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1.
The importance of thermal conduction on hot accretion flow is confirmed by observations of hot gas that surrounds Sgr A? and a few other nearby galactic nuclei. On the other hand, the existence of outflow in accretion flows is confirmed by observations and magnetohydrodynamic (MHD) simulations. In this research, we study the influence of both thermal conduction and outflow on hot accretion flows with ordered magnetic field. Since the inner regions of hot accretion flows are, in many cases, collisionless with an electron mean free path due to Coulomb collision larger than the radius, we use a saturated form of thermal conduction, as is appropriate for weakly collisional systems. We also consider the influence of outflow on accretion flow as a sink for mass, and the radial and the angular momentum, and energy taken away from or deposited into the inflow by outflow. The magnetic field is assumed to have a toroidal component and a vertical component as well as a stochastic component. We use a radially self-similar method to solve the integrated equations that govern the behavior of such accretion flows. The solutions show that with an ordered magnetic field, both the surface density and the sound speed decrease, while the radial and angular velocities increase. We found that a hot accretion flow with thermal conduction rotates more quickly and accretes more slowly than that without thermal conduction. Moreover, thermal conduction reduces the influences of the ordered magnetic field on the angular velocities and the sound speed. The study of this model with the magnitude of outflow parameters implies that the gas temperature decreases due to mass, angular momentum, and energy loss. This property of outflow decreases for high thermal conduction.  相似文献   

2.
Non-degenerate stars of essentially all spectral classes are soft X-ray sources. Their X-ray spectra have been important in constraining physical processes that heat plasma in stellar environments to temperatures exceeding one million degrees. Low-mass stars on the cooler part of the main sequence and their pre-main sequence predecessors define the dominant stellar population in the galaxy by number. Their X-ray spectra are reminiscent, in the broadest sense, of X-ray spectra from the solar corona. The Sun itself as a typical example of a main-sequence cool star has been a pivotal testbed for physical models to be applied to cool stars. X-ray emission from cool stars is indeed ascribed to magnetically trapped hot gas analogous to the solar coronal plasma, although plasma parameters such as temperature, density, and element abundances vary widely. Coronal structure, its thermal stratification and geometric extent can also be interpreted based on various spectral diagnostics. New features have been identified in pre-main sequence stars; some of these may be related to accretion shocks on the stellar surface, fluorescence on circumstellar disks due to X-ray irradiation, or shock heating in stellar outflows. Massive, hot stars clearly dominate the interaction with the galactic interstellar medium: they are the main sources of ionizing radiation, mechanical energy and chemical enrichment in galaxies. High-energy emission permits to probe some of the most important processes at work in these stars, and put constraints on their most peculiar feature: the stellar wind. Medium and high- resolution spectroscopy have shed new light on these objects as well. Here, we review recent advances in our understanding of cool and hot stars through the study of X-ray spectra, in particular high-resolution spectra now available from XMM-Newton and Chandra. We address issues related to coronal structure, flares, the composition of coronal plasma, X-ray production in accretion streams and outflows, X-rays from single OB-type stars, massive binaries, magnetic hot objects and evolved WR stars.  相似文献   

3.
The presence of an imposed vertical magnetic field may drastically influence the structure of thin accretion discs. If the field is sufficiently strong, the rotation law can depart from the Keplerian one. We consider the structure of a disc for a given eddy magnetic diffusivity but neglect details of the energy transport. The magnetic field is assumed to be in balance with the internal energy of the accretion flow. The thickness of the disc as well as the turbulent magnetic Prandtl number and the viscosity, α , are the key parameters of our model. The calculations show that the radial velocity can reach the sound speed for a magnetic disc if the thickness is comparable to that of a non-magnetic one. This leads to a strong amplification of the accretion rate for a given surface density. The inclination angle of the magnetic field lines can exceed the critical value 30° (required to launch cold jets) even for a relatively small magnetic Prandtl number of order unity. The toroidal magnetic fields induced at the disc surface are smaller than predicted in previous studies.  相似文献   

