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1.
Ultracool stars usually have active regions, which is confirmed by their high-power radiofrequency emission modulated by the star axial rotation. The interpretation of this emission is commonly based on the electron cyclotron maser mechanism realized in the active regions. A plasma mechanism of radiofrequency emission is not considered, because ultracool star atmospheres are tightly “pressed” against the star surface, and the plasma frequency is much lower than the electron gyrofrequency (fL ? fB) at the coronal levels. This paper explores active regions of ultracool stars for the possible existence of a system of coronal magnetic loops carrying electric current generated by photospheric convection. It is shown that current dissipation induces a temperature increase inside the loops to about 107 K, which causes an increase in the scale of height of the inhomogeneous atmosphere and, at the coronal levels, effectuates condition fL ? fB, at which the plasma mechanism of radiofrequency emission prevails over the electron cyclotron maser mechanism. The magnetic loop parameters, intensity of electric currents generated by the photospheric convection, and efficiency of plasma heating inside the magnetic loops are evaluated on the example of the brown dwarf TVLM513-46546. The scale of the height of the modified atmosphere, which appears to be comparable to the star radius, is calculated; it is shown that the soft X-ray flow created by the hot modified atmosphere inside a coronal magnetic loop is about equal to that observed for brown dwarf TVLM513-46546.  相似文献   

2.
At what evolutionary stage of the Sun as a star the accumulation of its matter from the interstellar medium ended and the dominant outflow of solar wind (SW) streams started is one of the main outstanding questions in the astrophysical aspect of the problem of SW origin. It is unknown when and how this happened in detail, although the onset of thermonuclear reactions in the solar interior undoubtedly played a crucial role in the energetics and dynamics of the star, which could lead to such a change of the regimes. Therefore, it is hypothesized that the accretion or plasma outflow from the star was determined by the preceding evolution, i.e., by the “memory“ and not just by the distribution of instantaneous density, temperature, magnetic field, and other macroscopic parameters of the system that consists of this star, its nearest stellar environment, and interstellar gas. Depending on this, neighboring stars can serve as donors or acceptors of interstellar gas. Some of them can simultaneously or alternately play both roles. The polytropic solution for centrally symmetric flows obtained by Bondi is degenerate in the sign of the radial velocity. It is suitable for describing the quasi-stationary regimes of both types. However, the theory of transient processes has not been developed. Hence, the question of whether there exist stars similar in their internal structure and parameters to the present-day Sun but without stellar wind emission or even with interstellar gas accretion can be answered only observationally. The possibility that such stars exist is not ruled out; it does not contradict any laws of nature. The plasma-physical aspect of the problem of SW origin concerns much shorter time intervals than the main evolutionary time scale measured in billions of years. Hence, this aspect of the problem has been studied much better, although it is also fairly complex and has not yet been solved in much detail due to the multi-scale character of the flow formation processes. The SW as a permanent supermagnetosonic plasma outflow in the radial direction appears against the background of much more powerful nonequilibrium and unsteady motions, which are ordered only partially in the upper solar atmosphere and corona (turbosphere). The instantaneous SW state is controlled by the fluxes of free energy, matter, and momentum that enter the corona from the convection zone and underlying solar atmospheric layers. Although the main physical mechanisms of the transport of free energy of the electromagnetic field and plasma are generally well known, they need a quantitative analysis as applied to specific realizations in frequent and rare particle collisions in the solar corona to ascertain the nonlocal processes of the formation of fields and plasma flows, including the SW.  相似文献   

3.
Stars with the same temperature, luminosity, chemical composition, and activity as the Sun may be designated “sunlike stars.” The variability of several dozen reasonably sunlike stars has been monitored regularly since 1966 in chromospheric Ca II HK emission, and since 1984 in the visual continuum; larger stellar samples have been observed less comprehensively. Similar solar time series exist. A comparison of solar variability with its stellar analogs indicates that the Sun's current behavior is not unusual among sunlike stars, although the amplitude of the Sun's cyclic variation, measured photometrically, may be smaller than that of its stellar analogs. If the ensemble characteristics of a sample of sunlike stars are representative of the Sun's possible behavior, then such samples may provide insight about states of solar activity, such as Maunder Minima, which have not been observed on the Sun during the contemporary era of space-based measurements. Future progress in solar–stellar research will likely require studies of (1) rigorously selected samples of sunlike field stars, and (2) larger samples from stellar clusters, observed using 4-m class telescopes equipped with multi-object spectrographs and CCD photometers.  相似文献   

