共查询到20条相似文献,搜索用时 15 毫秒
1.
A localized force-free current is proposed as a model for the observed coronal loops.An upper limit for the growth rate of kink instabilities in this model is found by solving numerically, in cylinder symmetry, the MHD equation of motion, with the boundary condition = 0 outside the loop.For various current densities a spectrum of kinks is found. These instabilities will disrupt the loops that are long or strongly twisted, on a time scale of a few seconds.The kinks in short or barely twisted loops are too long to fit. 相似文献
2.
We explore an accretion model for low luminosity AGN (LLAGN) that attributes the low radiative output to a low mass accretion
rate,
, rather than a low radiative efficiency. In this model, electrons are assumed to drain energy from the ions as a result of
collisionless plasma microinstabilities. Consequently, the accreting gas collapses to form a geometrically thin disk at small
radii and is able to cool before reaching the black hole. The accretion disk is not a standard disk, however, because the
radial disk structure is modified by a magnetic torque which drives a jet and which is primarily responsible for angular momentum
transport. We also include relativistic effects. We apply this model to the well known LLAGN M87 and calculate the combined
disk-jet steady-state broadband spectrum. A comparison between predicted and observed spectra indicates that M87 may be a
maximally spinning black hole accreting at a rate of ∼10−3
M
⊙ yr−1. This is about 6 orders of magnitude below the Eddington rate for the same radiative efficiency. Furthermore, the total jet
power inferred by our model is in remarkably good agreement with the value independently deduced from observations of the
M87 jet on kiloparsec scales.
* This paper has previously been published in Astrophysics and Space Science, vol. 310:3–4. 相似文献
3.
We explore an accretion model for low luminosity AGN (LLAGN) that attributes the low radiative output to a low mass accretion
rate,
, rather than a low radiative efficiency. In this model, electrons are assumed to drain energy from the ions as a result of
collisionless plasma microinstabilities. Consequently, the accreting gas collapses to form a geometrically thin disk at small
radii and is able to cool before reaching the black hole. The accretion disk is not a standard disk, however, because the
radial disk structure is modified by a magnetic torque which drives a jet and which is primarily responsible for angular momentum
transport. We also include relativistic effects. We apply this model to the well known LLAGN M87 and calculate the combined
disk-jet steady-state broadband spectrum. A comparison between predicted and observed spectra indicates that M87 may be a
maximally spinning black hole accreting at a rate of ∼10−3
M
⊙ yr−1. This is about 6 orders of magnitude below the Eddington rate for the same radiative efficiency. Furthermore, the total jet
power inferred by our model is in remarkably good agreement with the value independently deduced from observations of the
M87 jet on kiloparsec scales. 相似文献
4.
Properties of dispersive Alfvén waves: 1. Kinetics (very low, intermediate, and low density plasmas)
P. P. Malovichko 《Kinematics and Physics of Celestial Bodies》2013,29(6):269-284
The work is devoted to the study of the behavior of dispersive Alfvén waves, including inertial and kinetic Alfvén waves, in astrophysical plasma of very low, intermediate, and low pressure. New and full solutions were obtained. The solutions for “ordinary” and inertial Alfvén waves were shown to be particular cases of the general solution. The effect of the parameters of the astrophysical medium on the behavior and properties of dispersive Alfvén waves was analyzed. All the main wave characteristics were obtained, namely, the dispersion, fading, polarization, density perturbations, and charge density perturbations, whose consideration is essential for observation and detection of these waves and for more adequate understanding of their role in different astrophysical processes that occur in the space environment. 相似文献
5.
We have performed photometric B, V, and R observations of nine disk galaxies that presumably have abnormally low total mass-to-light (M/L) ratios for given color indices. Our data on surface photometry are used to analyze the possible causes of anomalous M/L estimates. In many cases, these can be the result of errors in photometry or rotational velocity determination but can also
reflect the real peculiarities of the stellar composition of galaxies. Comparison of the photometric and dynamical disk mass
estimates obtained by analyzing the rotational velocities shows that low M/L values for a given color index are probably real for some of the galaxies. This is primarily true of NGC 4826 (Sab), NGC
5347 (Sab), and NGC 6814 (Sb). The small number of such galaxies suggests that the stellar initial mass function is universal.
