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1.
The thermosolutal instability of a plasma with finite Larmor radius and Hall effects is studied. When the instability sets in as stationary convection, finite Larmor radius effects are always stabilizing forx(=k 2 d 2/2 greater than two and forx less than two, they have a stabilizing or destabilizing influence depending on the Larmor radius parameterN in the presence of Hall currents. On the other hand the Hall currents have both stabilizing and destabilizing effects on the thermosolutal instability forx less than two and forx greater than two depending on the Hall parameterM. The stable solute gradient is found to have a stabilizing effect on stationary convection. The case of overstability is also considered wherein the sufficient conditions for the non-existence of overstability are derived.  相似文献   

2.
The RS CVn-type eclipsing binary SZ Psc was observed in two coloursB andV, in 1981. The rate of change of the period was calculated using two different least-squares procedures. The amplitudes of the light variations at outside eclipses are 0 . m 181 and 0 . m 213 inB andV colours, respectively. The light curves of the system have been solved using a new approximation. The radius of the cooler component is only 82% of its Roche-radius and the system is detached one.  相似文献   

3.
The Caledonian four-body problem introduced in a recent paper by the authors is reduced to its simplest form, namely the symmetrical, four body double binary problem, by employing all possible symmetries. The problem is three-dimensional and involves initially two binaries, each binary having unequal masses but the same two masses as the other binary. It is shown that the simplicity of the model enables zero-velocity surfaces to be found from the energy integral and expressed in a three dimensional space in terms of three distances r 1, r 2, and r 12, where r 1 and r 2 are the distances of two bodies which form an initial binary from the four body systems centre of mass andr 12 is the separation between the two bodies.  相似文献   

4.
The evolution of the 27-day recurrence in the series of two solar indices (Wolf number WN and 10.7 cm radio flux F) and two geomagnetic indices (Dst and ζ, variance of the geomagnetic field recorded at a magnetic observatory) have been studied over the 1957 – 2007 time span. Spectral energies contained in two period domains (25 – 27.3 and 27.3 – 31 days), designated as E 1 and E 2, have been computed. Whereas the evolution of E 1 is the same for the four indices, that of E 2 is essentially different for WN and F on the one hand, Dst and ζ on the other hand. Some general conclusions on the dynamics of the solar outer layers are inferred from these results. First the solar activity, as measured by WN, and when averaged over a few years, evolves in the same way whatever the latitude. Second, two families of coronal holes (CHs) are identified; the rapidly and the slowly rotating CHs evolve quite differently.  相似文献   

5.
The detection of trailing arms in twenty spiral galaxies, completed by the 60s, allowed one to suggest the overwhelming prevalence of this sense of rotation. Pasha and Dambis (1982) have recently determined the direction of rotation in a further 54 galaxies and found two leading spirals, NGC 4490 and NGC 5395. The current sample of galaxies with an established sense of rotation contains 79 trailing and two leading systems. The two spirals found to be leading belong to M51 type galaxies.In this paper we present a more detailed investigation of the system NGC 4490/85 which confirms that the arms in NGC 4490 are leading. Statistics are invoked to estimate a probable percentageX p and an upper limitX up of galaxies with leading arms. It gives, for 79 trailing and two leading spirals, the valuesX p 3.5% andX up 8.2%.  相似文献   

6.
Galsgaard  K.  Priest  E.R.  Nordlund  Å. 《Solar physics》2000,193(1-2):1-16
In two dimensions magnetic energy release takes place at locations where the magnetic field strength becomes zero and has an x-point topology. The x-point topology can collapse into two y-points connected by a current sheet when the advection of magnetic flux into the x-point is larger than the dissipation of magnetic flux at the x-point. In three dimensions magnetic fields may also contain singularities in the form of three-dimensional null points. Three-dimensional nulls are created in pairs and are therefore, at least in the initial stages, always connected by at least one field line – the separator. The separator line is defined by the intersection of the fan planes of the two nulls. In the plane perpendicular to a single separator the field line topology locally has a two dimensional x-point structure. Using a numerical approach we find that the collapse of the separator can be initiated at the two nulls by a velocity shear across the fan plane. It is found that for a current concentration to connect the two nulls along the separator, the current sheet can only obtain two different orientations relative to the field line structure of the nulls. The sheet has to have an orientation midway between the fan plane and the spine axis of each null. As part of this process the spine axes are found to lose their identity by transforming into an integrated part of the separator surfaces that divide space into four magnetically independent regions around the current sheet.  相似文献   