4.
The levels of chromospheric and coronal activity of the Sun are compared with new vast observations of late-type stars. The solar chromosphere turned to be more powerful than in the main body of stars and the corona is considerably weakened. A wavelet analysis of activity indices and measurement results for the magnetic field of the Sun as a star was performed for several solar cycles. It was obtained that solar activity in the differential rotation differs from phenomena on less massive K stars with cycles and, in contrast to them, the large-scale magnetic field of the Sun is a regulating factor for active processes. These results can be naturally explained with the assumption that the activity of a star with a given mass depends on the depth of the lower base of the convective zone. This seems to require the development of knowledge about the two-level dynamo and a new approach to studying solar-type activity.  相似文献   

5.
Great things are expected of the GAIA Observatory, currently expected to launch in 2011. Gerry Gilmore explains how it will provide accurate measurements that will help us understand the formation of the Milky Way and the distribution of dark matter.
The GAIA Observatory, ESA's Cornerstone 6 mission, addresses the origin and evolution of our galaxy, and a host of other scientific challenges. GAIA will provide unprecedented positional and radial velocity measurements with the accuracies needed to produce a stereoscopic and kinematic census of about one billion stars in our galaxy and throughout the Local Group, about 1% of the galactic stellar population. Combined with astrophysical information for each star, provided by on-board multicolour photometry, these data will have the precision and depth necessary to address the three key questions which underlie the GAIA science case: l when did the stars in the Milky Way form? l when and how was the Milky Way assembled? l what is the distribution of dark matter in our galaxy? The accurate stellar data acquired for this purpose will also have an enormous impact on all areas of stellar astrophysics, including luminosity calibrations, structural studies, and the cosmic distance scale. Additional scientific products include detection and orbital classification of tens of thousands of extrasolar planetary systems, a comprehensive survey of objects ranging from huge numbers of minor bodies in our solar system, including near-Earth objects, through galaxies in the nearby universe, to some 500 000 distant quasars. GAIA will also provide several stringent new tests of general relativity and cosmology.  相似文献   

6.
Richard de Grijs reviews the picture of starbirth presented in this nearby, prototype starburst galaxy, a well-studied example of processes that must be widespread in the early universe.
The last tidal encounter between M82 and M81, some 500 Myr ago, had a major impact on what was probably an otherwise normal, quiescent disk galaxy. It caused a concentrated burst of star formation in the form of massive star clusters that decreased rapidly, within a few 100 Myr. The current starburst in the centre of the galaxy is likely to arise from large-scale propagating star formation. Alternatively, it may be related to late infall of tidally disrupted debris from M82 itself. Star formation here overall may, in fact, come from a combination of these two mechanisms, in the sense that the star formation in the active core is actively propagating, whilst the overall evolution of the starburst depends on tidal debris raining back onto the disk of the galaxy, giving rise to the present-day starburst.  相似文献   

7.
Astronomers met at Keele University, Staffordshire, in June to discuss how observations of extended horizontal branch stars have changed our view of hot star atmospheres, stellar winds, single and binary star evolution, stellar pulsations and the age estimates for elliptical galaxies. Pierre Maxted reports.  相似文献   

8.
The prevailing heat transfer processes—convection in the photosphere and wave propagation in the chromosphere—are principally different. Despite this fact, there is a direct link between these processes: it is precisely convective photospheric flows that excite intense Alfven waves in the chromosphere. A physical model explaining the effect of strong chromospheric and coronal heating is improved in this work. The model is based on synchronous propagation and interaction in the chromosphere of photospheric spicules and Alfven waves. The results of observations of the last decade and the analytical solution of the equations of magnetohydrodynamics are used. It is established that the heating of the solar atmospheric plasma proceeds not in the corona but in the upper chromosphere.  相似文献   

9.

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   

10.
The generation and evolution of the Sun’s magnetic field and other stars is usually related to the dynamo mechanism. This mechanism is based on the consideration of the joint influence of the α effect and differential rotation. Dynamo sources can be located at different depths of the convection zone and can have different intensities. Based on such a system, the dynamical system in the case of the stellar dynamo in a two-layer medium has been constructed with regard to meridional fluxes in order to model the double cycle that corresponds to the simultaneous presence of 22-year and quasi-biennial magnetic field oscillations. It has been indicated that the regime of mixed oscillations can originate because a dynamo wave moves oppositely to the meridional flows in the upper layer of the convection zone. This results in the deceleration of the toroidal field propagation and in the generation of slow oscillations. In deeper layers, the directions of a dynamo wave and meridional flows coincide with each other, as a result of which fast magnetic fields originate. Therefore, the total contribution of two oscillations with different frequencies corresponds to the appearance of quasi-biennial cycles against 22-year cycles. It has been indicated that the beating regime, which can be related to the secular oscillations of solar magnetic activity, originates in the system when the meridional flows are weak.  相似文献   

