32 research outputs found
Gravity induced neutrino-antineutrino oscillation: CPT and lepton number non-conservation under gravity
We introduce a new effect in the neutrino oscillation phase which shows the
neutrino-antineutrino oscillation is possible under gravity even if the rest
masses of the corresponding eigenstates are same. This is due to CPT violation
and possible to demonstrate if the neutrino mass eigenstates are expressed as a
combination of neutrino and antineutrino eigenstates, as of the neutral kaon
system, with the plausible breaking of lepton number conservation. For Majorana
neutrinos, this oscillation is expected to affect significantly the inner edge
of neutrino dominated accretion disks around a compact object by influencing
the neutrino sphere which controls the accretion dynamics, and then the related
type-II supernova evolution and the r-process nucleosynthesis. On the other
hand, in early universe, in presence of various lepton number violating
processes, this oscillation, we argue, might lead to neutrino asymmetry which
resulted baryogenesis from the B-L symmetry by electro-weak sphaleron
processes.Comment: 15 pages; Accepted for publication in Classical and Quantum Gravit
Mass of highly magnetized white dwarfs exceeding the Chandrasekhar limit: An analytical view
In recent years a number of white dwarfs has been observed with very high
surface magnetic fields. We can expect that the magnetic field in the core of
these stars would be much higher (~ 10^{14} G). In this paper, we analytically
study the effect of high magnetic field on relativistic cold electron, and
hence its effect on the stability and the mass-radius relation of a magnetic
white dwarf. In strong magnetic fields, the equation of state of the Fermi gas
is modified and Landau quantization comes into play. For relatively very high
magnetic fields (with respect to the energy density of matter) the number of
Landau levels is restricted to one or two. We analyse the equation of states
for magnetized electron degenerate gas analytically and attempt to understand
the conditions in which transitions from the zero-th Landau level to first
Landau level occur. We also find the effect of the strong magnetic field on the
star collapsing to a white dwarf, and the mass-radius relation of the resulting
star. We obtain an interesting theoretical result that it is possible to have
white dwarfs with mass more than the mass set by Chandrasekhar limit.Comment: 18 pages including 3 figures; to appear in Modern Physics Letters
Growing hydrodynamic modes in Keplerian accretion disks during secondary perturbations: Elliptical vortex effects
The origin of hydrodynamic turbulence, and in particular of an anomalously
enhanced angular momentum transport, in accretion disks is still an unsolved
problem. This is especially important for cold disk systems which are
practically neutral in charge and therefore turbulence can not be of
magnetohydrodynamic origin. While the flow must exhibit some instability and
then turbulence in support of the transfer of mass inward and angular momentum
outward, according to the linear perturbation theory, in absence of
magnetohydrodynamic effects, it should always be stable. We demonstrate that
the three-dimensional secondary disturbance to the primarily perturbed disk,
consisting of elliptical vortices, gives significantly large hydrodynamic
growth in such a system and hence may suggest a transition to an ultimately
turbulent state. This result is essentially applicable to accretion disks
around quiescent cataclysmic variables, in proto-planetary and star-forming
disks, the outer region of disks in active galactic nuclei, where the gas is
significantly cold and thus the magnetic Reynolds number is smaller than 10^4.Comment: 21 pages including 4 figures, aastex format; Accepted for publication
in The Astrophysical Journa
Bypass to Turbulence in Hydrodynamic Accretion Disks: An Eigenvalue Approach
Cold accretion disks such as those in star-forming systems, quiescent
cataclysmic variables, and some active galactic nuclei, are expected to have
neutral gas which does not couple well to magnetic fields. The turbulent
viscosity in such disks must be hydrodynamic in origin, not
magnetohydrodynamic. We investigate the growth of hydrodynamic perturbations in
a linear shear flow sandwiched between two parallel walls. The unperturbed flow
is similar to plane Couette flow but with a Coriolis force included. Although
there are no exponentially growing eigenmodes in this system, nevertheless,
because of the non-normal nature of the eigenmodes, it is possible to have a
large transient growth in the energy of perturbations. For a constant angular
momentum disk, we find that the perturbation with maximum growth has a
wave-vector in the vertical direction. The energy grows by more than a factor
of 100 for a Reynolds number R=300 and more than a factor of 1000 for R=1000.
