共查询到20条相似文献,搜索用时 234 毫秒
1.
Paolo Saracco Sandro D'Odorico Alan Moorwood Jean G. Cuby 《Astrophysics and Space Science》2001,276(2-4):991-998
Deep J- and K
s-band images covering a 5 × 5 arcmin area centred on the NTT Deep Field have been obtained during the Science Verification
of SOFI at the NTT. These images were made available via the Web in early June, 1998. The preliminary results we have obtained
by the analysis of these data are the following: (i) the counts continue to rise with no evidence of a turnover or of a flattening
down to the limits of the survey (K
s = 22.5 and J = 24 mag); (ii) we find a slope d log(N)/dm≈ 0.37, in agreement with most of the faintest surveys but much steeper than the Hawaii survey; (iii) fainter than K
s ≈ 19and J ≈ 20 mag, the median J-K colour of galaxies shows a break in its reddening trend turning toward bluer colours; (iv) faint bluer galaxies also display
a larger compactness index, and a smaller apparent size.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
2.
The energy density of Vaidya-Tikekar isentropic superdense star is found to be decreasing away from the center, only if the
parameter K is negative. The most general exact solution for the star is derived for all negative values of K in terms of circular and inverse circular functions. Which can further be expressed in terms of algebraic functions for K = 2-(n/δ)2 < 0 (n being integer andδ = 1,2,3 4). The energy conditions 0 ≤ p ≤ αρc
2, (α = 1 or 1/3) and adiabatic sound speed conditiondp dρ ≤ c
2, when applied at the center and at the boundary, restricted the parameters K and α such that .18 < −K −2287 and.004 ≤ α ≤ .86. The maximum mass of the star satisfying the strong energy condition (SEC),
(α = 1/3) is found to be3.82 Mq· at K=−2/3, while the same for the weak energy condition (WEC), (α =1) is 4.57 M_
⊙ atK=−>5/2. In each case the surface density is assumed to be 2 × 1014 gm cm-3. The solutions corresponding to K>0 (in fact K>1) are also made meaningful by considering the hypersurfaces t= constant as 3-hyperboloid by replacing the parameter R
2 by −R2 in Vaidya-Tikekar formalism. The solutions for the later case are also expressible in terms of algebraic functions for K=2-(n/δ2 > 1 (n being integer or zero and δ =1,2,3 4). The cases for which 0 < K < 1 do not possess negative energy density gradient and therefore are incapable of representing any physically plausible
star model. In totality the article provides all the physically plausible exact solutions for the Buchdahl static perfect
fluid spheres.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
3.
C.R. Benn M. Vigotti R. Carballo J.I. González-Serrano S.F. Sánchez 《Astrophysics and Space Science》2001,276(2-4):1037-1040
The broad range of optical–infrared colours of radio QSOs,1 < B-K < 6, has been cited as evidence for several mag of dust
extinction (Webster et al.. If such large extinctions are typical, the implications for our understanding of the space density of optically selected
QSOs are profound. We have previously found that the host galaxies of several of the reddest B3 QSOs are readily detectable
in K-band images. This suggests contamination of the K apparent magnitudes by starlight, i.e. the redness in B-Kmay be due to excess light in K, rather than to dust extinction of the B light. We have now imaged the B3 QSOs inUBVR, and we use the range of observed optical and optical–IR colours to place an upper limit on the amount of dust extinction
present, rest-frame A
V < 1.5 mag.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
4.
Toshio Fukushima 《Celestial Mechanics and Dynamical Astronomy》2009,105(4):305-328
As a preparation step to compute Jacobian elliptic functions efficiently, we created a fast method to calculate the complete
elliptic integral of the first and second kinds, K(m) and E(m), for the standard domain of the elliptic parameter, 0 < m < 1. For the case 0 < m < 0.9, the method utilizes 10 pairs of approximate polynomials of the order of 9–19 obtained by truncating Taylor series
expansions of the integrals. Otherwise, the associate integrals, K(1 − m) and E(1 − m), are first computed by a pair of the approximate polynomials and then transformed to K(m) and E(m) by means of Jacobi’s nome, q, and Legendre’s identity relation. In average, the new method runs more-than-twice faster than the existing methods including
Cody’s Chebyshev polynomial approximation of Hastings type and Innes’ formulation based on q-series expansions. Next, we invented a fast procedure to compute simultaneously three Jacobian elliptic functions, sn(u|m), cn(u|m), and dn(u|m), by repeated usage of the double argument formulae starting from the Maclaurin series expansions with respect to the elliptic
argument, u, after its domain is reduced to the standard range, 0 ≤ u < K(m)/4, with the help of the new method to compute K(m). The new procedure is 25–70% faster than the methods based on the Gauss transformation such as Bulirsch’s algorithm, sncndn, quoted in the Numerical Recipes even if the acceleration of computation of K(m) is not taken into account. 相似文献
5.