11.
Abstract

In this paper a method for solving the equation for the mean magnetic energy <BB> of a solar type dynamo with an axisymmetric convection zone geometry is developed and the main features of the method are described. This method is referred to as the finite magnetic energy method since it is based on the idea that the real magnetic field B of the dynamo remains finite only if <BB> remains finite. Ensemble averaging is used, which implies that fields of all spatial scales are included, small-scale as well as large-scale fields. The method yields an energy balance for the mean energy density ε ≡ B 2/8π of the dynamo, from which the relative energy production rates by the different dynamo processes can be inferred. An estimate for the r.m.s. field strength at the surface and at the base of the convection zone can be found by comparing the magnetic energy density and the outgoing flux at the surface with the observed values. We neglect resistive effects and present arguments indicating that this is a fair assumption for the solar convection zone. The model considerations and examples presented indicate that (1) the energy loss at the solar surface is almost instantaneous; (2) the convection in the convection zone takes place in the form of giant cells; (3) the r.m.s. field strength at the base of the solar convection zone is no more than a few hundred gauss; (4) the turbulent diffusion coefficient within the bulk of the convection zone is about 1014cm2s?1, which is an order of magnitude larger than usually adopted in solar mean field models.  相似文献   

12.
Solar faculae are among the most common manifestations of solar activity and can play an important role in the energy transfer from the lower solar atmosphere into the corona. However, the mechanisms by which energy is transferred remain insufficiently studied. Our work is based on observational data obtained with the AST telescope of the Sayan solar observatory. Simultaneous observations were performed in the Hα 6563 Å and FeI 6569 Å, BaII 4554 Å and FeI 4551.6 Å, and CaII 8542 Å and FeI 8538 Å pairs of spectral lines. The studies indicated that the spectral composition of the line-of-sight (LOS) velocity oscillations in faculae is inhomogeneous and is affected by the chromospheric network’s structural elements. The possible presence of short and low magnetic loops in the facula region makes it difficult to determine the unambiguous phase relations between chromospheric and photospheric oscillations. The LOS velocity oscillation signals registered at the chromospheric level both lead and lag behind the signals registered at the photospheric level. At the same time, signs of propagating waves are evidently registered in the chromosphere of individual faculae.  相似文献   

13.
If surface anomalies in the composition of the metallic-line A stars (Am stars) are due to a precipitation of planet-like bodies (planetoids) on them, then one should expect a correlation to exist between the overabundance of heavier-than-iron elements on these stars and their “standard” abundances in the solar system (since chondrites provide the “standard” level for these elements). However, an anticorrelation was revealed.Nevertheless, this fact supports the original suggestion on the origin of the metallicism of A stars, and can easily be explained within the author's hypothesis on the formation of the Sun from matter escaping from the proto-Jupiter. During the terminal stages of mass transfer, the matter was strongly depleted in refractories (forming the rocky core of Jupiter). Therefore the composition of the meteorites formed should not coincide with the primary composition of the matter. Thus the Sun's outer layers may also have a distorted composition. The author concludes that it is desirable to revise the “standard” abundances of elements heavier than iron.From a comparison of the surface composition of Am stars with the composition of lunar anorthosites and that of rocks in the upper zones of the Skaergaard intrusion (Greenland), the Am phenomenon may be seen to result from a precipitation of large geochemically differentiated planetoids onto a star. Such planetoids (including the Moon) condense in the cooled envelope of the primary component of a close binary stellar system.  相似文献   

14.
Thermal history of Mimas and Enceladus is investigated from the beginning of accretion to 400 Myr. The numerical model of convection combined with the parameterized theory is used. The following heat sources are included: short lived and long lived radioactive isotopes, accretion, serpentinization, and phase changes. The heat transfer processes are: conduction, solid state convection, and liquid state convection. We find that temperature of Mimas’ interior was significantly lower than that of Enceladus. If Mimas accreted 1.8 Myr after CAI then the internal melting and differentiation did not occur at all. Comparison of thermal models of Mimas and Enceladus indicates that conditions favorable for the start of tidal heating lasted for a short time (~107 yr) in Mimas and for ~108 yr in Enceladus. This could explain the Mimas—Enceladus paradox. In fact, in view of the chronology based on cometary impact rate, one cannot discard a possibility that also Mimas was for some time active and it has the interior differentiated on porous core and icy mantle.  相似文献   

