共查询到20条相似文献，搜索用时 953 毫秒
1.
Homogeneous anisotropic turbulence simulations are used to determine off-diagonal components of the Reynolds stress tensor and its parameterization in terms of turbulent viscosity and Λ-effect. The turbulence is forced in an anisotropic fashion by enhancing the strength of the forcing in the vertical direction. The Coriolis force is included with a rotation axis inclined relative to the vertical direction. The system studied here is significantly simpler than that of turbulent stratified convection which has often been used to study Reynolds stresses. Certain puzzling features of the results for convection, such as sign changes or highly concentrated latitude distributions, are not present in the simpler system considered here. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
2.
The availability of vector-magnetogram sequences with sufficient accuracy and cadence to estimate the temporal derivative
of the magnetic field allows us to use Faraday’s law to find an approximate solution for the electric field in the photosphere,
using a Poloidal–Toroidal Decomposition (PTD) of the magnetic field and its partial time derivative. Without additional information,
however, the electric field found from this technique is under-determined – Faraday’s law provides no information about the
electric field that can be derived from the gradient of a scalar potential. Here, we show how additional information in the
form of line-of-sight Doppler-flow measurements, and motions transverse to the line-of-sight determined with ad-hoc methods such as local correlation tracking, can be combined with the PTD solutions to provide much more accurate solutions
for the solar electric field, and therefore the Poynting flux of electromagnetic energy in the solar photosphere. Reliable,
accurate maps of the Poynting flux are essential for quantitative studies of the buildup of magnetic energy before flares
and coronal mass ejections. 相似文献
3.
Stefanie Komossa 《天文研究与技术》2003,(Z1)
1 Introduction1 .1 X rayprobesoftheblackholeregionofAGNX rayemissionoriginatesfromtheimmediatevicinityoftheblackhole .Thedetectionofluminous,hard ,power law likeX rayemission ,rapidvariability ,andtherecentdiscoveryofrelativisticeffectsintheiron Klineprofileprovidedexcellentevidenceforthepresenceofsupermassiveblackholes (SMBHs)inactivegalaxies.X rayobservationscurrentlyconstitutethemostpowerfulmeanstoexploretheblackholeregionofAGN .X raysatthecentersofAGNariseintheaccretion disk—coro… 相似文献
4.
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
5.
We study the multi-waveband non-thermal emission from the pulsar wind neb- ulae (PWNe) Vela X and G0.9 + 0.1 in the frame of a time-dependent model describing non-thermal radiation from the PWNe. In such a model, the relativistic wind of parti- cles driven by a central pulsar blows into the ambient medium and creates a termination shock that accelerates the particles to very high energy in a PWN. The non-thermal pho-tons in the PWN are produced both by synchrotron radiation and the inverse Compton process, with electrons coming directly from the pulsar magnetosphere and electrons be- ing accelerated at the termination shock. We apply this model to reproduce the observed multi-waveband photon spectra of Vela X and the G0.9+0.1, both of which have been detected emitting very high energy photons. Our results indicate that TeV photons are produced by the inverse Compton scattering of the high-energy electrons in the infrared photon field in both Vela X and PWN G0.9+0.1. The TeV photons from these two PWNe may have leptonic origins. 相似文献
6.
Darren J. Croton Glennys R. Farrar 《Monthly notices of the Royal Astronomical Society》2008,386(4):2285-2289
Void regions of the Universe offer a special environment for studying cosmology and galaxy formation, which may expose weaknesses in our understanding of these phenomena. Although galaxies in voids are observed to be predominately gas rich, star forming and blue, a subpopulation of bright-red void galaxies can also be found, whose star formation was shutdown long ago. Are the same processes that quench star formation in denser regions of the Universe also at work in voids?