Michel Hénon 《Celestial Mechanics and Dynamical Astronomy》2005,93(1-4):87-100
We present five families of periodic solutions of Hill’s problem which are asymmetric with respect to the horizontal ξ axis. In one of these families, the orbits are symmetric with respect to the vertical η axis; in the four others, the orbits are without any symmetry. Each family consists of two branches, which are mirror images
of each other with respect to the ξ axis. These two branches are joined at a maximum of Γ, where the family of asymmetric periodic solutions intersects a family
of symmetric (with respect to the ξ axis) periodic solutions. Both branches can be continued into second species families for Γ → − ∞. 相似文献
6.
S. Chatterjee 《Journal of Astrophysics and Astronomy》1991,12(4):269-280
We present here rigorous analytical solutions for the Boltzmann-Poisson equation concerning the distribution of stars above
the galactic plane. The number density of stars is considered to follow a behaviour n(m,0) ∼H(m - m0)m−x, wherem is the mass of a star andx an arbitrary exponent greater than 2 and also the velocity dispersion of the stars is assumed to behave as < v2(m)> ∼ m−θ the exponent θ being arbitrary and positive. It is shown that an analytic expression can be found for the gravitational field
Kz, in terms of confluent hypergeometric functions, the limiting trends being Kz∼z for z →0, while Kz
→ constant for z → infinity. We also study the behaviour of < |z(m)|2>,i.e. the dispersion of the distance from the galactic disc for the stars of massm. It is seen that the quantity < |z(m)|2>∼ mt-θ, for m→ t, while it departs significantly from this harmonic oscillator behaviour for stars of lighter masses. It is suggested
that observation of < |z(m)|2> can be used as a probe to findx and hence obtain information about the mass spectrum. 相似文献
7.
We investigate numerically the chemodynamical evolution of major disc–disc galaxy mergers in order to explore the origin of
the mass-dependent chemical, photometric and spectroscopic properties observed in elliptical galaxies. We investigate especially
the dependence of the fundamental properties on merger progenitor disc mass (M
d). Three main results are obtained in this study:– More massive (luminous) ellipticals formed by galaxy mergers between more
massive spirals have higher metallicity (Z) and thus show redder colours; the typical metallicity ranges from ∼ 1.0 solar abundance (Z∼ 0.02) for ellipticals formed by mergers with M
d = 1010
M
⊙to ∼ 2.0 solar (Z∼ 0.04) for those with M
d= 1012
M
⊙.– Both the Mg2 line index in the central part of ellipticals (R ≤ 0.1 R
e) and the radial gradient of Mg2 (δ Mg2 / δ log R) are more likely to be larger for massive ellipticals. δ Mg2 / δ log R correlates reasonably well with the central Mg2 in ellipticals. For most of the present merger models, ellipticals show a positive radial gradient of the Hβ line index. – Both M/L
B and M/L
K (where M, L
B, and L
K are the total stellar mass of galaxy mergers, the B-band and the K-band luminosities, respectively) depend on galactic mass in such a way that more massive ellipticals have larger M/L
B and smaller M/L
K.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
8.
We present a variety of well behaved classes of Charge Analogues of Tolman’s iv (1939). These solutions describe charged fluid
balls with positively finite central pressure, positively finite central density; their ratio is less than one and causality
condition is obeyed at the centre. The outmarch of pressure, density, pressure-density ratio and the adiabatic speed of sound
is monotonically decreasing, however, the electric intensity is monotonically increasing in nature. These solutions give us
wide range of parameter for every positive value of n for which the solution is well behaved hence, suitable for modeling
of super dense stars. keeping in view of well behaved nature of these solutions, one new class of solutions is being studied
extensively. Moreover, this class of solutions gives us wide range of constant K (0.3≤K≤0.91) for which the solution is well behaved hence, suitable for modeling of super dense stars like Strange Quark stars,
Neutron stars and Pulsars. For this class of solutions the mass of a star is maximized with all degree of suitability, compatible
with Quark stars, Neutron stars and Pulsars. By assuming the surface density ρ
b
=2×1014 g/cm3 (like, Brecher and Caporaso in Nature 259:377, 1976), corresponding to K=0.30 with X=0.39, the resulting well behaved model has the mass M=2.12M
Θ, radius r
b
≈15.27 km and moment of inertia I=4.482×1045 g cm2; for K=0.4 with X=0.31, the resulting well behaved model has the mass M=1.80M
Θ, radius r
b
≈14.65 km and moment of inertia I=3.454×1045 g cm2; and corresponding to K=0.91 with X=0.135, the resulting well behaved model has the mass M=0.83M
Θ, radius r
b
≈11.84 km and moment of inertia I=0.991×1045 g cm2. For n=0 we rediscovered Pant et al. (in Astrophys. Space Sci. 333:161, 2011b) well behaved solution. These values of masses and moment of inertia are found to be consistent with other models of Neutron
stars and Pulsars available in the literature and are applicable for the Crab and the Vela Pulsars. 相似文献
9.