15.
在宇宙中,众多恒星发出的光线充斥于万物之间.光压虽然早在1901年就被人们发现和试验证实,但一直未把它作为一种客观存在的自然力看待.其原因是(光压)斥力相互作用(自然力)主要表现在宇观物体(天体)上,在太阳系内的(太阳光压)斥力作用下,能否证明地球物质的宏观运动状态发生了变化及发生了什么样的变化,则是人们把(光压)斥力相互作用是否作为一种自然力看待的关键.宋贯一(1991)发现,太阳光压、地球的自转与公转组成了一套类似金属塑性压力加工中的一套天然"轧机",太阳的能量(动量)就是通过这种"轧制"式耦合物理机制传递给地球的,并依据这种耦合机制产生的地壳"轧展"效应特征,发现(1999)太阳光压斥力作用才是控制全球地壳运动和地震活动的唯一重要因素.宋贯一(1992)又发现,太阳光压、地球的自转与公转及赤黄交角的存在,组成了一套天然的日地间太阳能量(动量)相耦合的"摇摆"式物理机制,依据这种耦合机制特征,发现和证明(2006,2011)太阳光压斥力相互作用才是激发自转轴摆动并产生极移及地球自转速度季节性变化的主要力源.上述一系列发现,为(光压)斥力相互作用在自然界的客观存提供了具有说服力的证据.它与(万有)引力相相互作用一样,在宇宙的演化中起着同等重要的作用.  相似文献   

16.
A more detailed scenario of one stage (August–November 2004) of the quasibiennial MHD process “Origination ... and dissipation of the four-sector structure of the solar magnetic field” during the decline phase of cycle 23 has been constructed. It has been indicated that the following working hypothesis on the propagation of an MHD disturbance westward (in the direction of solar rotation) and eastward (toward the zone of active longitudes) with the displacement of the large-scale open solar magnetic field (LOSMF) from this zone can be constructed based on LOSMF model representations and data on sunspot formation, flares, active filaments, and coronal ejections as well as on the estimated contribution of sporadic energy release to the flare luminosity and kinetic energy of ejections: (1) The “explosion” of the LOSMF singularity and the formation in the explosion zone of an anemone active region (AR), which produced the satellite sunspot formation that continued west and east of the “anemone,” represented a powerful and energy-intensive source of MHD processes at this stage. (2) This resulted in the origination of two “governing” large-scale MHD processes, which regulated various usual manifestations of solar activity: the fast LOSMF along the neutral line in the solar atmosphere, strongly affecting the zone of active longitudes, and the slow LOSMF in the outer layers of the convection zone. The fronts of these processes were identified by powerful (about 1031 erg) coronal ejections. (3) The collision of a wave reflected from the zone of active longitudes with the eastern front of the hydromagnetic impulse of the convection zone resulted in an increase in LOSMF magnetic fluxes, origination of an active sector boundary in the zone of active longitudes, shear-convergent motions, and generation and destabilization of the flare-productive AR 10696 responsible for the heliospheric storm of November 3–10, 2004.  相似文献   

17.
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18.
The position of the auroral luminosity equatorward boundary during the interaction between the Earth’s magnetosphere and isolated solar wind streams from different solar sources has been statistically studied based on the ground and satellite observations of auroras. These studies continue the series of the works performed in order to develop the technique for predicting auroras based on the characteristics of the interplanetary medium and auroral disturbances. The dependences of the minimal position of the auroral luminosity equatorward boundary (Φ′) on the values of the azimuthal component of the interplanetary electric field (E y ) and AL indices of magnetic activity, averaged over 6 and 24 h, are presented. The distribution limits for each type of isolated solar wind streams on the Φ′-E y and Φ′-AL planes have been determined.  相似文献   

19.
The results of modeling the preflare situation in the solar corona, obtained using a numerical solution for a complete set of three-dimensional MHD equations, are reviewed. Any assumptions concerning the flare development character or the active region’s behavior before a flare are not introduced. The initial and boundary conditions on the photosphere are specified from magnetic field measurements before a flare. The photospheric field sources are approximated by magnetic dipoles. The usage of the PERESVET program indicated that a current sheet is formed in the vicinity of a singular magnetic field line in the corona. The sheet is formed due to disturbances coming from the photosphere. The energy necessary for a flare is stored in the current sheet magnetic field during 2–3 days. The main construction principles of the PERESVET program, which makes it possible to use the maps of a measured photospheric field as boundary conditions, are presented.  相似文献   

20.
The evolution of the dimensionless Ekman, Rossby, and Rayleigh numbers in the model of composite convection in the Earth’s, which that determine the force balance and magnetic field evolution, is considered on the basis of known estimates of the rate of solid core growth and variations in the day length. Various scenarios of this evolution, both in the past and future, are presented over time intervals comparable with the age of the core. It is shown, in particular, that the current intensity of convection in the fluid core continues to increase. This fact corresponds to the increase in the frequency of geomagnetic field inversions and the transition from a dipole field to a multipole one. A decrease in the volume of the liquid core leads to a decrease in the magnetic field strength.  相似文献   

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