首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 29 毫秒
1.
Recent numerical investigations of wave propagation near coronal magnetic null points (McLaughlin and Hood: Astron. Astrophys. 459, 641, 2006) have indicated how a fast MHD wave partially converts into a slow MHD wave as the disturbance passes from a low-β plasma to a high-β plasma. This is a complex process and a clear understanding of the conversion mechanism requires the detailed investigation of a simpler model. An investigation of mode conversion in a stratified, isothermal atmosphere with a uniform, vertical magnetic field is carried out, both numerically and analytically. In contrast to previous investigations of upward-propagating waves (Zhugzhda and Dzhalilov: Astron. Astrophys. 112, 16, 1982a; Cally: Astrophys. J. 548, 473, 2001), this paper studies the downward propagation of waves from a low-β to high-β environment. A simple expression for the amplitude of the transmitted wave is compared with the numerical solution.  相似文献   

2.
We present first results from the Near-infrared Integral Field Spectrograph (NIFS) located at Gemini North. For the active galaxies Cygnus A and Perseus A we observe rotationally-supported accretion disks and adduce the existence of massive central black holes and estimate their masses. In Cygnus A we also see remarkable high-excitation ionization cones dominated by photoionization from the central engine. In the T-Tauri stars HV Tau C and DG Tau we see highly-collimated bipolar outflows in the [Fe II] λ 1.644 micron line, surrounded by a slower molecular bipolar outflow seen in the H2 lines, in accordance with the model advocated by Pyo, T.-S., et al., Astrophys. J. 570, 724 (2002).  相似文献   

3.
Here the effect of rotation up to third order in the angular velocity of a star on the p, f and g modes is investigated. To do this, the third-order perturbation formalism presented by Soufi et al. (Astron. Astrophys. 334:911, 1998) and revised by Karami (Chin. J. Astron. Astrophys. 8:285, 2008), was used. I quantify by numerical calculations the effect of rotation on the oscillation frequencies of a uniformly rotating β-Cephei star with 12 M ⊙. For an equatorial velocity of 90 km s−1, it is found that the second- and third-order corrections for (l,m)=(5,−4), for instance, are of order of 0.07% of the frequency for radial order n=−3 and reaches up to 0.6% for n=−20.  相似文献   

4.
The Theory of Alfven drag (Drell et al. in J Geophys Res 70: 3131–3145 1965; Anselmo and Farinella in Icarus, 58, 182–185 1983) is applied here to show that the existence of a possible solar ring structure at a radial distance of 0.02 AU (~4R ⊙ , R ⊙  = radius of the sun) predicted by earlier authors (Brecher et al. in Nature 282, 50–52 1979; Rawal in Bull. Astr. Soc. India 6, 92–95 1978, Moon Planets 24, 407–414 1981, Moon Planets 31, 175–182 1984, J Astrophys Astr 10, 257–259 1989) may not survive Alfven drag produced during even moderate solar magnetic storms which take place from time to time through the age of the sun, but a possible solar ring structure at a radial distance of 0.13 AU (~27R ⊙ ) (Brecher et al. in Nature 282, 50–52 1979; Rawal in Bull. Astr. Soc. India 6, 92–95 1978, Moon Planets 24, 407–414 1981, Moon Planets 31, 175–182 1984, J Astrophys Astr 10, 257–259 1989) may survive intense Alfven drag produced during even strong magnetic storms of magnetic field value up to 1,000 G.  相似文献   

5.
The decrease in the rms contrast of time-averaged images with the averaging time is compared between four data sets: (1) a series of solar granulation images recorded at La Palma in 1993, (2) a series of artificial granulation images obtained in numerical simulations by Rieutord et al. (Nuovo Cimento 25, 523, 2002), (3) a similar series computed by Steffen and his colleagues (see Wedemeyer et al. in Astron. Astrophys. 44, 1121, 2004), (4) a random field with some parameters typical of the granulation, constructed by Rast (Astron. Astrophys. 392, L13, 2002). In addition, (5) a sequence of images was obtained from real granulation images by using a temporal and spatial shuffling procedure, and the contrast of the average of n images from this sequence as a function of n is analysed. The series (1) of real granulation images exhibits a considerably slower contrast decrease than do both the series (3) of simulated granulation images and the series (4) of random fields. Starting from some relatively short averaging times t, the behaviour of the contrast in series (3) and (4) resembles the t −1/2 statistical law, whereas the shuffled series (5) obeys the n −1/2 law from n=2 on. Series (2) demonstrates a peculiarly slow decline of contrast, which could be attributed to particular properties of the boundary conditions used in the simulations. Comparisons between the analysed contrast-variation laws indicate quite definitely that the brightness field of solar granulation contains a long-lived component, which could be associated with locally persistent dark intergranular holes and/or with the presence of quasi-regular structures. The suggestion that the random field (4) successfully reproduces the contrast-variation law for the real granulation (Rast in Astron. Astrophys. 392, L13, 2002) can be dismissed.  相似文献   

