共查询到20条相似文献，搜索用时 109 毫秒
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Colin B. Roald 《Monthly notices of the Royal Astronomical Society》1998,300(2):397-410
In an attempt to produce a simple representation of an interface dynamo, I examine a dynamo model composed of two one-dimensional (radially averaged) pseudo-spherical layers, one in the convection zone and possessing an α-effect, and the other in the tachocline and possessing an ω-effect. The two layers communicate by means of an analogue of Newton's law of cooling, and a dynamical back-reaction of the magnetic field on ω is provided. Extensive bifurcation diagrams are calculated for three separate values of η, the ratio of magnetic diffusivities of the two layers. I find recognizable similarities to, but also dramatic differences from, the comparable one-layer model examined by Roald &38; Thomas. In particular, the solar-like dynamo mode found previously is no longer stable in the two-layer version; in its place there is a sequence of periodic, quasi-periodic and chaotic modes probably created in a homoclinic bifurcation. These differences are important enough to provide support for the view that the solar dynamo cannot be meaningfully modelled in one dimension. 相似文献
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The possibility of magnetic flux expulsion from the Galaxy in superbubble (SB) explosions, important for the α –Ω dynamo, is considered. Special emphasis is put on investigation of the downsliding of the matter from the top of the shell formed by the SB explosion, which is able to influence the kinematics of the shell. It is shown that either Galactic gravity or the development of the Rayleigh–Taylor instabilities in the shell limit the SB expansion, thus making magnetic flux expulsion impossible. The effect of cosmic rays in the shell on the sliding is considered, and it is shown that it is negligible compared with Galactic gravity. Thus the question of the possible mechanism of flux expulsion in the α –Ω dynamo remains open. 相似文献
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Luca Casagrande Chris Flynn Laura Portinari Leo Girardi Raul Jimenez 《Monthly notices of the Royal Astronomical Society》2007,382(4):1516-1540
We use nearby K dwarf stars to measure the helium-to-metal enrichment ratio Δ Y /Δ Z , a diagnostic of the chemical history of the solar neighbourhood. Our sample of K dwarfs has homogeneously determined effective temperatures, bolometric luminosities and metallicities, allowing us to fit each star to the appropriate stellar isochrone and determine its helium content indirectly. We use a newly computed set of Padova isochrones which cover a wide range of helium and metal content.
Our theoretical isochrones have been checked against a congruous set of main-sequence binaries with accurately measured masses, to discuss and validate their range of applicability. We find that the stellar masses deduced from the isochrones are usually in excellent agreement with empirical measurements. Good agreement is also found with empirical mass-luminosity relations.
Despite fitting the masses of the stars very well, we find that anomalously low helium content (lower than primordial helium) is required to fit the luminosities and temperatures of the metal-poor K dwarfs, while more conventional values of the helium content are derived for the stars around solar metallicity.
We have investigated the effect of diffusion in stellar models and the assumption of local thermodynamic equilibrium (LTE) in deriving metallicities. Neither of these is able to resolve the low-helium problem alone and only marginally if the cumulated effects are included, unless we assume a mixing-length which is strongly decreasing with metallicity. Further work in stellar models is urgently needed.
The helium-to-metal enrichment ratio is found to be Δ Y /Δ Z = 2.1 ± 0.9 around and above solar metallicity, consistent with previous studies, whereas open problems still remain at the lowest metallicities. Finally, we determine the helium content for a set of planetary host stars. 相似文献
Our theoretical isochrones have been checked against a congruous set of main-sequence binaries with accurately measured masses, to discuss and validate their range of applicability. We find that the stellar masses deduced from the isochrones are usually in excellent agreement with empirical measurements. Good agreement is also found with empirical mass-luminosity relations.
Despite fitting the masses of the stars very well, we find that anomalously low helium content (lower than primordial helium) is required to fit the luminosities and temperatures of the metal-poor K dwarfs, while more conventional values of the helium content are derived for the stars around solar metallicity.
We have investigated the effect of diffusion in stellar models and the assumption of local thermodynamic equilibrium (LTE) in deriving metallicities. Neither of these is able to resolve the low-helium problem alone and only marginally if the cumulated effects are included, unless we assume a mixing-length which is strongly decreasing with metallicity. Further work in stellar models is urgently needed.
The helium-to-metal enrichment ratio is found to be Δ Y /Δ Z = 2.1 ± 0.9 around and above solar metallicity, consistent with previous studies, whereas open problems still remain at the lowest metallicities. Finally, we determine the helium content for a set of planetary host stars. 相似文献
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M. Briquet K. Lefever K. Uytterhoeven C. Aerts 《Monthly notices of the Royal Astronomical Society》2005,362(2):619-625
The central stars of highly evolved planetary nebulae (PNe) are expected to have closely similar absolute visual magnitudes MV . This enables us to determine approximate distances to these sources where one knows their central star visual magnitudes, and levels of extinction. We find that such an analysis implies values of D which are similar to those determined by Phillips; Cahn, Kaler & Stanghellin; Acker, and Daub. However, our distances are very much smaller than those of Zhang; Bensby & Lundstrom, and van de Steene & Zijlstra. The reasons for these differences are discussed, and can be traced to errors in the assumed relation between brightness temperature and radius.