However, a small fraction of galaxies probably may have a non-typical mass function “depleted” in low-mass stars. Such galaxies
require a more careful study. 相似文献
6.
P. Pergola K. Geurts C. Casaregola M. Andrenucci 《Celestial Mechanics and Dynamical Astronomy》2009,105(1-3):19-32
Application of low thrust propulsion to interconnect ballistic trajectories on invariant manifolds associated with multiple circular restricted three body systems has been investigated. Sun-planet three body models have been coupled to compute the two ballistic trajectories, where electric propulsion is used to interconnect these trajectories as no direct intersection in the Poincarè sections exists. The ability of a low thrust to provide the energy change required to transit the spacecraft between two systems has been assessed for some Earth to Mars transfers. The approach followed consists in a planetary escape on the unstable manifold starting from a periodic orbit around one of the two collinear libration points near the secondary body. Following the planetary escape and the subsequent coasting phase, the electric thruster is activated and executes an ad-hoc thrusting phase. The complete transfer design, composed of the three discussed phases, and possible applications to Earth–Mars missions is developed where the results are outlined in this paper. 相似文献
7.
8.
M. Mapelli B. Moore E. Ripamonti L. Mayer M. Colpi L. Giordano 《Monthly notices of the Royal Astronomical Society》2008,383(3):1223-1231
We simulate the collisional formation of a ring galaxy and we integrate its evolution up to 1.5 Gyr after the interaction. About 100–200 Myr after the collision, the simulated galaxy is very similar to observed ring galaxies (e.g. Cartwheel). After this stage, the ring keeps expanding and fades. Approximately 0.5–1 Gyr after the interaction, the disc becomes very large (∼100 kpc) and flat. Such extended discs have been observed only in giant low surface brightness galaxies (GLSBs). We compare various properties of our simulated galaxies (surface brightness profile, morphology, H i spectrum and rotation curve) with the observations of four well-known GLSBs (UGC 6614, Malin 1, Malin 2 and NGC 7589). The simulations match quite well the observations, suggesting that ring galaxies could be the progenitors of GLSBs. This result is crucial for the cold dark matter (CDM) model, as it was very difficult, so far, to explain the formation of GLSBs within the CDM scenario. 相似文献
9.
10.
Three localized sets of small arcuate ridges associated with slopes in the northern polar area of Mars (∼70°N latitude) are morphologically similar to sets of drop moraines left by episodes of advance and retreat of cold-based glaciers. Comparison with other glacial features on Mars shows that these features differ in important aspects from those associated with water–ice flow. Instead, we interpret these features to be due to perennial accumulation and flow of solid carbon dioxide during recent periods of very low spin-axis obliquity. 相似文献
11.
W. Bronkalla G. M. Richter I. D. Karachentsev V. E. Karachentseva 《Astronomische Nachrichten》1989,310(3):199-202
Wir möchten aufmerksam machen auf den relativ seltenen Fall einer anscheinend “isolierten” sphärischen Zwerggalaxie K 23 (α = 02h40m.5, δ = +16·31'). Am 6 m-Teleskop aufgenommene Platten wurden mit einem Microdensitometer abgetastet und die photometrischen Parameter sowie die Parameter eines angepaßten King-Modells bestimmt. Danach ist K 23 den bekannten Prototypen in der Lokalen Gruppe und der M81-Gruppe sehr ähnlich. Eine Analyse der räumlichen Galaxienverteilung zeigt, daß K 23 wahrscheinlich in der Brücke liegt, die den Lokalen Superhaufen mit dem Perseus-Pisces-Superhaufen verbindet, also nicht wirklich räumlich isoliert ist. 相似文献
12.
De Yu Wang Guang Li Huang C. -G. Fälthammar P. A. Lindqvist L. Blomberg Li Ting Song 《Astrophysics and Space Science》1996,240(2):175-186
Nonlinear kinetic Alfvén waves where m
e
/m
i
, have been solved both with and without the Poisson equation correction. It is found that the ratio of the perpendicular electric field and magnetic field, and the ratio of parallel and perpendicular electric field increase with deepening of the depressive density soliton. The former ratio may be larger than the Alfvén velocity in the case of a large amplitude solitary kinetic Alfvén wave. The Poisson equation correction is important for the nonlinear kinetic Alfvén wave propagating along the magnetic field, which solves a puzzle of Sagdeev potential to approach infinity in the limit ofK
x
0. This correction causes the solitary KAW possessing an electrostatic character along the direction of wave moving frame. These results have been compared with the observations from the Freja satellite in the low aurora. 相似文献
13.