4.
The observation of the hot gas surrounding Sgr A * and a few other nearby galactic nuclei imply that electron and proton mean free paths are comparable to the gas capture radius. So, the hot accretion flows are likely to proceed under week-collision conditions. Hence, thermal conduction has been suggested as a possible mechanism by which the sufficient extra heating is provided in hot advection-dominated accretion flow (ADAF) accretion discs. We consider the effects of thermal conduction in the presence of a toroidal magnetic field in an ADAF around a compact object. For a steady-state structure of such accretion flows, a set of self-similar solutions are presented. We find two types of solutions which represent high and slow accretion rate. They have different behaviours with saturated thermal conduction parameter, φ.  相似文献   

5.
Considering nonthermal e±-pair production, this paper is focused on the radial structure of hot one-temperature accretion flows with advection under the condition of small viscous parameter ( = 0.1) and low mass-accretion rates. Self-similar relations of hot flow with advection have been adopted. The authors have used a scheme, in which the two regions of accretion flow are calculated separately and then joined together. Some typical features of hot flow with advection have been confirmed, and several new results obtained: there exists a critical radius rcr; the local cooling rate is inversely proportional to the square of the mass of the central object; the e±-pair process affects significantly the radiation of hot flows with advection.  相似文献   

6.
The aim of this work is to study the effects of an external magnetic field generated by a magnetized compact star on the outflows of its accretion disc. For this purpose, we solve a set of magneto-hydrodynamic (MHD) equations for an accretion disc in spherical coordinates to consider the disc structure along the θ-direction. We also consider the magnetic field of a compact star beyond its surface as a dipolar field, producing a toroidal magnetic field inside the disc. We convert the equations to a set of ordinary differential equations (ODEs) as a function of the θ only by applying self-similar assumptions in the radial direction. Then, this set of equations is solved under symmetrical boundary conditions in the equatorial plane to obtain the velocity field. The results are considered in the gas-pressure-dominated (GPD) region and radiation-pressure-dominated (RPD) region as well. The dipolar field of the compact stars can significantly enhance the speed of outflows. It also can change the structure of the disc. The results of this work would be useful in the study of X-ray binaries, the origin of ultra-relativistic outflows, and jet formation around the compact stars.  相似文献   

7.
Observations and numerical magnetohydrodynamic (MHD) simulations indicate the existence of outflows and ordered large-scale magnetic fields in the inner region of hot accretion flows. In this paper, we present the self-similar solutions for advection-dominated accretion flows (ADAFs) with outflows and ordered magnetic fields. Stimulated by numerical simulations, we assume that the magnetic field has a strong toroidal component and a vertical component in addition to a stochastic component. We obtain the self-similar solutions to the equations describing the magnetized ADAFs, taking into account the dynamical effects of the outflow. We compare the results with the canonical ADAFs and find that the dynamical properties of ADAFs such as radial velocity, angular velocity and temperature can be significantly changed in the presence of ordered magnetic fields and outflows. The stronger the magnetic field is, the lower the temperature of the accretion flow will be and the faster the flow rotates. The relevance to observations is briefly discussed.  相似文献   

8.
The results of longitudinal magnetic field measurements B z in the hot accretion spot in three classical T Tauri stars (CTTS) are reported. In all three stars the magnetic field is detected at a level above 2σ in the formation region of the narrow component of the He I 5876 Å emission line. In the case of DS Tau the longitudinal field B z in the hot spot was also measured from the narrow emission components of the Na I D lines, implying +0.8 ± 0.3 kG, which is equal to the B z field component measured from the He I 5876 Å line. Our results suggest that the 6-m telescope of the Special Astrophysical Observatory can be used to study magnetic fields in the hot spots of CTTS with magnitudes down to 13m, making it possible to double the number of stars of this type with measured B z values in the accretion zone.  相似文献   

