共查询到20条相似文献,搜索用时 218 毫秒
1.
Virginia Trimble 《Journal of Astrophysics and Astronomy》1984,5(4):389-402
Supernovae of both Type I (hydrogen-poor) and Type II (hydrogen-rich) can be expected to occur among binary stars. Among massive
stars (>10 M•), the companion makes it more difficult for the primary to develop an unstable core of >1.4.M• while still retaining
the extended, hydrogen-rich envelope needed to make a typical Type II light curve. Among 1–10 M• stars, on the other hand,
a companion plays a vital role in currently popular models for Type I events, by transferring material to the primary after
it has become a stable white dwarf, and so driving it to conditions where either core collapse or explosive nuclear burning
will occur. Several difficulties (involving nucleosynthesis, numbers and lifetimes of progenitors, the mass-transfer mechanism,etc.) still exist in these models. Some of them are overcome by a recent, promising scenario in which the secondary also evolves
to a degenerate configuration, and the two white dwarfs spiral together to produce a hydrogen-free explosion, long after single
stars of the same initial masses have ceased to be capable of fireworks. 相似文献
2.
Carolus J. Schrijver Marc L. Derosa Thomas R. Metcalf Yang Liu Jim Mctiernan Stéphane Régnier Gherardo Valori Michael S. Wheatland Thomas Wiegelmann 《Solar physics》2006,235(1-2):161-190
We compare six algorithms for the computation of nonlinear force-free (NLFF) magnetic fields (including optimization, magnetofrictional,
Grad–Rubin based, and Green's function-based methods) by evaluating their performance in blind tests on analytical force-free-field
models for which boundary conditions are specified either for the entire surface area of a cubic volume or for an extended
lower boundary only. Figures of merit are used to compare the input vector field to the resulting model fields. Based on these
merit functions, we argue that all algorithms yield NLFF fields that agree best with the input field in the lower central
region of the volume, where the field and electrical currents are strongest and the effects of boundary conditions weakest.
The NLFF vector fields in the outer domains of the volume depend sensitively on the details of the specified boundary conditions;
best agreement is found if the field outside of the model volume is incorporated as part of the model boundary, either as
potential field boundaries on the side and top surfaces, or as a potential field in a skirt around the main volume of interest.
For input field (B) and modeled field (b), the best method included in our study yields an average relative vector error En = 〈 |B−b|〉/〈 |B|〉 of only 0.02 when all sides are specified and 0.14 for the case where only the lower boundary is specified, while the total
energy in the magnetic field is approximated to within 2%. The models converge towards the central, strong input field at
speeds that differ by a factor of one million per iteration step. The fastest-converging, best-performing model for these
analytical test cases is the Wheatland, Sturrock, and Roumeliotis (2000) optimization algorithm as implemented by Wiegelmann
(2004). 相似文献
3.
P. Amram C. Mendes de Oliveira H. Plana C. Balkowski J. Boulesteix C. Carignan 《Astrophysics and Space Science》2002,281(1-2):389-392
A long-term program based on Fabry-Perot Hα velocity field data of compact groups has been undertaken in order to analyse
the kinematics of the compact group galaxies and the extent of their dark halos. The data are taken at the ESO and the CFH
3.6 m telescopes. The main goals of our project are: • To determine the evolutionary stages of the studied groups, • To search
for tidal dwarf galaxy candidates in heavily interacting systems and • in combination with photometry available in the literature,
to determine the Tully-Fisher relation for the group galaxies. The sample contains examples of groups at a variety of dynamical
stages: from a false group that is in fact one single irregular galaxy with several star-forming blobs (e.g. H18) to a group
whose members are strongly interacting and possibly forming tidal dwarf galaxies (e.g. H92) to a group in the final process
of merging. We have so far identified five HCG classes: 1. merging groups2. strongly interacting, 3. weakly interacting, 4.
non-groups, 5. single irregular galaxies. The ones which can be used in the TF studies are those in classes (2) and (3), for
which comprehensive rotation curves can be derived.