Finally, we determine that the binary companions of such stars can be no brighter than MV ∼ 6 mag , implying a spectral type of K0 or later in the case of main-sequence stars. 相似文献
Finally, we determine that the binary companions of such stars can be no brighter than M
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G. Handler R. R. Shobbrook T. Mokgwetsi 《Monthly notices of the Royal Astronomical Society》2005,362(2):612-618
We have carried out a three-site photometric campaign for the β Cephei star θ Oph from 2003 April to August. 245 h of differential photoelectric u v y photometry were obtained during 77 clear nights. The frequency analysis of our measurements has resulted in the detection of seven pulsation modes within a narrow frequency interval between 7.116 and 7.973 c d−1 . No combination or harmonic frequencies have been found. We have performed a mode identification of the individual pulsations from our colour photometry that shows the presence of one radial mode, one rotationally split ℓ= 1 triplet and possibly three components of a rotationally split ℓ= 2 quintuplet. We discuss the implications of our findings and point out the similarity of the pulsation spectrum of θ Oph to that of another β Cephei star, V836 Cen. 相似文献
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Michael Feast Patricia Whitelock Freddy Marang 《Monthly notices of the Royal Astronomical Society》2001,322(4):741-748
Spectra (1951–78) of the central object in η Car, taken by A. D. Thackeray, reveal three previously unrecorded epochs of low excitation. Since 1948, at least, these states have occurred regularly in the 2020-d cycle proposed by Damineli et al. They last about 10 per cent of each cycle. Early slit spectra (1899–1919) suggest that at that time the object was always in a low state. JHKL photometry is reported for the period 1994–2000. This shows that the secular increase in brightness found in 1972–94 has continued and its rate has increased at the shorter wavelengths. Modulation of the infrared brightness in a period near 2020 d continues. There is a dip in the JHKL light curves near 1998.0, coincident with a dip in the X-ray light curve. Evidence is given that this dip in the infrared repeats in the 2020-d cycle. As suggested by Whitelock & Laney, the dip is best interpreted as an eclipse phenomenon in an interacting binary system; the object eclipsed being a bright region ('hotspot'), possibly on a circumstellar disc or produced by interacting stellar winds. The eclipse coincides in phase and duration with the state of low excitation. It is presumably caused by a plasma column and/or by one of the stars in the system. 相似文献
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B. Weferling 《Monthly notices of the Royal Astronomical Society》2005,362(1):263-267
Relations are presented between the optical depth of the atmosphere (τ) in the millimetre and submillimetre windows at 350, 450, 850 and 1250 μm. These relations are derived by an analysis of 849 skydips at 850 μm performed with the submillimetre camera SCUBA at the James Clerk Maxwell Telescope (JCMT) and polynomial fits to over 19 000 measurements of a tipping radiometer operating at 350 μm. The relations allow for an improved calibration of millimetre and submillimetre observations on Mauna Kea, Hawai'i, and for better monitoring of atmospheric conditions. 相似文献
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We carried out a series of linear stability analyses of the radial and low-degree non-radial p modes for stellar models with initial masses of . The stellar models were computed by using convective overshoot distance , 0.25 and 0.40 H P . Our numerical results show that the β Cephei instability strip forms a horn-shaped region pointing upwards near the main sequence on the Hertzsprung–Russell diagram (HRD). The lower part of the instability strip for the radial modes join the zero-age main-sequence (ZAMS) at , while the top of the instability strip extends up to . The instability strip for the non-radial modes is even wider. The overall instability strip is dominated by the radial and non-radial fundamental modes. The first overtone (the radial-order index is also pulsationally unstable. We have shown that the β Cephei stability is almost independent of the overshoot parameter d over used for the stellar models, while it depends critically on the metal abundance. With decreasing metal abundance, the instability region shrinks and eventually disappears for . 相似文献
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Chris Koen Ruby Van Rooyen Francois Van Wyk Freddy Marang 《Monthly notices of the Royal Astronomical Society》1999,309(4):1051-1062
HD 23194, a member of the Pleiades, was found to pulsate with a period of about 30 min. The literature on the star is reviewed, and it is concluded that it may be a marginal Am star in a binary system. HD 95321 is an evolved Am ( ρ Puppis) star with a 5.1-h periodicity. Mode identification of its pulsation, based on multicolour photometry, suggests that the oscillation is probably non-radial with ℓ=2. We also report on the discovery of six other new δ Scuti stars, some of which may be pulsating in gravity modes. 相似文献
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B. E. J. Pagel & L. Portinari 《Monthly notices of the Royal Astronomical Society》1998,298(3):747-752
The slope Δ Y /Δ Z is a quantity of interest in relation to stellar evolution, the initial mass function and the determination of the primordial helium abundance. In this paper we estimate Δ Y /Δ Z from fine structure in the main sequence of nearby stars from Hipparcos data for stars with Z ≤ Z⊙. We find a value of about 3, which is consistent with what has been found in extragalactic H II regions and with stellar models for suitable upper limits to the initial masses of supernovae according to the initial mass function slope adopted. 相似文献