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191.
Nucleosynthetic yields and production rates of helium and heavy elements are derived using new initial mass functions which
take into account the recent revisions in O star counts and the stellar models of Maeder (1981a, b) which incorporate the
effects of massloss on evolution. The current production rates are significantly higher than the earlier results due to Chiosi
& Caimmi (1979) and Chiosi (1979), and a near-uniform birthrate operating over the history of the galactic disc explains the
currently observed abundances. However, the yields are incompatibly high, and to obtain agreement it is necessary to assume
that stars above a certain mass do not explode but proceed to total collapse. Further confirmation of this idea comes from
the consideration of the specific yields and production rates of oxygen, carbon and iron and the constraints imposed by the
observational enrichment history in the disc as discussed by Twarog & Wheeler (1982). Substantial amounts of4He and14C, amongst the primary synthesis species, are contributed by the intermediate mass stars in their wind phases. If substantial
numbers of them exploded as Type I SN, their contribution to the yields of12C and56Fe would be far in excess of the requirements of galactic nucleosynthesis. Either efficient massloss precludes such catastrophic
ends for these stars, or the current stellar models are sufficiently in error to leave room for substantial revisions in the
specific yields. The proposed upward revision of the12C (α,γ)16O rate may produce the necessary changes in stellar yields to provide a solution to this problem. Stars that produce most
of the metals in the Galaxy are the same ones that contribute most to the observed supernova rate. 相似文献
192.
Data from the Pulkovo spectrophotometric data base on the absolute quasimonochromatic fluxes from oCet in the 320–1080 nm range are used to determine the physical parameters of this star in different phases of its light curve. The continuum emission layer is found to expand between the phases of the cycle corresponding to the rising and falling branches of the light curve. The average expansion velocity is 32 km/s. By the time the star’s brightness has fallen by roughly three magnitudes, its radius has increased by almost a factor of three. Over this same time the temperature of the layer has fallen from 3000 K to 2200 K. For this expansion velocity, the calculated mass rate loss is ⊙ M /year.__________Translated from Astrofizika, Vol. 48, No. 2, pp. 175–189 (May 2005). 相似文献
193.
Magnetic stratigraphy observed in ferromanganese crust 总被引:1,自引:0,他引:1
Summary. A sample of ferromanganese crust deposit (SCHW-1D) was cut into two sets of 1 mm slices for palaeomagnetic study. Alternating field and thermal demagnetization, and isothermal remanent magnetization analyses were performed on each thin slice. The results reveal the presence of a stable magnetism and both normal and reversed polarity intervals in the specimens. The observed polarity intervals cannot be confidently correlated with the geomagnetic polarity time-scale of the last 10 Myr due to the polarity overlap inherent in the sampling technique. But the results confirm the slow accretion rate of ferromanganese deposit determined by 10 Be method and suggest potential use of ferromanganese deposits in palaeomagnetic and tectonic studies. 相似文献
194.
Summary. Exact spherical harmonic expansions are given for calculating the gravitational and magnetic fields associated with certain uniform solids of revolution. The figures are those made by rotating a conic section about one of its principal axes. The coefficients in the expansions can be computed accurately and efficiently and this approach leads to a very satisfactory method for calculating the fields of geological bodies with approximate circular symmetry about a vertical axis. A complete theory of convergence is given for the expansions. Somewhat unexpectedly, the sphere of convergence is determined by the location of a number of equivalent point or line sources that lie within the body or on its edges. 相似文献
195.
196.
Valentin D. Ivanov G. Chauvin C. Foellmi M. Hartung N. Huélamo C. Melo D. Nürnberger M. Sterzik 《Astrophysics and Space Science》2006,304(1-4):247-249
The multiplicity of early-type stars is still not well established. The derived binary fraction is different for individual star forming regions, suggesting a connection with the age and the environment conditions. The few studies that have investigated this connection do not provide conclusive results. To fill in this gap, we started the first detailed adaptive-optic-assisted imaging survey of early-type field stars to derive their multiplicity in a homogeneous way. The sample has been extracted from the Hipparcos Catalog and consists of 341 BA-type stars within ∼300 pc from the Sun. We report the current status of the survey and describe a Monte-Carlo simulation that estimates the completeness of our companion detection. 相似文献
197.
We present results of investigations into chromospheric velocity oscillations in sunspots, carried out at the Sayan Solar
Observatory. It is shown that the “chevron” structures in the space-time diagrams demonstrate wavetrain properties. Such structures
are indicators of a propagating wave process and they are typical of many sunspots. In the authors’ opinion, three-minute
umbral oscillations are not the source of running penumbral waves (RPW). It is very likely that umbral oscillations and RPW
initially propagate along different magnetic field lines. We explain the decrease in RPW propagation velocity and frequency
in the outer penumbra, as compared with the inner, by the combined action of different frequency modes. To better reveal the
properties of these modes, frequency filtering was used. Our measurements of the RPW (five-minute mode) wavelength and RPW
propagation velocity in different sunspots vary from 12″ to 30″ and from 28 to 60 – 70 km s−1 correspondingly. 相似文献
198.
199.
200.
Report of the International Astronomical Union Division I Working Group on Precession and the Ecliptic 总被引:1,自引:0,他引:1
J. L. Hilton N. Capitaine J. Chapront J. M. Ferrandiz A. Fienga T. Fukushima J. Getino P. Mathews J.-L. Simon M. Soffel J. Vondrak P. Wallace J. Williams 《Celestial Mechanics and Dynamical Astronomy》2006,94(3):351-367
The IAU Working Group on Precession and the Equinox looked at several solutions for replacing the precession part of the IAU
2000A precession–nutation model, which is not consistent with dynamical theory. These comparisons show that the (Capitaine
et al., Astron. Astrophys., 412, 2003a) precession theory, P03, is both consistent with dynamical theory and the solution most compatible with the IAU 2000A
nutation model. Thus, the working group recommends the adoption of the P03 precession theory for use with the IAU 2000A nutation.
The two greatest sources of uncertainty in the precession theory are the rate of change of the Earth’s dynamical flattening,
ΔJ2, and the precession rates (i.e. the constants of integration used in deriving the precession). The combined uncertainties
limit the accuracy in the precession theory to approximately 2 mas cent−2.
Given that there are difficulties with the traditional angles used to parameterize the precession, zA, ζA, and θA, the working group has decided that the choice of parameters should be left to the user. We provide a consistent set of parameters
that may be used with either the traditional rotation matrix, or those rotation matrices described in (Capitaine et al., Astron.
Astrophys., 412, 2003a) and (Fukushima Astron. J., 126, 2003).
We recommend that the ecliptic pole be explicitly defined by the mean orbital angular momentum vector of the Earth–Moon barycenter
in the Barycentric Celestial Reference System (BCRS), and explicitly state that this definition is being used to avoid confusion
with previous definitions of the ecliptic.
Finally, we recommend that the terms precession of the equator and precession of the ecliptic replace the terms lunisolar precession and planetary precession, respectively. 相似文献