This revised version was published online in September 2006 with corrections to the Cover Date. 相似文献
4.
Anirudh Pradhan Hassan Amirhashchi Rekha Jaiswal 《Astrophysics and Space Science》2011,334(2):249-260
A new class of dark energy models in a Locally Rotationally Symmetric Bianchi type-II (LRS B-II) space-time with variable
equation of state (EoS) parameter and constant deceleration parameter have been investigated in the present paper. The Einstein’s
field equations have been solved by applying a variation law for generalized Hubble’s parameter given by Berman: Nuovo Cimento
74:182 (1983) which generates two types of solutions for the average scale factor, one is of power-law type and other is of the exponential-law
form. Using these two forms, Einstein’s field equations are solved separately that correspond to expanding singular and non-singular
models of the universe respectively. The dark energy EoS parameter ω is found to be time dependent and its existing range for both models is in good agreement with the three recent observations
of (i) SNe Ia data (Knop et al.: Astrophys. J. 598:102 (2003)), (ii) SNe Ia data collaborated with CMBR anisotropy and galaxy clustering statistics (Tegmark et al.: Astrophys. J. 606:702 (2004)) and latest (iii) a combination of cosmological datasets coming from CMB anisotropies, luminosity distances of high redshift
type Ia supernovae and galaxy clustering (Hinshaw et al.: Astrophys. J. Suppl. 180:225 (2009); Komatsu et al. 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 behaviour of the universe have also been discussed in detail. 相似文献
5.
Empirical evidence for both stellar mass black holes (M
•<102
M
⊙) and supermassive black holes (SMBHs, M
•>105
M
⊙) is well established. Moreover, every galaxy with a bulge appears to host a SMBH, whose mass is correlated with the bulge
mass, and even more strongly with the central stellar velocity dispersion σ
c
, the M
•–σ relation. On the other hand, evidence for “intermediate-mass” black holes (IMBHs, with masses in the range 100–105 M
⊙) is relatively sparse, with only a few mass measurements reported in globular clusters (GCs), dwarf galaxies and low-mass
AGNs. We explore the question of whether globular clusters extend the M
•–σ relationship for galaxies to lower black hole masses and find that available data for globular clusters are consistent with
the extrapolation of this relationship. We use this extrapolated M
•–σ relationship to predict the putative black hole masses of those globular clusters where existence of central IMBH was proposed.
We discuss how globular clusters can be used as a constraint on theories making specific predictions for the low-mass end
of the M
•–σ relation. 相似文献
6.
XMM-Newton is a major X-ray observatory of the European Space Agency (ESA). Its observing time is open to astronomers from the whole scientific community on a peer reviewed competitive basis. The Science Operations Centre, located at ESA’s premises in Villafranca del Castillo, Spain, is responsible for the instrument operations, as well as for all the tasks related to facilitating the scientific exploitation of the data which the mission has been producing since its launch in December 1999. Among them, one may list:
Everyone can be an XMM-Newton observer. So far, astronomers from 36 countries submitted observing programs. Public data can be accessed by every scientist in the world through the XMM-Newton Science Archive (XSA).Despite all these efforts, one can’t help noticing an asymmetric level of scientific exploitation in the realm of X-ray astronomy between developing and developed countries. The latter have traditionally enjoyed the comparative advantage of deeper know-how, deriving from direct experience in hardware and mission development. The XMM-Newton Science Operations Centre’s efforts act to alleviate this situation through, for example, increasing the usage of the web for data and information dissemination, as well as by supporting actively such initiatives as the COSPAR Capacity-Building Workshops, specifically designed to create long-lasting bridges between researchers in developing and developed countries. 相似文献
• | distribution of scientific data in different formats, from raw telemetry, up to processed and calibrated high-level science products, such as images, spectra, source lists, etc; |
• | development and distribution of dedicated science analysis software, as well as of continuously updated instrument calibration; |
• | regular organisation of training workshops (free of cost), for potential users of XMM-Newton data, where the procedures and techniques to successfully reduce and analyze XMM-Newton data are introduced; |
• | access to the data through state-of-the-art, in-house-developed archival facilities, either through the Internet or via CD-ROM; |
• | continuously updated documentation on all aspects of spacecraft and instrument operations, data reduction and analysis; |
• | maintenance of a comprehensive set of project web pages; |
• | a competent and responsive HelpDesk, providing dedicated support to individual XMM-Newton users. |
7.
