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1.
Classical nova outbursts occur in binary systems containing a white dwarf accretor and a Roche-lobe-filling main-sequence star. The outburst is due to a thermonuclear runaway in the accreted material on the surface of the white dwarf, and results in the ejection of up to 10–4
M
of material at velocities of several hundred to a few thousand kilometres per second. There is now strong evidence that the mass ejection takes place via a wind with secularly increasing velocity. The fast ejecta catches up with slower moving material ejected earlier in the outburst, forming a layer of shock-heated gas which gives rise to a short burst of soft X-ray emission. This emission was observed in V838 Her (Nova Herculis 1991), and was succesfully accounted for by the interacting winds model. In this paper, we present 2.5-D numerical hydrodynamics calculations of interacting winds in novae which consider the effects of the binary system on shaping the mass-loss, and show that many of the features seen in the optical shells of novae many years after outburst can be accounted for.This author is supported by a PPARC research assistantship 相似文献
2.
In this paper we review the possibilities for
magnetohydrodynamic processes to handle the angular momentum transport
in accretion disks. Traditionally the angular momentum transport has
been considered to be the result of turbulent viscosity in the disk,
although the Keplerian flow in accretion disks is linearly stable towards
hydrodynamic perturbations. It is on the other hand linearly unstable
to some magnetohydrodynamic (MHD) instabilities.
The most important instabilities are the Parker and Balbus-Hawley
instabilities that are related to the magnetic buoyancy and the shear
flow, respectively. We discuss these instabilities not only in the
traditional MHD framework, but also in the context of slender flux
tubes, that reduce the complexity of the problem while keeping most of
the stability properties of the complete problem. In the non-linear
regime the instabilities produce turbulence. Recent numerical
simulations describe the generation of magnetic fields by a dynamo in
the resulting turbulent flow. Eventually such a dynamo may generate a
global magnetic field in the disk. The relation of the MHD-turbulence
to observations of accretion disks is still obscure. It is commonly
believed that magnetic fields can be highly efficient in transporting
the angular momentum, but emission lines, short-time scale variability
and non-thermal radiation, which a stellar astronomer would take as
signs of magnetic variability, are more commonly observed during periods
of low accretion rates.
Received October 12, 1995 / Accepted November 16, 1995 相似文献
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There is a disconcerting global trend of retiring telescopes of modest aperture, supplanting them instead with fewer expensive telescopes of quite large aperture. As a consequence, the available time and feasibility of following transient objects in astrophysics is diminishing. We show the utility of having a suite of small to moderate aperture telescopes capable of conducting imaging and spectroscopic observations in a service queue mode. The example we provide is the high-cadence early observations of the classical nova V475 Scuti (2003) carried out with the SMARTS suite of telescopes located at CTIO. 相似文献
7.
We have radically re-assessed the conditions required for the formation and growth of carbon grains in the ejecta of novae. The stability and hence the ultimate fate of the grains is primarily determined by the degree to which they are annealed by the nova's ultraviolet radiation field. 相似文献
8.
V.P. Kozhevnikov 《New Astronomy》2012,17(1):38-42
The cataclysmic variable V378 Peg is known since 15 years. Although V378 Peg is a rather bright star (14 mag), it underwent no detailed study. We performed photometric observations of V378 Peg during 75 h with the goal to detect periodic brightness variations. The obtained light-curves clearly showed changes with a period of about 3 h. The Fourier analysis reveals that this oscillation occurs with a period of 3.238 h and a semiamplitude of 0.07 mag. Although the detected oscillation possesses certain coherence, it appears to have a slightly unstable period or phase. Therefore, the detected period cannot be the orbital period of the V378 Peg system. Because such instability is typical of superhumps, we must consider the detected oscillation as superhumps. Furthermore, V378 Peg shows no outbursts and has to be a nova-like variable rather than a dwarf nova. Hence, the detected superhumps have to be regarded as permanent superhumps. Because superhump periods in cataclysmic variables are close to orbital periods, we can find the place of V378 Peg in the orbital period distribution of cataclysmic variables. V378 Peg is a permanent superhump system above the upper edge of the 2-3 h period gap in the orbital period distribution. 相似文献
9.
I discuss the status of the soft X-ray transient model. First, I discuss and then compare with observations the assumption that the geometrically thin disc evaporates into an ADAF. Second, I address the problems created by the recent determinations of the distance to SS Cyg, according to which the disc instability model does not apply to this famous dwarf-nova, thus casting doubt on the application of this model to any system at all. 相似文献
10.
We investigated spherically symmetric solution for nonrelativistic cosmological fluid equations and thermodynamic equation of state for Newtonian stars. It was shown that the assumption of a polytropic equation, , at the center of the star only suffices to integrate the equations explicitly. Our exact solution yields many fruitful results such as stellar stability, spherical oscillation and collapses of stars. Pressure, temperature, and density profiles inside stars were obtained. Central densities, pressures and temperatures of the Newtonian stars such as Sun, Jupiter and Saturn were also calculated. Collapse and expansion mechanism was explained by the heat transfer mechanism inside star. The upper bound value of white dwarf mass obtained by the Newtonian cosmological fluid equations turns out to be comparable to the static limit of Chandrasekhar one. Motion of the Universe was also discussed within the framework of Newtonian mechanics. Our calculation results without considering nuclear reactions inside stars may be applicable to the formation of protostars. 相似文献