31,781 research outputs found
On trapped surface formation in gravitational collapse II
Further to our consideration on trapped surfaces in gravitational collapse,
where pressures were allowed to be negative while satisfying weak energy
condition to avoid trapped surface formation, we discuss here several other
attempts of similar nature in this direction. Certain astrophysical aspects are
pointed out towards examining the physical realization of such a possibility in
realistic gravitational collapse
Geometric phase for neutrino propagation in magnetic field
The geometric phase for neutrinos propagating in an adiabatically varying
magnetic field in matter is calculated. It is shown that for neutrino
propagation in sufficiently large magnetic field the neutrino eigenstates
develop a significant geometric phase. The geometric phase varies from 2
for magnetic fields fraction of a micro gauss to for fields gauss or more. The variation of geometric phase with magnetic field
parameters is shown and its phenomenological implications are discussed
Two Step Filament Eruption During 14-15 March 2015
We present here an interesting two-step filament eruption during 14-15 March
2015. The filament was located in NOAA AR 12297 and associated with a halo
Coronal Mass Ejection (CME). We use observations from the Atmospheric Imaging
Assembly (AIA) and Heliospheric Magnetic Imager (HMI) instruments onboard the
Solar Dynamics Observatory (SDO), and from the Solar and Heliospheric
Observatory (SOHO) Large Angle and Spectrometric Coronagraph (LASCO). We also
use H-alpha data from the Global Oscillation Network Group (GONG) telescope and
the Kanzelhoehe Solar Observatory. The filament shows a first step eruption on
14 March 2015 and it stops its rise at a projected altitude ~ 125 Mm on the
solar disk. It remains at this height for ~ 12 hrs. Finally it eruptes on 15
March 2015 and produced a halo CME. We also find jet activity in the active
region during both days, which could help the filament de-stabilization and
eruption. The decay index is calculated to understand this two-step eruption.
The eruption could be due to the presence of successive
instability-stability-instability zones as the filament is rising.Comment: 11 pages, 7 figures, accepted for the publication in Solar Physic
Stability of Naked Singularity arising in gravitational collapse of Type I matter fields
Considering gravitational collapse of Type I matter fields, we prove that,
given an arbitrary - mass function and a -
function (through the corresponding - metric function
), there exist infinitely many choices of energy distribution
function such that the `true' initial data () leads
the collapse to the formation of naked singularity. We further prove that the
occurrence of such a naked singularity is stable with respect to small changes
in the initial data. We remark that though the initial data leading to both
black hole and naked singularity form a "big" subset of the true initial data
set, their occurrence is not generic. The terms `stability' and `genericity'
are appropriately defined following the theory of dynamical systems. The
particular case of radial pressure has been illustrated in details
to get clear picture of how naked singularity is formed and how, it is stable
with respect to initial data.Comment: 16 pages, no figure, Latex, submitted to Praman
Tropical rainforest bird community structure in relation to altitude, tree species composition, and null models in the Western Ghats, India
Studies of species distributions on elevational gradients are essential to
understand principles of community organisation as well as to conserve species
in montane regions. This study examined the patterns of species richness,
abundance, composition, range sizes, and distribution of rainforest birds at 14
sites along an elevational gradient (500-1400 m) in the Kalakad-Mundanthurai
Tiger Reserve (KMTR) of the Western Ghats, India. In contrast to theoretical
expectation, resident bird species richness did not change significantly with
elevation although the species composition changed substantially (<10%
similarity) between the lowest and highest elevation sites. Constancy in
species richness was possibly due to relative constancy in productivity and
lack of elevational trends in vegetation structure. Elevational range size of
birds, expected to increase with elevation according to Rapoport's rule, was
found to show a contrasting inverse U-shaped pattern because species with
narrow elevational distributions, including endemics, occurred at both ends of
the gradient (below 800 m and above 1,200 m). Bird species composition also did
not vary randomly along the gradient as assessed using a hierarchy of null
models of community assembly, from completely unconstrained models to ones with
species richness and range-size distribution restrictions. Instead, bird
community composition was significantly correlated with elevation and tree
species composition of sites, indicating the influence of deterministic factors
on bird community structure. Conservation of low- and high-elevation areas and
maintenance of tree species composition against habitat alteration are
important for bird conservation in the southern Western Ghats rainforests.Comment: 36 pages, 5 figures, two tables (including one in the appendix)
Submitted to the Journal of the Bombay Natural History Society (JBNHS
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