One goal of helioseismology is to determine the subsurface structure of sunspots. In order to do so, it is important to understand
first the near-surface effects of sunspots on solar waves, which are dominant. Here we construct simplified, cylindrically-symmetric
sunspot models that are designed to capture the magnetic and thermodynamics effects coming from about 500 km below the quiet-Sun
τ
5000=1 level to the lower chromosphere. We use a combination of existing semi-empirical models of sunspot thermodynamic structure
(density, temperature, pressure): the umbral model of Maltby et al. (1986, Astrophys. J. 306, 284) and the penumbral model of Ding and Fang (1989, Astron. Astrophys. 225, 204). The OPAL equation-of-state tables are used to derive the sound-speed profile. We smoothly merge the near-surface properties
to the quiet-Sun values about 1 Mm below the surface. The umbral and penumbral radii are free parameters. The magnetic field
is added to the thermodynamic structure, without requiring magnetostatic equilibrium. The vertical component of the magnetic
field is assumed to have a Gaussian horizontal profile, with a maximum surface field strength fixed by surface observations.
The full magnetic-field vector is solenoidal and determined by the on-axis vertical field, which, at the surface, is chosen
such that the field inclination is 45° at the umbral – penumbral boundary. We construct a particular sunspot model based on
SOHO/MDI observations of the sunspot in active region NOAA 9787. The helioseismic signature of the model sunspot is studied
using numerical simulations of the propagation of f, p
1, and p
2 wave packets. These simulations are compared against cross-covariances of the observed wave field. We find that the sunspot
model gives a helioseismic signature that is similar to the observations. 相似文献
8.
In a density-stratified turbulent medium, the cross helicity 〈u′⋅B′〉 is considered as a result of the interaction of the velocity fluctuations and a large-scale magnetic field. By means of
a quasilinear theory and by numerical simulations, we find the cross helicity and the mean vertical magnetic field to be anti-correlated.
In the high-conductivity limit the ratio of the helicity and the mean magnetic field equals the ratio of the magnetic eddy
diffusivity and the (known) density scale height. The result can be used to predict that the cross helicity at the solar surface
will exceed the value of 1 gauss km s−1. Its sign is anti-correlated to that of the radial mean magnetic field. Alternatively, we can use our result to determine
the value of the turbulent magnetic diffusivity from observations of the cross helicity. 相似文献
9.
The question about the interpretation of numerical experiments on magnetic reconnection in solar flares is considered. A correspondence
between the standard classification of magnetohydrodynamic discontinuities and the parameters characterizing the mass flux
through a discontinuity and the magnetic field configuration has been established within a classical formulation of the problem
on discontinuous magnetohydrodynamic flows. A pictorial graphical representation of the relationship between the angles of
the magnetic field vector relative to the normal to the discontinuity plane on both its sides has also been found. The relations
between the parameters of a two-dimensional discontinuous flow have the simplest form in a frame of reference where the magnetic
field lines (B) are parallel to the matter velocity (u)—the deHoffmann-Teller frame. The question about the transformation of the magnetic field configuration when passing to a
“laboratory” frame of reference where (v · B) ≠ 0, i.e., an electric field is present, is considered in this connection. The result is applied to the analytical solution
of the problem on the magnetic field structure in the vicinity of a reconnecting current sheet obtained previously by Bezrodnykh
et al. The regions of nonevolutionary shocks are shown to appear near the endpoints of a current sheet with reverse currents. 相似文献
10.