《天文和天体物理学研究(英文版)》2015,(6)
When observing blazars, we identified a new eclipsing binary named GSC03517–00663. The light curves of GSC 03517–00663 are typical of EW-type light curves. Based on the observation using the 1 m telescope at the Weihai Observatory of Shandong University, complete VRI light curves were determined. Then, we analyzed the multiple light curves using the Wilson-Devinney program. It was found that GSC 03517–00663 has a mass ratio of q = 0.164 and a degree of contact of f = 69.2%. GSC 03517–00663 is a deep, low mass ratio overcontact binary. The light curves of GSC 03517–00663 show a strong O'Connell effect, which was explained by employing a dark spot on the secondary component. 相似文献
14.
Beno?t Noyelles 《Celestial Mechanics and Dynamical Astronomy》2012,112(4):353-383
This paper presents a study of the Poincaré–Hough model of rotation of the synchronous natural satellites, in which these
bodies are assumed to be composed of a rigid mantle and a triaxial cavity filled with inviscid fluid of constant uniform density
and vorticity. In considering an Io-like body on a low eccentricity orbit, we describe the different possible behaviors of
the system, depending on the size, polar flattening and shape of the core. We use for that the numerical tool. We propagate
numerically the Hamilton equations of the system, before expressing the resulting variables under a quasi-periodic representation.
This expression is obtained numerically by frequency analysis. This allows us to characterise the equilibria of the system,
and to distinguish the causes of their time variations. We show that, even without orbital eccentricity, the system can have
complex behaviors, in particular when the core is highly flattened. In such a case, the polar motion is forced by several
degrees and longitudinal librations appear. This is due to splitting of the equilibrium position of the polar motion. We also
get a shift of the obliquity when the polar flattening of the core is small. 相似文献
15.
Jian-Nan Zhang A-Li Luo Yong-Heng Zhao 《天体物理学报》2009,(6):712-724
PLS (Partial Least Squares regression) is introduced into an automatic estimation of fundamental stellar spectral parameters. It extracts the most correlative spectral component to the parameters (Teff, log g and [Fe/H]), and sets up a linear regression function from spectra to the corresponding parameters. Considering the properties of stellar spectra and the PLS algorithm, we present a piecewise PLS regression method for estimation of stellar parameters, which is composed of one PLS model for Teff, and seven PLS models for log g and [Fe/H] estimation. Its performance is investigated by large experiments on flux calibrated spectra and continuum normalized spectra at different signal-to-noise ratios (SNRs) and resolutions. The results show that the piecewise PLS method is robust for spectra at the medium resolution of 0.23 nm. For low resolution 0.5 nm and 1 nm spectra, it achieves competitive results at higher SNR. Experiments using ELODIE spectra of 0.23 nm resolution illustrate that our piecewise PLS models trained with MILES spectra are efficient for O ~ G stars: for flux calibrated spectra, the systematic offsets are 3.8%, 0.14 dex, and -0.09 dex for Teff, log g and [Fe/H], with error scatters of 5.2%, 0.44 dex and 0.38 dex, respectively; for continuum normalized spectra, the systematic offsets are 3.8%, 0.12dex, and -0.13 dex for Teff, log g and [Fe/H], with error scatters of 5.2%, 0.49 dex and 0.41 dex, respectively. The PLS method is rapid, easy to use and does not rely as strongly on the tightness of a parameter grid of templates to reach high precision as Artificial Neural Networks or minimum distance methods do. 相似文献
16.
Nadjinangar Gotoum Christophe Nkem Kamel Hammami M. Ahamat?Charfadine L. C. Owono?Owono Nejm-Eddine Jaidane 《Astrophysics and Space Science》2011,332(1):209-217
We report on the calculation of collision induced rotational excitation cross sections and rate coefficients of AlF by He
atom at low temperature. These quantities were obtained by first computing the interaction potential energy surface (PES)
of the AlF(X
1Σ+)-He(1
S) van der Waals complex at the ab initio Coupled Cluster with Single and Double and perturbative Triple excitations [CCSD(T)] level of theory. The aug-cc-pVQZ Gaussian
basis, to which was added a set of bond functions, was used for that purpose. The calculations account for basis set superposition
errors (BSSE). The interaction potential presents a minimum of ∼24 cm−1 below the AlF-He dissociation limit. The PES was fitted on a basis of Legendre polynomial functions to allow for the calculation
of cross sections in the close-coupling (CC) approach. By averaging these cross sections over a Maxwell-Boltzmann velocity
distribution, rate coefficients were inferred at low temperatures (T≤300 K). From our computations, a propensity towards ΔJ=1 transitions is observed. 相似文献
17.