9.
《New Astronomy》2002,7(4):155-160
We report first results from a series of N-body/gasdynamical simulations designed to study the origin of galaxy morphologies in a cold dark matter-dominated universe. The simulations include star formation and feedback and have numerical resolution sufficiently high to allow for a direct investigation of the morphology of simulated galaxies. We find, in agreement with previous theoretical work, that the presence of the main morphological components of galaxies—disks, spheroids, bars—is regulated by the mode of gas accretion and intimately linked to discrete accretion events. In the case we present, disks arise from the smooth deposition of cooled gas at the center of dark halos, spheroids result from the stirring of preexisting disks during mergers, and bars are triggered by tides generated by satellites. This demonstrates that morphology is a transient phenomenon within the lifetime of a galaxy and that the Hubble sequence reflects the varied accretion histories of galaxies in hierarchical formation scenarios. In particular, we demonstrate directly that disk/bulge systems can be built and rebuilt by the smooth accretion of gas onto the remnant of a major merger and that the present-day remnants of late dissipative mergers between disks are spheroidal stellar systems with structure resembling that of field ellipticals. The perplexing variety of galaxy morphologies is thus highly suggestive of—and may actually even demand—a universe where structures have evolved hierarchically.  相似文献   

10.
A high density of massive dark objects (MDOs), probably supermassive black holes, in the centres of nearby galaxies has been inferred from recent observations. There are various indications that much of the accretion responsible for producing these objects took place in dust-enshrouded active galactic nuclei (AGNs). If so, then measurements of the intensity of background radiation and the source counts in the far-infrared and submillimetre wavebands constrain the temperature of dust in these AGNs. An additional constraint comes from the hard X-ray background, if this is produced by accretion. One possibility is that the dust shrouds surrounding the accreting AGNs are cold, about 30 K. In this event, the dusty AGNs could be some subset of the population of luminous distant sources discovered at 850 μm using the SCUBA array on the James Clerk Maxwell Telescope, as proposed by Almaini, Lawrence & Boyle. An alternative is that the dust shrouds surrounding the accreting AGNs are much hotter (>60 K). These values are closer to the dust temperatures of a number of well-studied low-redshift ultraluminous galaxies that are thought to derive their power from accretion. If the local MDO density is close to the maximum permitted, then cold sources cannot produce this density without the submillimetre background being overproduced if they accrete at high radiative efficiency, and thus a hot population is required. If the dust-enshrouded accretion occurred at similar redshifts to that taking place in unobscured optical quasars, then a significant fraction of the far-infrared background radiation measured by COBE at 140 μm, but very little of the submillimetre background at 850 μm, may have been produced by hot dust-enshrouded AGNs which may have already been seen in recent Chandra X-ray surveys.  相似文献   

11.
Identifying generic physical mechanisms responsible for the generation of magnetic fields and turbulence in differentially rotating flows is fundamental to understand the dynamics of astrophysical objects such as accretion disks and stars. In this paper, we discuss the concept of subcritical dynamo action and its hydrodynamic analogue exemplified by the process of nonlinear transition to turbulence in non‐rotating wall‐bounded shear flows. To illustrate this idea, we describe some recent results on nonlinear hydrodynamic transition to turbulence and nonlinear dynamo action in rotating shear flows pertaining to the problem of turbulent angular momentum transport in accretion disks. We argue that this concept is very generic and should be applicable to many astrophysical problems involving a shear flow and non‐axisymmetric instabilities of shearinduced axisymmetric toroidal velocity or magnetic fields, such as Kelvin‐Helmholtz, magnetorotational, Tayler or global magnetoshear instabilities. In the light of several recent numerical results, we finally suggest that, similarly to a standard linear instability, subcritical MHD dynamo processes in high‐Reynolds number shear flows could act as a large‐scale driving mechanism of turbulent flows that would in turn generate an independent small‐scale dynamo. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