P. R. Vishwanath 《Journal of Astrophysics and Astronomy》2002,23(1-2):45-51
The field of Very High Energy (VHE) gamma ray astronomy using the Atmospheric Cerenkov Technique has entered an interesting
phase with detection of various galactic and extragalactic sources. Among galactic sources, only the Crab nebula has been
established as a standard candle. Most observations on pulsars are in agreement as to the necessity for the GeV spectra to
steepen at < 200 GeV. While the Imaging method for increase of sensitivity has been successful with many results, an alternate
technique —Wavefront Sampling Technique- is also being used by an increasing number of experiments. The recently commissioned experiment at Pachmarhi (PACT) in India
is presented as an example of this technique. Preliminary results from this experiment show detection of VHE γ-ray emission
from (a) the Crab nebula at a high significance and (b) Crab and Geminga pulsars at > 1.5 TeV which could be the second component of the Outer Gap pulsar models. 相似文献
11.
A possible correlation between the magnetic and velocity fields has been analyzed based on the SOHO/MDI magnetograms and Dopplergrams.
It is found that the observed large-scale weak magnetic field (weaker than 50 G (gauss)) is correlated with the velocity statistically.
The curves of u⋅b with latitude, where u and b are the velocity and magnetic fields in a rectangular region (±15○ in longitude, ±45○ in latitude) on the Sun, show the same patterns in the years 2000, 2004, and 2007. The patterns indicate that u and b are positively correlated near the equator but are anti-correlated at the middle latitudes. For a strong magnetic field between
50 G and 3000 G, the curves of u⋅b with latitude show the same tendencies at the middle latitudes. Near the equator, however, the slope of the curve is positive
in 2000 and is negative in 2004 and 2007. In addition, we give an estimation for the amplitude of the cross helicity h
χ
(hc=[`(u·b)]h_{\chi}=\overline{\mathbf{u}\cdot\mathbf{b}}) inferred from the MDI data, which is of the order of 103 G m s−1 near the center of the solar disk. 相似文献
12.
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
13.
The quantum field model is used to study the correlation functions of velocity and magnetic fluctuations in helical developed
MHD turbulence of solar wind which is generated by random forces with mixed noise correlators. The exponential increase of
the magnetic fluctuations is stabilized by spontaneous symmetry breaking mechanism, which leads to the creation of homogeneous
magnetic field 〈E〉, and consequently, gives rise to the α-effect. The maximum value of the α-effect is determined in the Kolmogorov universal
regime and its contribution to the proton acceleration is estimated. The contribution of the α-effect to ∼100 MeV proton acceleration
is discussed and compared with the 2nd Fermi acceleration mechanism.
This article was submitted by the authors in English. 相似文献
14.
Hypersurface–homogeneous cosmological models containing a bulk viscous fluid with time varying G and Λ have been presented. We have shown that the field equations are solvable for any arbitrary cosmic scale function. The
viscosity coefficient of bulk viscous fluid is assumed to be a power function of the energy density. Exact solutions of Einstein’s
field equations are obtained which represent an expanding, shearing and accelerating/decelerating models of the universe.
The physical and kinematical behaviours of the models are also discussed. 相似文献
15.
