919 research outputs found
Gravitational Lens Systems to probe Extragalactic Magnetic Fields
The Faraday rotation measurements of multiply-imaged gravitational lens
systems can be effectively used to probe the existence of large-scale ordered
magnetic fields in lensing galaxies and galaxy clusters. The available sample
of lens systems appears to suggest the presence of a coherent large-scale
magnetic field in giant elliptical galaxies somewhat similar to the spiral
galaxies.Comment: 11 pages, 1 figur
Photometric studies of some starburst galaxies
We present the results of a detailed morphological analysis of ten starburst
galaxies selected from the Markarian catalogue of uv-excess objects. CCD
surface photometry of these galaxies was carried out based on observations made
in B, V (Johnson) and R, I (Kron-Cousins) band passes. We present the radial
variations of surface brightness, ellipticity, position angle and the colour
indices for each galaxy obtained using ellipse fitting isophotal analysis. The
residual images constructed for extracting the fine structure are also
presented. A variety of morphological types are found to host the starburst
phenomenon. The star formation activity is not confined to the nuclear region
alone, but it also occurs at various locations in the galaxy and is seen as
clumpy regions. The colour index and the residual images are used for deriving
information about the sites of enhanced star formation activity and the
triggers of the starburst. The luminosity profiles show an exponential
behaviour in the outer region. The disk scale lengths and the half-light radii
are derived. The contribution of the burst component has been estimated and the
colours of the burst component are presented. Strong isophotal twisting is
detected in all the S0 and E galaxies: Mrk 1002, Mrk 1308 and Mrk 14, in the
sample. This is accompanied by boxiness in some cases, suggesting that a merger
is responsible for the starburst activity in these galaxies. In case of
isolated spirals, a bar or a central oval distortion appear to be the likely
trigger for the starburst.Comment: 12 pages of text and 28 figures. Uses aastex. To be published in A&A
On the inversion of the density gradient at the fringe of the convection zone
Introduction. It is well known that the total pressure and the temperature increase as one goes inward from the surface to the center of a star. That the density, on the other hand, does not necessarily increase with depth below the surface was pointed out by Hoyle and Schwarzschild (1955) and was borne out quite clearly by the numerical integrations of the solar surface layers by Faulkner, Griffiths, and Hoyle (1963). The question was raised by Tayler and Gough (1963) as to whether the density gradient inversion was real or whether it was due to the particular
model of convection adopted by Faulkner et al. It is the purpose of this note to show that the inversion is indeed genuine and results from the steep temperature gradient that exists in the outermost layers of the convection zone where convection is not fully efficient and carries only a fraction (<1/2) of the total energy flux. Also, the electron pressure-temperature plane can be divided into regions where dp/dT is negative and positive. The dividing line depends, in an insensitive manner, on the assumed model and efficiency of convection. In the case of the
sun it is the hydrogen ionization at about 10^4 °K that causes the opacity to go up sharply and as a result the temperature gradient steepens there by inverting the
density gradient. The inversion necessarily results in a Rayleigh-Taylor instability
Does Viscosity turn inflation into the CMB and
Consideration of the entropy production in the creation of the CMB leads to a
simple model of the evolution of the universe during this period which suggests
a connection between the small observed acceleration term and the early
inflation of a closed universe. From this we find an unexpected relationship
between the Omega's of cosmology and calculate the total volume of the
universe.Comment: 8 pages , 2 figure
Particle Creation in Kaluza-Klein Cosmology
We exactly calculate the particle number of scalar fields which are
created from an initial vacuum in certain higher-dimensional cosmological
models. The spacetimes in these models are the four-dimensional Chitre-Hartle
or radiation-dominated universe with extra spaces which are static or power-law
contracting. Except for some models in which no particles could be produced,
the distribution of created particles shows a thermal behavior, at least in the
limit of high three-dimensional "momentum" . In some models, does not
depend on the magnitude of the extra-dimensional "momentum" if is
nonvanishing. A cutoff momentum may emerge in some models, and particles
with could not be produced. We also discuss these results.Comment: Latex 11 page
Role of the scalar field in gravitational lensing
A static and circularly symmetric lens characterized by mass and scalar
charge parameters is constructed. For the small values of the scalar charge to
the mass ratio, the gravitational lensing is qualitatively similar to the case
of the Schwarzschild lens; however, for large values of this ratio the lensing
characteristics are significantly different. The main features are the
existence of two or nil Einstein ring(s) and a radial critical curve, formation
of two or four images and possibility of detecting three images near the lens
for sources located at relatively large angular positions. Such a novel lens
may also be treated as a naked singularity lens.Comment: 11 pages using A&A LaTex, 3 figures included, misprints corrected,
changes in section five, two references added Journal- ref : Astronomy &
Astrophysics, 337(1) 1 (1998
Solar internal rotation rate and the latitudinal variation of the tachocline
A new set of accurately measured frequencies of solar oscillations are used
to infer the rotation rate inside the Sun, as a function of radial distance as
well as latitude. We have adopted a regularized least squares technique with
iterative refinement for both 1.5D inversion using the splitting coefficients
and 2D inversion using individual m splittings. The inferred rotation rate
agrees well with earlier estimates showing a shear layer just below the surface
and another one around the base of the convection zone. The tachocline or the
transition layer where the rotation rate changes from differential rotation in
the convection zone to almost latitudinally independent rotation rate in the
radiative interior is studied in detail. No compelling evidence for any
latitudinal variation in position and width of tachocline is found though it
appears that the tachocline probably shifts to slightly larger radial distance
at higher latitudes and possibly becomes thicker also. However, these
variations are within the estimated errors and more accurate data would be
needed to make a definitive statement about latitudinal variations.Comment: 15 pages, MNRAS-TeX, 15 figure
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