We compare the luminosity function of void galaxies in the 2dF Galaxy Redshift Survey, to those from a galaxy formation model built on the Millennium simulation. We show that a global star formation suppression mechanism in the form of low-luminosity 'radio-mode' active galactic nuclei (AGN) heating is sufficient to reproduce the observed population of void early types. Radio-mode heating is environment independent other than its dependence on dark matter halo mass, where, above a critical mass threshold of approximately Mvir ∼ 1012.5 M⊙ , gas cooling on to the galaxy is suppressed and star formation subsequently fades. In the Millennium simulation, the void halo mass function is shifted with respect to denser environments, but still maintains a high-mass tail above this critical threshold. In such void haloes, radio-mode heating remains efficient and red galaxies are found; collectively these galaxies match the observed space density without any modification to the model. Consequently, galaxies living in vastly different large-scale environments but hosted by haloes of similar mass are predicted to have similar properties, consistent with observations. 相似文献
We compare the luminosity function of void galaxies in the 2dF Galaxy Redshift Survey, to those from a galaxy formation model built on the Millennium simulation. We show that a global star formation suppression mechanism in the form of low-luminosity 'radio-mode' active galactic nuclei (AGN) heating is sufficient to reproduce the observed population of void early types. Radio-mode heating is environment independent other than its dependence on dark matter halo mass, where, above a critical mass threshold of approximately M
7.
C. Barbieri F. Tamburini G. Anzolin A. Bianchini E. Mari A. Sponselli G. Umbriaco M. Prasciolu F. Romanato P. Villoresi 《Earth, Moon, and Planets》2009,105(2-4):283-288
The properties of optical vortices produced with spiral phase plates have recently found interesting applications in astronomical coronagraphy. Here we review the characteristics of the optical vortex coronagraph. Our simulations show that the intensity of an on-axis star can be fainted by 10 orders of magnitude, thus allowing the detection of close faint sources like extrasolar planets. We also discuss the expected coronagraphic performances achievable with a stepped spiral phase plate. 相似文献
8.
Solar Physics - The generally low interplanetary magnetic field magnitude around the minimum between Solar Cycles 23 and 24 (SC 23/24 minimum) allows us to identify weak and small solar wind... 相似文献
9.
W. J. Maciel 《Astrophysics and Space Science》1995,229(2):203-213
Measurements of the equivalent width of the interstellar Lyman line from IUE spectra in the direction of the planetary nebulae NGC 7009 and BD+30°3639 are used to infer the H column density in these directions. Hydrogen 21 cm profiles are also used in connection with the ultraviolet data so that the expected rotation velocities and distances can be determined. The results are compared with recently published distances, in an attempt to distinguish between the short and long PN distance scales as applied to these objects. 相似文献
10.
The thermal character of inner horizon in a Reissner-Nordstrom black hole is studied via Hamilton-Jacobi method. There is
“Hawking absorption” as a quantum effect near the inner horizon, and a negative temperature of the inner horizon was attained
by choosing an observer outside the black hole. Using a redefined entropy of the black hole, we give a new expression of Bekenstein-Smarr
formula. The redefined entropy satisfies Nernst Theorem, so it can be regarded as Planck absolute entropy of the Reissner-Nordstrom
black hole. 相似文献
11.
C. Johan Masreliez 《Astrophysics and Space Science》2010,326(2):281-291
The phenomenon of inertia is modeled by a new process, which semi-discretely changes the metrical scale of spacetime. This
process explains inertia as a curved spacetime phenomenon, since all accelerating trajectories take place on geodesics of
general relativity. By this approach inertial reference frames are Minkowskian manifolds imbedded in a five-dimensional space.
It would imply a revised interpretation of special relativity, which preserves a universal temporal reference. If acceleration
curves spacetime, it would also imply the existence of an energy-momentum tensor with net energy, which under certain conditions
could become negative. 相似文献
12.
Ahmad Rami El-Nabulsi 《Astrophysics and Space Science》2010,326(2):175-179
We discuss in this letter a new cosmological model including both a non-linear derivative interaction and a Gauss–Bonnet invariant. The first interaction term appears in the theory with the Galilean shift symmetry, whereas the second term is motivated from string theory and plays a crucial role in Chern–Simons gravitational theories as well as in the renormalization of quantum field theories in curved spacetime. It is shown that the presence of both the cubic interaction and the Gauss–Bonnet invariant term supply amazingly prosperous cosmological phenomenologies, which are explored in some detail. 相似文献
13.