V.E. Margoniner 《Astrophysics and Space Science》2001,276(2-4):741-748
We present CCD photometry of 16 Abell clusters and one cluster candidate found in POSS-II field 861. The images were taken
at the 0.9 m Telescope at Cerro Tololo, in the g, r and i filters of the Gunn–Thuan system. We tested the idea proposed by Garilli et al. (1996) that there is a population of unusually red galaxies which could be associated with either the field or clusters.
Garilli et al. (1996) suggest that these galaxies have anomalously red colours, but we find that these objects are all near the limiting
magnitude of the images (20m<r<22m) and have colours that are consistent with those expected for stars or field galaxies at z∼0.7.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
10.
Naoteru Gouda Takuji Tsujimoto Yukiyasu Kobayashi Tadashi Nakajima Naoki Yasuda Hideo Matsuhara 《Astrophysics and Space Science》2002,280(1-2):89-94
JASMINE is the name of a Japanese infrared (K-band) scanning astrometric satellite. JASMINE (I and/or II-project) is planned to be launched between 2013 and 2017 and will
measure parallaxes and proper motions with the precision of 10μas at K≃ 12 - 15 mag. JASMINE will observe a few hundred million stars belonging to the disk and the bulge components of our Galaxy,
which are hidden by the interstellar dust extinction in optical bands. Furthermore, JASMINE will also obtain photometry of
stars in K, J and H-bands. The main objective of JASMINE is to study the most fundamental structure and evolution of the disk and the bulge components
of the Milky Way Galaxy.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
11.
P. K. Manoharan 《Solar physics》2006,235(1-2):345-368
Knowledge of the radial evolution of the coronal mass ejection (CME) is important for the understanding of its arrival at
the near-Earth space and of its interaction with the disturbed/ambient solar wind in the course of its travel to 1 AU and
further. In this paper, the radial evolution of 30 large CMEs (angular width > 150∘, i.e., halo and partial halo CMEs) has been investigated between the Sun and the Earth using (i) the white-light images of
the near-Sun region from the Large Angle Spectroscopic Coronagraph (LASCO) onboard SOHO mission and (ii) the interplanetary scintillation (IPS) images of the inner heliosphere obtained from the Ooty Radio Telescope (ORT). In the LASCO field of view at heliocentric
distances R≤30 solar radii (R⊙), these CMEs cover an order of magnitude range of initial speeds, VCME≈260–2600 km s−1. Following results have been obtained from the speed evolution of these CMEs in the Sun–Earth distance range: (1) the speed
profile of the CME shows dependence on its initial speed; (2) the propagation of the CME goes through continuous changes,
which depend on the interaction of the CME with the surrounding solar wind encountered on the way; (3) the radial-speed profiles
obtained by combining the LASCO and IPS images yield the factual view of the propagation of CMEs in the inner heliosphere
and transit times and speeds at 1 AU computed from these profiles are in good agreement with the actual measurements; (4)
the mean travel time curve for different initial speeds and the shape of the radial-speed profiles suggest that up to a distance
of ∼80 R⊙, the internal energy of the CME (or the expansion of the CME) dominates and however, at larger distances, the CME's interaction
with the solar wind controls the propagation; (5) most of the CMEs tend to attain the speed of the ambient flow at 1 AU or
further out of the Earth's orbit. The results of this study are useful to quantify the drag force imposed on a CME by the
interaction with the ambient solar wind and it is essential in modeling the CME propagation. This study also has a great importance
in understanding the prediction of CME-associated space weather at the near-Earth environment. 相似文献
12.