6.
We show in this article that charged fluid with pressure derived by Bijalwan (Astrophys. Space. Sci. doi:, 2011a) can be used to model classical electron, quark, neutron stars and pulsar with charge matter, quasi black hole, white dwarf, super-dense star etc. Recent analysis by Bijalwan (Astrophys. Space. Sci., 2011d) that all charged fluid solutions in terms of pressure mimic the classical electron model are partially correct because solutions by Bijalwan (Astrophys. Space. Sci. doi:, 2011a) may possess a neutral counterpart. In this paper we characterized solutions in terms of pressure for charged fluids that have and do not have a well behaved neutral counter part considering same spatial component of metric e λ for neutral and charged fluids. We discussed solution by Gupta and Maurya (Astrophys. Space Sci. 331(1):135–144, 2010a) and solutions by Bijalwan (Astrophys. Space Sci. doi:, 2011b; Astrophys. Space Sci. doi:, 2011c; Astrophys. Space Sci., 2011d) such that charged fluids possess and do not possess a neutral counterpart as special cases, respectively. For brevity, we only present some analytical results in this paper.  相似文献   

7.
The scale invariance model (Heinz, S. and Sunyaev, R.A.: 2003, MNRAS 343, L59) can be used to derive robust scaling relations between the radio luminosity from accreting black holes and the black hole mass and accretion rate. These relations agree well with the recently found “fundamental plane” of black hole activity (Merloni, A., Heinz, S. and Di Matteo, T.: 2003, MNRAS 345, 1057). This relation provides a new, powerful tool for the comparison of jets from black holes of different masses and accretion rates. The regression coefficients of this relation contain information about the nature of the X-ray emission mechanism driving the correlation. We argue that X-ray synchrotron emission from the base of the jets is unlikely to be the dominant contribution to the X-ray spectrum in most of the sources.  相似文献   

8.
Various relations are found between the key parameters of black holes and active galactic nuclei. Some have a statistical property, others follow from the theoretical consideration of the evolution of these objects. In this paper we use a recently discovered empirical relation between the characteristic frequency of quasi-periodic oscillations of radiation ν br of black holes, their masses and matter accretion rates to determine the magnetic field strength B H at the black hole event horizon. Since the characteristic frequency can be determined from observations, the use of a new relation for the estimations of magnetic field B H can yield more definite results, since we are decreasing the number of the unknown or poorly-determined parameters of objects (it especially concerns the accretion rate Ṁ). The typical values which we have found are B H ≃ 108G for the stellar mass black holes, and B H ≃ 104G for the supermassive black holes. Besides, we demonstrate that if the linear polarization of an object is caused by the radiation of a magnetized accretion disk, then the degree of observable polarization is p ∼ ν br −1/2.  相似文献   