7.
The Vista Magellanic Cloud (VMC, PI M.R. Cioni) survey is collecting K S -band time series photometry of the system formed by the two Magellanic Clouds (MC) and the “bridge” that connects them. These data are used to build K S -band light curves of the MC RR Lyrae stars and Classical Cepheids and determine absolute distances and the 3D geometry of the whole system using the K-band period luminosity (PLK S ), the period–luminosity–color (PLC) and the Wesenhiet relations applicable to these types of variables. As an example of the survey potential we present results from the VMC observations of two fields centered respectively on the South Ecliptic Pole and the 30 Doradus star forming region of the Large Magellanic Cloud. The VMC K S -band light curves of the RR Lyrae stars in these two regions have very good photometric quality with typical errors for the individual data points in the range of ∼0.02 to 0.05 mag. The Cepheids have excellent light curves (typical errors of ∼0.01 mag). The average K S magnitudes derived for both types of variables were used to derive PLK S relations that are in general good agreement within the errors with the literature data, and show a smaller scatter than previous studies.  相似文献   

8.
Magnetic activity in the photosphere and chromosphere of the M dwarf EY Dra is studied and possible correlations between the two are investigated using photometric observations in the V and R bands and optical and near infrared spectroscopy. The longitudinal spot configuration in the photosphere is obtained from the V band photometry, and the chromospheric structures are investigated using variations in the Hα line profile and observations of the Paschen β line. The shape of the V band light‐curve indicates two active regions on the stellar surface, about 0.4 in phase apart. The spectroscopic observations show enhanced Hα emission observed close to the phases of the photometrically detected starspots. This could indicate chromospheric plages associated with the photospheric starspots. Some indications of prominence structures are also seen. The chromospheric pressure is limited to log mTR < –4 based on the non‐detection of emission in the Paschen β wavelength region. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

9.
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10.
We compare theW velocity dispersions of Brosche, Schwan & Schwarz (2001) with more recent results. The increase with the distance |z | from the galactic plane is confirmed, although perhaps with reduced amplitude. This could be interpreted either as one homogenous population or as a superposition of (at least) two populations with two constant dispersions and two scale heights. For each of the possibilities we propose a simple model. Combined with two observational variants for the velocity variation, we obtain surface densities up to |z | = 250 pc ranging from 5 to 44 M⊙ pc–2. Thus the case for considerable dark matter in the neighbourhood of the galactic plane is not supported  相似文献   

11.
The motions of comets and neutron stars have been integrated over five billion years in the Galactic potential to determine a gamma-ray burst distribution, presuming that bursts are the result of interactions between these two families of objects. The comets originate in two distinct populations - one from ejection by stars in the Galactic disk, and the other from ejection by stars in globular clusters. No choice of the free parameters resulted in agreement with both the isotropy data and the log(N >F)-log(F) data.  相似文献   

12.
We discuss the equilibrium solutions of four different types of collinear four-body problems having two pairs of equal masses. Two of these four-body models are symmetric about the center-of-mass while the other two are non-symmetric. We define two mass ratios as μ 1 = m 1/M T and μ 2 = m 2/M T, where m 1 and m 2 are the two unequal masses and M T is the total mass of the system. We discuss the existence of continuous family of equilibrium solutions for all the four types of four-body problems.  相似文献   

13.
Algebraic expressions are obtained for the interaction potential energy of a pair of galaxies in which one is disk shaped and the other spherical. The density distribution in the disk galaxy is represented by a polynomial in ascending powers of the distance from the centre of the disk while the density distribution in the spherical galaxy is represented by the superposition of spherical polytropes of integral indices. The basic functions required for obtaining the interaction potential energy of a coplanar disk-sphere pair of galaxies are tabulated. The forces of attraction between a coplanar disk-sphere pair of galaxies are shown graphically for two density models of disk and spherical galaxies. An overlapping coplanar disk-sphere pair of galaxies attract just like two mass-points at a certain separation,r c, of their centres. The force of attraction is less than that of two mass-points having masses equal to the masses of the two galaxies, if the separation of the centres is less thanr c, and greater if the separation is greater thanr c.For a typical coplanar disk-sphere pair of galaxies (the density of the disk is represented by Model II and of the sphere by a polytropic indexn=4) of equal radii, we note the following. At a separation of 0.79R, R being the common radius of the two galaxies, the force of attraction between the pair is the same as if the entire mass of each galaxy is concentrated at its centre. The mass-point model for the two galaxies will overestimate the force of attraction by more than a factor of 10 if the separation is less than 0.36R. For separation greater than the radii of the galaxies the mass-point model will underestimate the force but the departure in this case is less than 33%.  相似文献   

14.
We analyze multiple-wavelength observations of a two-ribbon flare exhibiting apparent expansion motion of the flare ribbons in the lower atmosphere and rising motion of X-ray emission at the top of newly-formed flare loops. We evaluate magnetic reconnection rate in terms of V r B r by measuring the ribbon-expansion velocity (V r) and the chromospheric magnetic field (B r) swept by the ribbons. We also measure the velocity (V t) of the apparent rising motion of the loop-top X-ray source, and estimate the mean magnetic field (B t) at the top of newly-formed flare loops using the relation 〈V t B t〉≈〈V r B r〉, namely, conservation of reconnection flux along flare loops. For this flare, B t is found to be 120 and 60 G, respectively, during two emission peaks five minutes apart in the impulsive phase. An estimate of the magnetic field in flare loops is also achieved by analyzing the microwave and hard X-ray spectral observations, yielding B=250 and 120 G at the two emission peaks, respectively. The measured B from the microwave spectrum is an appropriately-weighted value of magnetic field from the loop top to the loop leg. Therefore, the two methods to evaluate coronal magnetic field in flaring loops produce fully-consistent results in this event.  相似文献   