12.
The problem of arbitrary amplitude electron-acoustic solitary (EAS) waves in a plasma having cold fluid electrons, hot superthermal electrons and stationary ions is addressed. The domain of their allowable Mach numbers enlarges as the spectral index κ increases revealing therefore that the “maxwellisation” process of the hot component favors the propagation of the EAS waves. As the superthermal character of the plasma is increased, the potential pulse amplitude increases while its width is narrowed, i.e, the superthermal effects makes the electron-acoustic solitary structure more spiky. As the spectral index κ decreases, the hot electrons are locally expelled and pushed out of the region of the soliton’s localization. A decrease of the fractional number density of the hot electrons relative to that of the cold ones number density would lead to an increase of the depth as well as the width of the localized EAS wave. Our results should help to understand the salient features of large amplitude localized structures that may occur in the plasma sheet boundary layer and may provide an explanation for the strong spiky waveforms that have been observed in auroral electric fields.  相似文献   

13.
Infrared dark clouds (IRDCs) have dense elongated clumps and filaments with the favorable viewing condition of being on the near-side of a bright mid-infrared background. The clumps usually have multiple cores around the center. In this work, we study the effect of magnetic field morphology on the structure of massive IRDC clumps. To achieve this goal, we consider an axisymmetric isothermal oblate IRDC clump, embedded into a constant external magnetic field. We assume a polynomial function for the magnetic field morphology inside the clump. We use the numerical iterative methods to solve the equations: the successive over-relaxation method to find the magnetic and gravitational fluxes, and then the biconjugate gradient method to find the optimized values of mass and current densities. The results show that the IRDC clump will be very elongated along the perpendicular direction of the external magnetic field lines. Also, the assumption of choosing of a polynomial function for the magnetic field morphology leads to the formation of dense regions around the center. The greater the density of the central region, the larger the density of these dense regions and the closer to the center. The presence of these dense regions can lead to the formation of cores at these points.  相似文献   

14.
We study the effects of winds on advection dominated accretion flows in the presence of a global magnetic field under a self-similar treatment. The disk gas is assumed to be isothermal. For a steady state structure of such accretion flows a set of self similar solutions are presented. We consider the wind in a general magnetic field with three components (r,φ,z) in advection-dominated accretion flows. The mass-accretion rate $\dot{M}$ decreases with radius r as $\dot{M}\propto r^{s+1/2}$ , where s is an arbitrary constant. We will see, by increasing the wind parameter s, radial and rotational velocities increase.  相似文献   

15.
The angular distribution of low-frequency radiation after a single scattering by relativistic electrons with an isotropic velocity distribution differs markedly from the Rayleigh angular function. In particular, the scattering by an ensemble of ultrarelativistic electrons is described by the law p=1?cosα, where α is the scattering angle. Thus, photons are mostly scattered backward. We discuss some consequences of this fact for astrophysical problems. We show that a hot atmosphere of scattering electrons is more reflective than a cold one: the fraction of incident photons reflected after a single scattering can be larger than that in the former case by up to 50%. This must affect the photon exchange between cold accretion disks and hot coronae (or advective flows) near relativistic compact objects, as well as the rate of cooling (through multiple inverse-Compton scattering of the photons supplied from outside) of optically thick clouds of relativistic electrons in compact radio sources. Scattering asymmetry also causes the spatial diffusion of photons to proceed more slowly in a hot plasma than in a cold one, which affects the shapes of Comptonization spectra and the time delay in the detection of soft and hard radiation from variable X-ray sources.  相似文献   

16.
A study is made of axisymmetric, low sonic-Mach-number flows of a viscous fluid with angular momentum outside of a black-hole. The viscosity is an eddy viscosity due to turbulence in the sheared flows. Self-similar solutions arise naturally, reducing the Navier-Stokes equations to a set of nonlinear ordinary differential equations. These equations are solved analytically for flows of constant specific angular momentum and numerically for more general flows. For flows with non-constant specific angular momentum, the momentum flux density includes a planar discontinuity which is interpreted as an accretion disc. In general, two flow regions appear on each side of the disk, corresponding to accretion onto the disk and jet-like outflows along the ±z-axes. Physical interpretations of the solutions show that these flows arise in response to point sources of axial momentum at the origin directed in the ±z-directions. The power needed to maintain this momentum input is assumed to come from the mass accretion onto the black hole.The hydrodynamic flows are generalized to include a magnetic field. In the limit of infinite electrical conductivity, the possible types of flow patterns are the same as in hydrodynamic case. The magnetic field alters the relative amounts of reversible and irreversible momentum and angular momentum transport by the flow. For a flow with turbulent viscosity, the magnetic field acts to reduce the level of the turbulence and the effective value of the eddy viscosity.  相似文献   