Turbulence can be easily excited in such a disk, as found in previous numerical
simulations. For a Keplerian disk, on the other hand, similar vertical
perturbations grow by no more than a factor of 4, explaining why the same
simulations did not find turbulence in this system. However, certain other
two-dimensional perturbations with no vertical structure do exhibit modest
growth. For the optimum two-dimensional perturbation, the energy grows by a
factor of ~100 for R~10^4.5 and by a factor of 1000 for R~10^6. It is
conceivable that these two-dimensional disturbances might lead to
self-sustained turbulence. The Reynolds numbers of cold astrophysical disks are
much larger even than 10^6, therefore, hydrodynamic turbulence may be possible
in disks.Comment: 39 pages including 9 figures; Final version to appear in The
Astrophysical Journa
Magnetohydrodynamic stability of stochastically driven accretion flows
We investigate the evolution of magnetohydrodynamic (or hydromagnetic as coined by Chandrasekhar) perturbations in the presence of stochastic noise in rotating shear flows. The particular emphasis is the flows whose angular velocity decreases but specific angular momentum increases with increasing radial coordinate. Such flows, however, are Rayleigh stable but must be turbulent in order to explain astrophysical observed data and, hence, reveal a mismatch between the linear theory and observations and experiments. The mismatch seems to have been resolved, at least in certain regimes, in the presence of a weak magnetic field, revealing magnetorotational instability. The present work explores the effects of stochastic noise on such magnetohydrodynamic flows, in order to resolve the above mismatch generically for the hot flows. We essentially concentrate on a small section of such a flow which is nothing but a plane shear flow supplemented by the Coriolis effect, mimicking a small section of an astrophysical accretion disk around a compact object. It is found that such stochastically driven flows exhibit large temporal and spatial autocorrelations and cross-correlations of perturbation and, hence, large energy dissipations of perturbation, which generate instability. Interestingly, autocorrelations and cross-correlations appear independent of background angular velocity profiles, which are Rayleigh stable, indicating their universality. This work initiates our attempt to understand the evolution of three-dimensional hydromagnetic perturbations in rotating shear flows in the presence of stochastic noise. © 2013 American Physical Society
Violation of Chandrasekhar Mass Limit: The Exciting Potential of Strongly Magnetized White Dwarfs
We consider a relativistic, degenerate, electron gas under the influence of a
strong magnetic field, which describes magnetized white dwarfs. Landau
quantization changes the density of states available to the electrons, thus
modifying the underlying equation of state. In the presence of very strong
magnetic fields a maximum of either one, two or three Landau level(s) is/are
occupied. We obtain the mass-radius relations for such white dwarfs and their
detailed investigation leads us to propose the existence of white dwarfs having
a mass ~2.3M_Sun, which overwhelmingly exceeds the Chandrasekhar mass limit.Comment: 10 pages including 4 figures; received Honorable Mention for the
Gravity Research Foundation 2012 Awards for Essays on Gravitation; version to
appear in IJMP
Scalar and Spinor Perturbation to the Kerr-NUT Spacetime
We study the scalar and spinor perturbation, namely the Klein-Gordan and
Dirac equations, in the Kerr-NUT space-time. The metric is invariant under the
duality transformation involving the exchange of mass and NUT parameters on one
hand and radial and angle coordinates on the other. We show that this
invariance is also shared by the scalar and spinor perturbation equations.