15.
The kinematic turbulent dynamo equations are studied in the presence of a large-scale velocity field. The two length-scales approach is employed and solutions of the equations are found in the limit of small bulk motion and shear, and for large Reynolds numberR m . The regeneration term is calculated up to second-order in 1/R m using cyclonic convective turbulent velocity field.  相似文献   

16.
The eclipsing variable RT Lac, which is classified as an RS CVn-type binary by Hall, was observed in two colours,B andV, during the summer seasons of 1978 and 1979. The observations made during two successive seasons indicate that, outside eclipses, the system has brightened by about 0.12 and 0.15 magnitudes on average inB andV, respectively. Moreover, RT Lac was even fainter when observed by Milone in 1965. While the total brightness of the system remained unchanged at mid-secondary, it increased at mid-primary and outside eclipses. This may be interpreted as indicating that the cooler component is responsible for the peculiar light variations.  相似文献   

17.
A closed form solution to the minimum DVtot2{\Delta V_{\rm tot}^2} Lambert problem between two assigned positions in two distinct orbits is presented. Motivation comes from the need of computing optimal orbit transfer matrices to solve re-configuration problems of satellite constellations and the complexity associated in facing these problems with the minimization of DVtot{\Delta V_{\rm tot}}. Extensive numerical tests show that the difference in fuel consumption between the solutions obtained by minimizing DVtot2{\Delta V_{\rm tot}^2} and DVtot{\Delta V_{\rm tot}} does not exceed 17%. The DVtot2{\Delta V_{\rm tot}^2} solution can be adopted as starting point to find the minimum DVtot{\Delta V_{\rm tot}}. The solving equation for minimum DVtot2{\Delta V_{\rm tot}^2} Lambert problem is a quartic polynomial in term of the angular momentum modulus of the optimal transfer orbit. The root selection is discussed and the singular case, occurring when the initial and final radii are parallel, is analytically solved. A numerical example for the general case (orbit transfer “pork-chop” between two non-coplanar elliptical orbits) and two examples for the singular case (Hohmann and GTO transfers) are provided.  相似文献   

18.
We study the dynamical evolution of an f(R) model of gravity in a viscous and anisotropic background which is given by a Bianchi type-I model of the Universe. We find viable forms of f(R) gravity in which one is exactly the Einsteinian model of gravity with a cosmological constant and other two are power law f(R) models. We show that these two power law models are stable with a suitable choice of parameters. We also examine three potentials which exhibit the potential effect of f(R) models in the context of scalar tensor theory. By solving different aspects of the model and finding the physical quantities in the Jordan frame, we show that the equation of state parameter satisfy the dominant energy condition. At last we show that the two power law f(R) models behave like quintessence model at late times and also the shear coefficient viscosity tends to zero at late times.  相似文献   

19.
In radiative transfer, the intensities of radiation from the bounding faces of a scattering atmosphere of finite optical thickness can be expressed in terms of Chandrasekhar’s X- and Y-functions. The nonlinear nonhomogeneous coupled integral equations which the X- and Y-functions satisfy in the real plane are meromorphically extended to the complex plane to frame linear nonhomogeneous coupled singular integral equations. These singular integral equations are then transformed into nonhomogeneous Riemann–Hilbert problems using Plemelj’s formulae. Solutions of those Riemann–Hilbert problems are obtained using the theory of linear singular integral equations. New forms of linear nonhomogeneous decoupled expressions are derived for X- and Y-functions in the complex plane and real plane. Solutions of these two expressions are obtained in terms of one known N-function and two new unknown functions N 1- and N 2- in the complex plane for both nonconservative and conservative cases. The N 1- and N 2-functions are expressed in terms of the known N-function using the theory of contour integration. The unknown constants are derived from the solutions of Fredholm integral equations of the second kind uniquely using the new linear decoupled constraints. The expressions for the H-function for a semi-infinite atmosphere are obtained as a limiting case.  相似文献   

20.
We report on the X-ray and optical observation of the two unidentified EGRET sources 3EG J0616-3310 and 3EG J1249-8330. The X-ray coverage performed by the ESA space telescope XMM–Newton provided ∼150 X-ray sources within each of the two γ-ray error-circles. The optical follow-up carried on with the Wide Field Imager at the ESO/MPG 2.2 m telescope have found no candidate counterpart for 125 of these X-ray sources. Among these, we have selected 9 sources with f X/f opt≥100, which we consider promising INS candidates.  相似文献   

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