《天文和天体物理学研究(英文版)》2015,(8)
We present a sample of about 120 000 red clump candidates selected from the LAMOST DR2 catalog based on the empirical distribution model in the effective temperature vs. surface gravity plane. Although, in general, red clump stars are considered as standard candles, they do not exactly stay in a narrow range of absolute magnitude, but may have a range of more than one magnitude depending on their initial mass. Consequently, conventional oversimplified distance estimations with the assumption of a fixed luminosity may lead to systematic bias related to the initial mass or age, which can potentially affect the study of the evolution of the Galaxy with red clump stars. We therefore employ an isochrone-based method to estimate the absolute magnitude of red clump stars from their observed surface gravities, effective temperatures and metallicities. We verify that the estimation removes the systematics well and provides initial mass/age estimates that are independent of distance with accuracy better than 10%. 相似文献
14.
To search for a possible atmosphere on Pluto and Triton, spectra of these objects as well as comparison stars were obtained with a three-stage Varo image tube for the spectral region from 6800 to 9000 Å. Ratio spectra indicate an absorption feature near 8900 Å, although the steeply diminishing response of the image tube at that wavelength casts some doubt on the reality of this feature. The feature appears more definitive in the spectrum of Pluto and less certain in the spectrum of Triton. The absorption was analyzed using our recently determined band-model parameters for methane. Under the assumption of a pressure higher than 0.01 atm an abundance of 3 m-amagat was determined. For pressures limited by the methane abundance itself, an abundance of 50 m-amagat and a pressure of 10?3 atm was derived (using g = 0.20 g⊕ for both Pluto and Triton). This pressure is close to the pressure that can be expected from the equilibrium vapor pressure of a methane frost. If the absorption at 8900 Å is spurious, our analysis will be applicable as an upper limit for the presence of methane gas on Pluto or Triton. 相似文献
15.
HongZhang Cheng－ZhiZhang 《中国天文和天体物理学报》2001,1(3):275-280
The recent Galileo spacecraft explored Jupiter and its satellite system and provided us with new geodetic data. In order to discuss the dynamical parameters and secular tidal effect of Io, the theory of synchronous satellite is described in detail. Using the new geodetic data of Io, two sets of Io‘s internal structure models are constructed based on the asthenosphere assumption. The liberation parameters α，β，γ and dynamical flattening H are calculated for the models of Io. A comparison of Io with the Moon indicates the they are quite different in many characteristics in spite of the fact that they are approximately equal in mass and size and that they both orbit synchronously. 相似文献
16.
Radio observations of some active regions (ARs) obtained with the Nobeyama radioheliograph at λ=1.76cm are used for estimating the magnetic field strength in the upper chromosphere, based on thermal bremsstrahlung. The results are compared with the magnetic field strength in the photosphere from observations with the Solar Magnetic Field Telescope (SMFT) at Huairou Solar Observing Station of Beijing Astronomical Observatory. The difference in the magnetic field strength between the two layers seems reasonable. The solar radio maps of active regions obtained with the Nobeyama radioheliograph, both in total intensity (I-map) and in circular polarizations (V-map), are compared with the optical magnetograms obtained with the SMFT. The comparison between the radio map in circular polarization and the longitudinal photospheric magnetogram of a plage region suggest that the radio map in circular polarization is a kind of magnetogram of the upper chromosphere. The comparison of the radio map in total intensity with the photospheric vector magnetogram of an AR shows that the radio map in total intensity gives indications of magnetic loops in the corona, thus we have a method of defining the coronal magnetic structure from the radio I-maps at λ=1.76 cm. Analysing the I-maps, we identified three components: (a) a compact bright source; (b) a narrow elongated structure connecting two main magnetic islands of opposite polarities (observed in both the optical and radio magnetograms); (c) a wide, diffuse, weak component that corresponds to a wide structure in the solar active region which shows in most cases an S or a reversed S contour, which is probably due to the differential rotation of the Sun. The last two components suggest coronal loops on different spatial scales above the neutral line of the longitudinal photospheric magnetic field. 相似文献
17.