9.
A self-consistent method has been evolved to infer physical parameters like density, radiation field and abundances using line and continuum radiations as diagnostics. For that purpose, we first calculate the temperatures of graphite and silicate grains using the model of Li and Draine (Astrophys. J. 554:778, 2001) by solving self-consistently the energy balance for G 0 (1–104) times the radiation field following Weingartner and Draine (Astrophys. J. Suppl. Ser. 134:263, 2001). Consequently, infrared emission fluxes are also obtained. To keep it simple, this is presented in the empirical form of parameters T D and wavelength. The same model of the grain is adopted for photoelectric heating of gas using the formalism of Weingartner and Draine (Astrophys. J. Suppl. Ser. 134:263, 2001) (hereafter referred to as WD) and Bakes and Tielens (Astrophys. J. 427:822, 1994) (hereafter referred to as BT) for radiation field cited above in the range (6<hν≤13.6 eV). Temperature and abundances are determined using our own code for PDR very similar to cloudy code. All the possible sources of heating and cooling are considered for setting up the thermal balance. For the gas phase abundances that vary with depth in the cloud due to dust, self- and mutual shielding, chemical balance is solved. Most of the photoionization, photodissociation or chemical reaction rates are taken from UMIST database. We present an analysis of the cooling lines of singly ionized carbon [CII] at 158 μm and neutral oxygen [OI], at 63 μm and far infrared (FIR) continuum for a variety of star forming galaxies. Method of analysis of observational data is different from that of Malhotra et al. (Astrophys. J. 561:766, 2001). The radiation field G 0, density N h and abundance of carbon are obtained through best fit of observed and calculated intensities for lines and continuum radiations.  相似文献   

10.
We present dark energy models in an anisotropic Bianchi type-VI0 (B-VI0) space-time with a variable equation of state (EoS). The EoS for dark energy ω is found to be time dependent and its existing range for derived models is in good agreement with the recent observations of SNe Ia data (Knop et al. in Astrophys. J. 598:102 2003), SNe Ia data with CMBR anisotropy and galaxy clustering statistics (Tegmark et al. in Astrophys. J. 606:702, 2004b) and latest a combination of cosmological datasets coming from CMB anisotropies, luminosity distances of high redshift type Ia supernovae and galaxy clustering (Hinshaw et al. in Astrophys. J. Suppl. 180:225, 2009; Komatsu et al. in Astrophys. J. Suppl. 180:330, 2009). The cosmological constant Λ is found to be a positive decreasing function of time and it approaches a small positive value at late time (i.e. the present epoch) which is corroborated by results from recent supernovae Ia observations. The physical and geometric aspects of the models are also discussed in detail.  相似文献   

11.
We model thermal evolution of magnetars with a phenomenological heat source in a spherical internal layer and compare the results with observations of persistent thermal radiation from magnetars. We show that the heat source should be located in the outer magnetar’s crust, at densities ρ≲5×1011 g cm−3, and the heating rate should be ∼1020 erg cm−3 s−1. Heating deeper layers is extremely inefficient because the thermal energy is mainly radiated away by neutrinos and does not warm up the surface to the magnetar’s level. This deep heating requires too much energy; it is inconsistent with the energy budget of neutron stars.   相似文献   

12.
A Monte Carlo approach to solving a stochastic-jump transition model for active-region energy (Wheatland and Glukhov: Astrophys. J. 494, 858, 1998; Wheatland: Astrophys. J. 679, 1621, 2008) is described. The new method numerically solves the stochastic differential equation describing the model, rather than the equivalent master equation. This has the advantages of allowing more efficient numerical solution, the modeling of time-dependent situations, and investigation of details of event statistics. The Monte Carlo approach is illustrated by application to a Gaussian test case and to the class of flare-like models presented in Wheatland (Astrophys. J. 679, 1621, 2008), which are steady-state models with constant rates of energy supply, and power-law distributed jump transition rates. These models have two free parameters: an index (δ), which defines the dependence of the jump transition rates on active-region energy, and a nondimensional ratio ( ) of total flaring rate to rate of energy supply. For the nondimensional mean energy of the active-region satisfies , resulting in a power-law distribution of flare events over many decades of energy. The Monte Carlo method is used to explore the behavior of the waiting-time distributions for the flare-like models. The models with δ≠0 are found to have waiting times that depart significantly from simple Poisson behavior when . The original model from Wheatland and Glukhov (Astrophys. J. 494, 858, 1998), with δ=0 (i.e., no dependence of transition rates on active-region energy), is identified as being most consistent with observed flare statistics.  相似文献   