V. Ripepi M. I. Moretti G. Clementini M. Marconi M. R. Cioni J. B. Marquette P. Tisserand 《Astrophysics and Space Science》2012,341(1):51-56
The Vista Magellanic Cloud (VMC, PI M.R. Cioni) survey is collecting K S -band time series photometry of the system formed by the two Magellanic Clouds (MC) and the “bridge” that connects them. These data are used to build K S -band light curves of the MC RR Lyrae stars and Classical Cepheids and determine absolute distances and the 3D geometry of the whole system using the K-band period luminosity (PLK S ), the period–luminosity–color (PLC) and the Wesenhiet relations applicable to these types of variables. As an example of the survey potential we present results from the VMC observations of two fields centered respectively on the South Ecliptic Pole and the 30 Doradus star forming region of the Large Magellanic Cloud. The VMC K S -band light curves of the RR Lyrae stars in these two regions have very good photometric quality with typical errors for the individual data points in the range of ∼0.02 to 0.05 mag. The Cepheids have excellent light curves (typical errors of ∼0.01 mag). The average K S magnitudes derived for both types of variables were used to derive PLK S relations that are in general good agreement within the errors with the literature data, and show a smaller scatter than previous studies. 相似文献
13.
We describe technical aspects of an astrometric and photometric survey of the North Celestial Cap (NCC), from the Pole (δ=90°) to δ=80°, in support of the TAUVEX mission. This region, at galactic latitudes from ∼17° to ∼37°, has poor coverage in modern
CCD-based surveys. The observations are performed with the Wise Observatory one-meter reflector and with a new mosaic CCD
camera (LAIWO) that images in the Johnson–Cousins R and I bands a one-square-degree field with sub-arcsec pixels. The images are treated using IRAF and SExtractor to produce a final
catalogue of sources. The astrometry, based on the USNO-A2.0 catalogue, is good to ∼1 arcsec and the photometry is good to
∼0.1 mag for point sources brighter than R=20.0 or I=19.1 mag. The limiting magnitudes of the survey, defined at photometric errors smaller than 0.15 mag, are 20.6 mag (R) and 19.6 (I). We separate stars from non-stellar objects based on the object shapes in the R and I bands, attempting to reproduce the SDSS star/galaxy dichotomy. The completeness test indicates that the catalogue is complete
to the limiting magnitudes. 相似文献
14.
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
15.
A. V. Getling 《Solar physics》2006,239(1-2):93-111
Two series of solar-granulation images – the La Palma series of 5 June 1993 and the SOHO MDI series of 17 – 18 January 1997
– are analysed both qualitatively and quantitatively. New evidence is presented for the existence of long-lived, quasi-regular
structures (first reported by Getling and Brandt, Astron. Astrophys. 382, L5 (paper I), 2002), which no longer appear unusual in images averaged over 1 – 2-hour time intervals. Such structures appear
as families of light and dark concentric rings or families of light and dark parallel strips (“ridges” and “trenches” in the
brightness distributions). In some cases, rings are combined with radial “spokes” and can thus form “web” patterns. The characteristic
width of a ridge or trench is somewhat larger than the typical size of granules. Running-average movies constructed from the
series of images are used to seek such structures. An algorithm is developed to obtain, for automatically selected centres,
the radial distributions of the azimuthally averaged intensity, which highlight the concentric-ring patterns. We also present
a time-averaged granulation image processed with a software package intended for the detection of geological structures in
aerospace images. A technique of running-average-based correlations between the brightness variations at various points of
the granular field is developed and indications are found for a dynamical link between the emergence and sinking of hot and
cool parcels of the solar plasma. In particular, such a correlation analysis confirms our suggestion that granules – overheated
blobs – may repeatedly emerge on the solar surface. Based on our study, the critical remarks by Rast (Astron. Astrophys. 392, L13, 2002) on the original paper by Getling and Brandt (paper I) can be dismissed. 相似文献
16.
We present diagrams depicting the expected inter-dependences of two key kinematical parameters of radio knots in the parsec-scale
jets of blazars, deduced from VLBI observations. The two parameters are the apparent speed (υ
app = cβ
app) and the effective Doppler boosting factor (δ
eff) of the relativistically moving radio knot. A novel aspect of these analytical computations of β-δ diagrams is that they are made for parsec-scale jets having a conical shape, with modest opening angles (ω up to 10°), in accord with the VLBI observations of the nuclei of the nearest radio galaxies. Another motivating factor is
the recent finding that consideration of a conical geometry can have important implications for the interpretation of a variety
of radio observations of blazar jets. In addition to uniform jet flows (i.e., those having a uniform bulk Lorentz factor,
Γ), computational results are also presented for stratified jets where an ultra-relativistic central spine along the jet axis
is surrounded by a slower moving sheath, possibly arising from a velocity shear. 相似文献
17.