18.
Abundance Gradient from Open Clusters and Implications for the Galactic Disk Evolution 总被引：3，自引：0，他引：3
Jin-Liang Hou * Rui-Xiang Chang Li Chen Shanghai Astronomical Observatory Chinese Academy of Sciences Shanghai National Astronomical Observatories Chinese Academy of Sciences Beijing Galactic structure dynamics Received Jun 《中国天文和天体物理学报》2002,2(1):17-32
1 INTRODUCTIONRadial abundance gradient along the Galactic disk constitutes one of the most importantobservational constraillts fOr models of the evolution of the Galactic disk. The existence of sucha gradient is now well established, through radio and optical observations of HII regions, diskstars, pIanetary nebulae (see Henry and Worthey 1999 for a detailed review) and open clusters(Friel 1995, 1999). An average gradiellt of dlog(X/H)/dR ~ --0.06 dex kpc--' is observed inthe Milky … 相似文献
19.
Dmitry A. Pavlov James G. Williams Vladimir V. Suvorkin 《Celestial Mechanics and Dynamical Astronomy》2016,126(1-3):61-88
The aim of this work is to combine the model of orbital and rotational motion of the Moon developed for DE430 with up-to-date astronomical, geodynamical, and geo- and selenophysical models. The parameters of the orbit and physical libration are determined in this work from lunar laser ranging (LLR) observations made at different observatories in 1970–2013. Parameters of other models are taken from solutions that were obtained independently from LLR. A new implementation of the DE430 lunar model, including the liquid core equations, was done within the EPM ephemeris. The postfit residuals of LLR observations make evident that the terrestrial models and solutions recommended by the IERS Conventions are compatible with the lunar theory. That includes: EGM2008 gravitational potential with conventional corrections and variations from solid and ocean tides; displacement of stations due to solid and ocean loading tides; and precession-nutation model. Usage of these models in the solution for LLR observations has allowed us to reduce the number of parameters to be fit. The fixed model of tidal variations of the geopotential has resulted in a lesser value of Moon’s extra eccentricity rate, as compared to the original DE430 model with two fit parameters. A mixed model of lunar gravitational potential was used, with some coefficients determined from LLR observations, and other taken from the GL660b solution obtained from the GRAIL spacecraft mission. Solutions obtain accurate positions for the ranging stations and the five retroreflectors. Station motion is derived for sites with long data spans. Dissipation is detected at the lunar fluid core-solid mantle boundary demonstrating that a fluid core is present. Tidal dissipation is strong at both Earth and Moon. Consequently, the lunar semimajor axis is expanding by 38.20 mm/yr, the tidal acceleration in mean longitude is \(-25.90 {{}^{\prime \prime }}/\mathrm{cy}^2\), and the eccentricity is increasing by \(1.48\times 10^{-11}\) each year. 相似文献
20.
In this work, we report on the intense flaring activity from Mkn-421 in X-ray and γ-ray regimes simultaneously observed by Swift-XRT/BAT and Fermi-LAT satellite telescopes in February 2010. With the aim of understanding the underlying physics of the flaring state in Mkn-421, we have performed a detailed spectral analysis of Swift/XRT and Fermi/LAT observations of Mkn-421 during February 12–25, 2010 (MJD 55239–55252). Over this period, we study the daily light curves and spectral variability of the source in 1–10 keV, 0.1–1 GeV and 1–100 GeV energy bands. We have performed the spectral analysis of Swift-XRT and Fermi/LAT observations to study the spectral evolution in the X-ray and gamma-ray energy domains respectively. We also compute the fractional variability amplitude in both the energy bands during the above period. We study trends between spectral parameters and physical insights provided by the parameter responsible for X-ray and γ-ray emission from the source. We search for energetic features phenomenologically linked to the single zone SSC model for blazar emission. We also produce the broad band SED with a leptonic single zone SSC model for the source. 相似文献