13.
A time-dependent model for the energy of a flaring solar active region is presented based on an existing stochastic jump-transition model (Wheatland and Glukhov in Astrophys. J. 494, 858, 1998; Wheatland in Astrophys. J. 679, 1621, 2008 and Solar Phys. 255, 211, 2009). The magnetic free energy of an active region is assumed to vary in time due to a prescribed (deterministic) rate of energy input and prescribed (random) jumps downwards in energy due to flares. The existing model reproduces observed flare statistics, in particular flare frequency – size and waiting-time distributions, but modeling presented to date has considered only the time-independent choices of constant energy input and constant flare-transition rates with a power-law distribution in energy. These choices may be appropriate for a solar active region producing a constant mean rate of flares. However, many solar active regions exhibit time variation in their flare productivity, as exemplified by NOAA active region (AR) 11029, observed during October – November 2009 (Wheatland in Astrophys. J. 710, 1324, 2010). Time variation is incorporated into the jump-transition model for two cases: (1) a step change in the rates of flare transitions, and (2) a step change in the rate of energy supply to the system. Analytic arguments are presented describing the qualitative behavior of the system in the two cases. In each case the system adjusts by shifting to a new stationary state over a relaxation time which is estimated analytically. The model exhibits flare-like event statistics. In each case the frequency – energy distribution is a power law for flare energies less than a time-dependent rollover set by the largest energy the system is likely to attain at a given time. The rollover is not observed if the mean free energy of the system is sufficiently large. For Case 1, the model exhibits a double exponential waiting-time distribution, corresponding to flaring at a constant mean rate during two intervals (before and after the step change), if the average energy of the system is large. For Case 2 the waiting-time distribution is a simple exponential, again provided the average energy of the system is large. Monte Carlo simulations of Case 1 are presented which confirm the estimate for the relaxation time and the expected forms of the frequency – energy and waiting-time distributions. The simulation results provide a qualitative model for observed flare statistics in AR 11029.  相似文献   

14.
We studied the characteristics of the zebra-associated spike-like bursts that were recorded with high time resolution at 1420 MHz in four intervals (from 12:45 to 12:48 UT) during 5 August 2003. Our detailed analysis is based on the selection of more than 500 such spike-like bursts and it is, at least to our knowledge, the first study devoted to such short-lived bursts. Their characteristics are different from those pertinent to “normal” spike bursts, as presented in the paper by Güdel and Benz (Astron. Astrophys. 231, 202, 1990); in particular, their duration (about 7.4 ms at half power) is shorter, so they should be members of the SSS (super short structures) family (Magdalenić et al., Astrophys. J. 642, L77, 2006). The bursts were generally strongly R-polarized; however, during the decaying part of interval I a low R-polarized and L-polarized bursts were also present. This change of polarization shows a trend that resembles the peculiar form of the zebra lines in the spectral dominion (“V” like). A global statistical analysis on the bursts observed in the two polarimetric channels shows that the highest cross-correlation coefficient (about 0.5) was pertinent to interval I. The zebras and the bursts can be interpreted by the same double plasma resonance process as proposed by Bárta and Karlicky (Astron. Astrophys. 379, 1045, 2001) and Karlicky et al. (Astron. Astrophys. 375, 638, 2001); in particular, the spikes are generated by the interruption of this process by assumed turbulence (density or magnetic field variations). This process should be present in the region close to the reconnection site (e.g., in the plasma reconnection outflows) where the density and the magnetic field vary strongly.  相似文献   

15.
The peculiar galaxy NGC 4650 A (α=12h 42m. 1; = δ—40° 26′; 1950·0) has been studied by means of direct and spectral observations with the ESO 3·6-m telescope. It is interpreted as a prolate, elliptical galaxy surrounded by a warped ring of H II regions, dust and stars. The distance is 47 Mpc (H 0=55 km s−1 Mpc−1). The ring is seen nearly edge-on (inclination 85°) and it rotates. It has a diameter of about 21 kpc and is bluer than the elliptical galaxy for which the (M/L v) ratio is ∼12 in solar units. The observed configuration may be the result of interaction with the nearby galaxy, NGC 4650.  相似文献   

16.
Assuming the time-dependent equation of state p=λ(t)ρ, five dimensional cosmological models with viscous fluid for an open universe (k=−1) and flat universe (k=0) are presented. Exact solutions in the context of the rest mass varying theory of gravity proposed by Wesson (Astron. Astrophys. 119, 145, 1983) are obtained. It is found that the phenomenon of isotropisation takes place in this theory, i.e. the mass scale factor A(t) which characterizes the rest mass of a typical particle is evolving with cosmic time just as the spatial scale factor R(t). It is further found that rest mass is approximately constant in the present universe.  相似文献   