The results of morphological and spectral study of the galaxies Kaz 5, Kaz 92, and Kaz 390 are presented. The observations
were made on the 2.6-m telescope at the Byurakan Observatory with the VAGR microlenses spectrograph. Isophotes of the images
of the galaxies are constructed in the Hα, [NII] λλ6584 , 6548, and [SII] λλ6731, 6717 emission lines and in the continuum. The masses of Kaz 5 and Kaz 92 are determined to be 8.6 × 108 M⊙ and 6.1 × 109 M⊙ , respectively. The mass of the gaseous component in the centers of regions I and IV of Kaz 390, which encompass a 1 pixel
area, are also determined. The morphological structure of the central region of Kaz 5 in the observed spectral range, λλ6400–6800?, differs completely from the structure of the same part of the galaxy observed with the 6-m and 2.5-m telescopes. It is shown
that these differences in the structure in images of Kaz 5 are mostly explained by the comparatively low resolution of the
telescope in combination with the VAGR spectrograph. Absorbing matter also contributes to this effect. It is also shown that
a “deficit” of nitrogen has been observed in the region of Kaz 390 studied here (a circle of diameter 40″).
Translated from Astrofizika, Vol. 52, No. 1, pp. 63–74 (February 2009). 相似文献
18.
It is shown (1) that the coefficients Ai of the limb darkening functions I(μ)/Icenter = P5 (μ) = ∑Ai μi (i = 0... 5; μ = cos ϑ), which had been published by Neckel and Labs (Solar Phys. 153, 91, 1994), can well be approximated by analytical functions of wavelength λ, and (2) that at first sight purely formal extrapolation
of the functions P5(μ) to the very limb (μ = 0.0) is not meaningless: in combination with absolute intensities for the disk center these functions
yield ‘limb intensities’ which all correspond to almost the same ‘limb temperature’, Tlimb≈4746 K. Together these results lead to ‘reference functions’ which can quickly yield rather reliable values of the Sun's
continuum intensities, for any values of μ and λ. 相似文献
19.
Neeraj Pant 《Astrophysics and Space Science》2011,332(2):403-408
The paper presents a class of interior solutions of Einstein–Maxwell field equations of general relativity for a static, spherically
symmetric distribution of the charged fluid. This class of solutions describes well behaved charged fluid balls. The class
of solutions gives us wide range of parameter K (0≤K≤42) for which the solution is well behaved hence, suitable for modeling of super dense star. For this solution the mass of
a star is maximized with all degree of suitability and by assuming the surface density ρ
b
=2×1014 g/cm3. Corresponding to K=2 and X=0.30, the maximum mass of the star comes out to be 4.96 M
Θ with linear dimension 34.16 km and central redshift and surface redshift 2.1033 and 0.683 respectively. In absence of the
charge we are left behind with the well behaved fourth model of Durgapal (J. Phys., A, Math. Gen. 15:2637, 1982). 相似文献
20.
The stability of strange dwarfs for quark cores with M
0core
/M
⨀ = 10−4, has been studied by calculating, in each individual case, a series of strange dwarfs with configurations in which 5 ⋅ 10−4, 10−3, 5 ⋅ 10−3, 10−2, 1.31 ⋅ 10−2, 1.6 ⋅ 10−2, 1.7 ⋅ 10−2, 2 ⋅ 10−2, ranges from the values in white dwarfs to ρ
drip
= 4.3 ⋅ 1011 g/cm3, at which free neutrons are produced in the crust. For the series with M
0core
/M
⨀ < 0.0131, stability is lost when ρ
tr
< ρ
drip
. For the series with M
0core
/M
⨀ > 0.0131, the equality ρ
tr
= ρ
drip
is reached before the strange dwarf attains its maximum mass. Although the frequency of the radial pulsations in the fundamental
mode obeys ω02 > 0 for these configurations, they are unstable with respect to transitions into a strange star state with the same total
number of baryons and a radius on the order of that of neutron stars. An energy on the order of the energy in a supernova
explosion is released during these transitions. It is shown that the gravitational red shift of white and strange dwarfs are
substantially different for low and limiting (high) masses. 相似文献