet stars are reviewed. The numbers of known WR stars in the Galaxy, the LMC and the SMC are 185, 114, and 9, respectively. Distances and galactic distributions determined by various authors are compared. The single star and binary distributions are discussed in the light of evolutionary studies. 相似文献
55.
The status of the continuing effort to construct radiation driven wind models for O-Stars atmospheres is reviewed. Emphasis is given to several problems relating to the fomation of UV line spectra the use of accurate atomic data, the inclusion of EUV radiation by shock heated matter, the simulation of photospheric line blocking.A new tool for O-star diagnostics is presented. This is based on the use of wind models to calculate synthetic high resolution spectra covering the observable UV region. A comparison with observed spectra then gives physical constraints on the properties of stellar winds and stellar parameters, additionally abundances can be determined.The astrophysical potential of this method is demonstrated by an application to two Of-stars, the galactic O4f-star -Puppis and the LMC O3f-star Melnick 42. With regard to effective temperatures and gravities, the results from the application of classical methods to the analysis of photospheric lines are only partially verified. Explanations for the shortcomings of classical NLTE methods are discussed. 相似文献
56.
Wolf-Rainer Hamann 《Space Science Reviews》1993,66(1-4):237-251
Stratified Non-LTE models for expanding atmospheres became available in the recent years. They are based on the idealizing assumptions of spherical symmetry, stationarity and radiative equilibrium. From a critical discussion we conclude that this standard model is basically adequate for describing real Wolf-Rayet atmospheres and hence can be applied for quantitative spectral analyses of their spectra.By means of these models, the fundamental parameters have been determined meanwhile for the majority of the known Galactic WR stars. Most of them populate a vertical strip in the Herzsprung-Russell diagram at effective temperatures of 35 kK, the luminosities ranging from 104.5 to 105.9
L
. Only early-type WN stars with strong lines and WC stars are hotter. The chemical composition of WR atmospheres corresponds to nuclear-processed material (WN: hydrogen burning in the CNO cycle; WC: helium burning). Hydrogen is depleted but still detectable in the cooler part of the WN subclass.Different scenarios for the evolutionary formation of the Wolf-Rayet stars are discussed in the light of the empirical data provided from the spectral analyses. Post-red-supergiant evolution can principally explain the basic observational properties, except the rather low luminosities of a considerable fraction of WN stars. Among the alternative scenarios, close-binary evolution can theoretically produce the least-luminous WN stars. However, final conclusions about the evolutionary formation of the WR stars are not yet possible. 相似文献
57.
In this review we present the main results obtained by the ISO satellite on the abundance and spatial distribution of water
vapor in the direction of molecular clouds, evolved stars, galaxies, and in the bodies of our Solar System. We also discuss
the modeling of H2O and the difficulties found in the interpretation of the data, the need of collisional rates and the perspectives that future
high angular and high spectral resolution observations of H2O with the Herschel Space Observatory will open. 相似文献
58.
V. M. Lipunov I. E. Panchenko K. A. Postnov M. E. Prokhorov 《Space Science Reviews》1995,74(3-4):369-372
Theoretical logN-logS distributions and (V/V
max) tests of gamma-ray bursts in the model of coalescence of neutron star (NS+NS) and/or NS+black hole (NS+BH) binaries are calculated for a flat Universe (=1) with different values of the cosmological constant and under various assumptions about the star formation history. The observed logN-logS distribution and value of (V/V
max)=0.33 for 411 bursts with knownC
max/C
lim from the 2d BATSE catalogue are best fitted with a model for which = 0.2 and primary star formation occurs at redshiftsz5–6. 相似文献
59.
S. M. Haser D. J. Lennon R. -P. Kudritzki J. Puls N. R. Walborn L. Bianchi J. B. Hutchings 《Space Science Reviews》1993,66(1-4):179-182
Using the Hubble Space Telescope (HST) and the Faint Object Spectrograph (FOS) high signal to noise spectrograms were obtained for 15 OB stars in the Magellanic Clouds***, three of which are of spectral type O3. The data cover the spectral region from 1150 A – 2300 A with a resolution of /1 A. One O8.5 supergiant, OB78#231, in M31is also included in this work. These data are a substantial improvement on previous high resolution IUE observations in the Magellanic Clouds (Walborn et al. 1985 and references therein) because of the smaller aperture and the much better signal to noise ratio, while no high resolution UV spectra of O stars in M31 have been obtained before. In this paper we discuss various morphological aspects of the spectra, concerning metallicity and the stellar winds, compared to galactic analogues. 相似文献
60.
The fundamental properties of 24 Galactic WN stars are determined from analyses of their optical, UV and IR spectra using sophisticated model atmosphere codes (Hillier, 1987, 1990). Terminal velocities, stellar luminosities, temperatures, mass loss rates and abundances of hydrogen, helium, carbon, nitrogen and oxygen are determined. Stellar parameters are derived using diagnostic lines and interstellar reddenings found from fitting theoretical continua to observed energy distributions.Our results confirm that the parameters of WN stars span a large range in temperature (T*=30–90,000 K), luminosity (log L*/L=4.8–5.9), mass loss (M=0.9–12×10–5 M yr–1) and terminal velocity (v
=630–3300 km s–1). Hydrogen abundances are determined, and found to be low in WNEw and WNEs stars (<15% by mass) and considerable in most WNL stars (1–50%). Metal abundances are also determined with the nitrogen content found to lie in the range N/He=1–5×10–3 (by number) for all subtypes, and C/N 0.02 in broad agreement with the predictions of Maeder (1991). Enhanced O/N and O/C is found for HD 104994 (WN3p) suggesting a peculiar evolutionary history. Our results suggest that single WNL+abs stars may represent an evolutionary stage immediately after the Of phase. Since some WNE stars exist with non-negligible hydrogen contents (e.g. WR136) evolution may proceed directly from WNL+abs to WNE in some cases, circumventing the luminous blue variable (LBV) or red supergiant (RSG) stage. 相似文献