17.
A first theoretical work is presented to study the propagation of two-solitons in an electron acoustic waves (EAWs) within the theoretical framework of the Tsallis statistical mechanics. For this purpose, cylindrical and spherical Korteweg-de Vries (KdV) equations are derived for electron acoustic solitary waves (EASWs) in an unmagnetized three species plasma system comprised of cold electrons, immobile ions and hot electrons featuring Tsallis statistics by employing the standard reductive perturbation method. The effects of electron nonextensivity and the fractional number density of the hot electrons relative to that of the cold ones number density (α) on the profiles of two-soliton structures are investigated numerically. Results would be helpful for understanding the localized structures that may occur in space plasmas.  相似文献   

18.
A model for the inner regions of accretion flows is presented where, owing to disc instabilities, cold and dense material is clumped into deep sheets or rings. Surrounding these density enhancements is hot, tenuous gas where coronal dissipation processes occur. We expect this situation to be most relevant when the accretion rate is close to Eddington and the disc is radiation-pressure-dominated, and so may apply to narrow-line Seyfert 1 (NLS1) galaxies. In this scenario, the hard X-ray source is obscured for most observers, and so the detected X-ray emission would be dominated by reflection off the walls of the sheets. A simple Comptonization calculation shows that the large photon-indices characteristic of NLS1s would be a natural outcome of two reprocessors closely surrounding the hard X-ray source. We test this model by fitting the XMM-Newton spectrum of the NLS1 1H  0707–495  between 0.5 and 11 keV with reflection-dominated ionized disc models. A very good fit is found with three different reflectors each subject to the same  Γ=2.35  power law. An iron overabundance is still required to fit the sharp drop in the spectrum at around 7 keV. We note that even a small corrugation of the accretion disc may result in  Γ>2  and a strong reflection component in the observed spectrum. Therefore, this model may also explain the strength and the variability characteristics of the MCG–6-30-15 Fe K α line. The idea needs to be tested with further broad-band XMM-Newton observations of NLS1s.  相似文献   

19.
E. Y. Zlotnik 《Solar physics》2013,284(2):579-588
Solar radio emission is a significant source of information regarding coronal plasma parameters and the processes occurring in the solar atmosphere. High resolution frequency, space, and time observations together with the developed theory make it possible to retrieve physical conditions in the radiation source and recognize the radiation mechanisms responsible for various kinds of solar radio emission. In particular, the high brightness temperature of many bursts testifies to coherent radiation mechanisms, that is, to plasma instabilities in the corona. As an example, the fine structure of solar radio spectra looking like a set of quasi-harmonic stripes of enhanced and lowered radiation, which is observed against the type IV continuum at the post-flare phase of activity, is considered. It is shown that such emission arises from a trap-like source filled with a weakly anisotropic equilibrium plasma and a small addition of electrons which have a shortage of small velocities perpendicular to the magnetic field. For many recorded events with the mentioned fine spectral structure the instability processes responsible for the observed features are recognized. Namely, the background type IV continuum is due to the loss-cone instability of hot non-equilibrium electrons, and the enhanced striped radiation results from the double-plasma-resonance effect in the regions where the plasma frequency f p coincides with the harmonics of electron gyrofrequency f B ; f p=sf B . Estimations of the electron number density and magnetic field in the coronal magnetic traps, as well as the electron number density and velocities of hot electrons necessary to excite the radiation with the observed fine structure, are given. It is also shown that in some cases several ensembles of non-equilibrium electrons can coexist in magnetic traps during solar flares and that its radio signature sensitively depends on the parameters of the distribution functions of the various ensembles.  相似文献   

20.
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