This contribution is a follow-up to the recent paper of Kuznetsov et al. (Contrib. Astron. Obs. Skalnaté Pleso
36, 85, 2006) on the ground level enhancement (GLE) on 20 January 2005. We focused on a study of Forbush decrease (FD) of 17 – 18 and
21 – 22 January 2005, respectively. The data from the neutron monitor at Lomnicky Štít (1 min counts) and from the Geomagnetic
Observatory in Hurbanovo, both in Slovakia, were used as the basis for our investigation. The data on magnetic field and solar
wind from GOES 10 and 12, SOHO-CELIAS, ACE and WIND satellites were used for better understanding of the global evolution
of the event. The magnetic field is transformed to the RTN (Radial – Tangential – Normal) system where only the disturbed
part of the field is compared, i.e., daily variations and a constant part are subtracted. The field reduction method is described. Our results are temporal vector
diagrams of variation of all parameters at all positions from where we used the data. The amplitudes of |B| exceed 100 nT and variations during the arrival of the wavefront of CME take place simultaneously at the ground-based station
and at GOES satellites. The character of the variations is as if there would be regions with the dominant electric charge
of opposite signs, or electric currents with different orientations in the CME. On the basis of the values v
p and n
p and using certain assumptions we determined the mass of CME on 17 January and 21 January, respectively, of 1012 kg. A decrease of the cosmic ray level runs suddenly (during 10 minutes), starting, however, about two hours after a sudden
change of the magnetic field. 相似文献
16.
It was discovered some years ago by Schiff that the equations divE = 4πQ and curlB - (1/c) ∂E/∂t = (4π/c)J for fields in vacuum do not carry over without change from an inertial frame to a frame with rotating axes of space coordinates,
even for a region with all velocities of orderv≪c. However, the belief that all four of the field equations are invariant under such conditions is still prevalent and causes
misconceptions in physical applications, including astrophysical and geophysical ones. The purpose of the present paper is
therefore to call attention to Schiff's discovery, discussing its basis and its extension to fields in material media, and
to interpret the additional terms that must be added to the equations in order to obtain valid transformations to rotating
axes of coordinates. 相似文献
17.
A new class of plane-symmetric inhomogeneous cosmological models of perfect fluid distribution with electro-magnetic field
based on Lyra’s geometry is obtained by considering a time dependent displacement field. The source of the magnetic field
is due to an electric current produced along the z-axis. Only F
12 is a non-vanishing component of electromagnetic field tensor. To get the deterministic solutions, the free gravitational
field is assumed to be of Petrov type-II non-degenerate. It has been found that the displacement vector β(t) behaves like cosmological term Λ which is consistent with the recent observations of type Ia supernovae. It is also observed
that β(t) affects entropy. Some geometric and physical behaviour of the models are also discussed in presence of magnetic field.
相似文献
18.
Bermann [Nuovo Cimento B (1983), 74, 182] presented a law of variation of Hubble’s parameter that yields constant deceleration
parameter models of the Universe. In this paper, we study some cosmological models with negative constant deceleration parameter
within the framework of Lyra geometry.
PACS Nos: 98.80 cq, 04.20 jb, 04.50 相似文献
19.
In a previous study (Cane and Richardson, J. Geophys. Res.
108(A4), SSH6-1, 2003), we investigated the occurrence of interplanetary coronal mass ejections in the near-Earth solar wind during 1996 – 2002,
corresponding to the increasing and maximum phases of solar cycle 23, and provided a “comprehensive” catalog of these events.
In this paper, we present a revised and updated catalog of the ≈300 near-Earth ICMEs in 1996 – 2009, encompassing the complete
cycle 23, and summarize their basic properties and geomagnetic effects. In particular, solar wind composition and charge state
observations are now considered when identifying the ICMEs. In general, these additional data confirm the earlier identifications
based predominantly on other solar wind plasma and magnetic field parameters. However, the boundaries of ICME-like plasma
based on charge state/composition data may deviate significantly from those based on conventional plasma/magnetic field parameters.
Furthermore, the much studied “magnetic clouds”, with flux-rope-like magnetic field configurations, may form just a substructure
of the total ICME interval. 相似文献
20.
The gravitational field of a global monopole in the context of Einstein–Cartan theory is investigated. A comparison is made
with the corresponding results predicted by general relativity.
PACS Nos: 98.80cq; 04.20 jb; 04.50 相似文献