17.
Individual tidal torque λ 2,E 2 and apsidal-motion k 2 constants were calculated for 112 close eclipsing binaries (CEBs) with Detached components belonging to the Main Sequence (DMS-type) from the catalogue by Svechnikov and Perevozkina (Catalogue of orbital elements, masses and luminosities of variable stars of DMS-type and some results of its statistical treatment, Ural State University Press, Yekaterinburg, pp. 1–5, 1999) and for 95 detached binaries taken from the catalogue by Torres et al. (Astron. Astrophys. Rev. 18:67, 2010) on the base of theoretical evolutionary stellar models including tidal torque constants by Claret (Astron. Astrophys. 424:919, 2004). A method of the inversion of model track grid into isochrones was formulated as a complex interpolation procedure for DMS-binaries data. Sets of isochrones were computed in k 2–M, k 2–R, λ 2–M, λ 2–R, E 2–M, and E 2–R planes. Calculated tidal torque constants allow to test stellar structure theory by comparing observed and estimated values of apsidal motion period and analyzing the correlation between timescales of synchronization, circularization, magnetic braking, as well as nuclear burning of DMS-components.  相似文献   

18.
We develop a diagnostic tool for determination of the electron densities in solar prominences using eclipse data. The method is based on analysis of the hydrogen Balmer-line intensities (namely Hα and Hβ) and the white-light emission due to Thomson scattering on the prominence electrons. Our approach represents a generalization of the ratio method already used by Koutchmy, Lebecq, and Stellmacher (Astron. Astrophys. 119, 261, 1983). In this paper we use an extended grid of non-LTE prominence models of Gouttebroze, Heinzel, and Vial (Astron. Astrophys. Suppl. Ser. 99, 513, 1993) and derive various useful relations between prominence radiation properties and electron densities. Simultaneously, an effective geometrical thickness of the prominence can also be obtained. As an example we apply our general technique to original eclipse data of Koutchmy, Lebecq, and Stellmacher (Astron. Astrophys. 119, 261, 1983). Finally, we use our results to determine the color of prominences as it should be seen during total eclipses.  相似文献   

19.
Ly α and Ly β line profiles in a solar prominence were observed with high spatial and spectral resolution with SOHO/SUMER. Within a 60-arcsec scan, we measure a very large variety of profiles: not only reversed and nonreversed profiles but also red-peaked and blue-peaked ones in both lines. Such a spatial variability is probably related to both the fine structure in prominences and the different orientations of mass motions. The usage of integrated-intensity cuts along the SUMER slit allowed us to categorize the prominence in three regions. We computed average profiles and integrated intensities in these lines in the range 2.36 – 42.3 W m−2 sr−1 for Ly α and 0.027 – 0.237 W m−2 sr−1 for Ly β. As shown by theoretical modeling, the Ly α/Ly β ratio is very sensitive to geometrical and thermodynamic properties of fine structure in prominences. For some pixels, and in both lines, we found agreement between observed intensities and those predicted by one-dimensional models. But a close examination of the profiles indicated a rather systematic disagreement concerning their detailed shapes. The disagreement between observations and thread models (with ambipolar diffusion) leads us to speculate about the importance of the temperature gradient between the cool and coronal regions. This gradient could depend on the orientation of field lines as proposed by Heinzel, Anzer, and Gunár (Astron. Astrophys. 442, 331, 2005).  相似文献   

20.
Almost all galaxies have massive central black holes in their centers with masses typically ranging from ～105 to ～109 M⊙. However, the origin and evolution of these objects and their connection with the hosting galaxies are not completely understood yet. In this work we analyze the mass accretion rate of supermassive black holes (SMBH’s) and the mean Eddington ratio (MER) of type 1 AGN using data from the Sloan Sky Survey. For this purpose we improve the method for constructing the subsample of SMBH, taking into account the survey flux limit and the bias of the sample. It was observed that the mean bolometric luminosity of the active black holes can be represented by a function composed by a power law in mass and a like-Schechter function in redshift. Our results also show that both the mean Eddington ratio and the mass accretion rate are proportional to this function.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号