Ashutosh K. Singh K. K. Singh A. K. Singh Lalmani 《Astrophysics and Space Science》2011,331(2):459-468
A unique night-time natural electromagnetic disturbances in the VLF/ELF range received during a magnetically quite period
at a low latitude Indian ground station, Jammu (geomag. lat. 19°26′ N, L=1.17) has been reported. During the routine observation
of VLF waves at Jammu, whistlers and different types of VLF/ELF emissions such as whistlers of varying dispersion confined
to a small band limited frequency range, hisslers, pulsing hiss, discrete chorus emissions of rising and falling tones with
multiple bands, oscillating tone discrete emission, whistler-triggered hook and discrete chorus risers emissions, etc. have
been observed simultaneously during the quiet period on a single night. Such type of unique simultaneous observations has
never been reported from any of the low latitude ground stations and this is the first observation of its kind. The results
are discussed in the light of recorded features of whistlers and emissions. Generation and propagation mechanism are discussed
briefly. Plasma parameters are further derived from the dispersion analysis of nighttime whistlers and emissions recorded
simultaneously during magnetically quiet periods. 相似文献
18.
This paper presents an alternative interpretation for the wide scatter and apparent lack of anti-correlation in the relationship between the spectral luminosity (L ν ) and synchrotron peak frequency (ν peak ) in a sample of BL Lac Objects contained in Wu et al. (Astron. Astrophys. 466:43, 2007) compilation. The apparent lack of correlation between the parameters contradicts the blazar sequence proposed by Fossati et al. (in Mon. Not. R. Astron. Soc. 299:433, 1998), which predicts a general decline in L ν with increasing ν peak . Analysis of the radio luminosity and synchrotron peak frequency data of the sample reveals a strong selection effect, due to Malmquist bias. We show that a clear anti-correlation (r~?0.7) between the radio luminosity at synchrotron peak (L peak ) and ν peak exists for the BL Lac sample above some redshift cut-off (z c =0.3), which may correspond to the flux limit of the sample. The results are not only in agreement with FRI–BL Lac unification, but also suggest that the present data is consistent with the blazar sequence. 相似文献
19.
We have computed and studied the mosaic correlation maps of the ILC WMAP microwave background data with the positions of infrared
and submillimeter sources. Using the histograms of the signal values in pixels and angular power spectra, we studied the statistical
properties of these maps. We discovered similar behavior of a number of harmonics in the maps of correlations with the FSC
IRAS, 2MASX and Planck catalog objects. The most prominent multipoles among them, which may reflect the actual distribution
of radiation sources are the ℓ = 6 for the FSC and Planck data, and ℓ = 3 for the Planck source data. 相似文献
20.
Recently a number of studies have identified small lunar geologic structures to be <100 Ma in age using standard remote sensing techniques. Here we present new crater size frequency distributions (CSFDs) and model ages using craters D > 10 m for five small target units: one irregular mare patch (IMP) in Mare Nubium and four regions located on lunar wrinkle ridges in Mare Humorum. For comparison we also date another IMP found in a recent study in Mare Tranquillitatis (Braden et al. 2014 ). Absolute model age (AMA) derivation corresponds to 46 ± 5 Ma and 22 ± 1 Ma for Nubium and Sosigenes IMP, respectively. We show that for IMPs and in nearby control mare regions, similar production-like cumulative log–log SFD slopes of −3 are observed. In contrast, control mare regions in Mare Humorum exhibit shallower equilibrium slopes from −1.83 to −2. Three out of four wrinkle ridges appear to be in equilibrium but with crater lifetimes lower than on the corresponding maria. Low crater frequencies on one wrinkle ridge result in an age of 8.6 ± 1 Ma. This study region contains 80% fresh craters, which suggests that the crater population is still in production indicative of a recent resurfacing event. 相似文献