Further, by the duality transformation, one can go from the Kerr to the dual
Kerr solution, and vice versa, and the same applies to the perturbation
equations. In particular, it turns out that the potential barriers felt by the
incoming scalar and spinor fields are higher for the dual Kerr than that for
the Kerr. We also comment on existence of horizon and singularity.Comment: 31 pages including 20 figures, RevTeX style: Final version to appear
in Classical and Quantum Gravit
Accretion of Chaplygin gas upon black holes: Formation of faster outflowing winds
We study the accretion of modified Chaplygin gas upon different types of
black hole. Modified Chaplygin gas is one of the best candidates for a combined
model of dark matter and dark energy. In addition, from a field theoretical
point of view the modified Chaplygin gas model is equivalent to that of a
scalar field having a self-interacting potential. We formulate the equations
related to both spherical accretion and disc accretion, and respective winds.
The corresponding numerical solutions of the flow, particularly of velocity,
are presented and are analyzed. We show that the accretion-wind system of
modified Chaplygin gas dramatically alters the wind solutions, producing faster
winds, upon changes in physical parameters, while accretion solutions
qualitatively remain unaffected. This implies that modified Chaplygin gas is
more prone to produce outflow which is the natural consequence of the dark
energy into the system.Comment: 21 pages including 7 figures; published in Classical and Quantum
Gravit
Stochastically driven instability in rotating shear flows
Origin of hydrodynamic turbulence in rotating shear flows is investigated. The particular emphasis is on flows whose angular velocities decrease but specific angular momenta increase with increasing radial coordinate. Such flows are Rayleigh stable, but must be turbulent in order to explain observed data. Such a mismatch between the linear theory and observations/experiments is more severe when any hydromagnetic/magnetohydrodynamic instability and the corresponding turbulence therein is ruled out. The present work explores the effect of stochastic noise on such hydrodynamic flows. We focus on a small section of such a flow which is essentially a plane shear flow supplemented by the Coriolis effect. This also mimics a small section of an astrophysical accretion disk. It is found that such stochastically driven flows exhibit large temporal and spatial correlations of perturbation velocities, and hence large energy dissipations, that presumably generate instability. A range of angular velocity profiles (for the steady flow), starting with the constant angular momentum to that of the constant circular velocity are explored. It is shown that the growth and roughness exponents calculated from the contour (envelope) of the perturbed flows are all identical, revealing a unique universality class for the stochastically forced hydrodynamics of rotating shear flows. This work, to the best of our knowledge, is the first attempt to understand origin of instability and turbulence in the three-dimensional Rayleigh stable rotating shear flows by introducing additive stochastic noise to the underlying linearized governing equations. This has important implications in resolving the turbulence problem in astrophysical hydrodynamic flows such as accretion disks
Assessment of occupational health problems and physiological stress among the brick field workers of West Bengal, India
Objectives: The brick field industry is one of the oldest industries in India, which employs a large number of workers of poor socioeconomic status. The main aim of the present investigation is i) to determine the prevalence of musculoskeletal disorders among brick field workers, ii) to determine the prevalence of respiratory disorders and physiological stress among brick field workers compared to control workers. Material and Methods: For this study, a total of 220 brick field workers and 130 control subjects were selected randomly. The control subjects were mainly involved in hand-intensive jobs. The Modified Nordic Questionnaire was applied to assess the discomfort felt among both groups of workers. Thermal stress was also assessed by measuring the WBGT index. The pulmonary functions were checked using the spirometry. Physiological assessment of the workload was carried out by recording the heart rate and blood pressure of the workers prior to work and just after work in the field. Results: Brick field workers suffered from pain especially in the lower back (98%), hands (93%), knees (86%), wrists (85%), shoulders (76%) and neck (65%). Among the brick-making activities, brick field workers felt discomfort during spading for mud collection (98%), carrying bricks (95%) and molding (87%). The results showed a significantly lower p value < 0.001 in FVC, FEV1, FEV1/FVC ratio and PEFR in brick field workers compared to the control group. The post-activity heart rate of the brick field workers was 148.6 beats/min, whereas the systolic and diastolic blood pressure results were 152.8 and 78.5 mm/Hg, respectively. Conclusions: This study concludes that health of the brick field workers was highly affected due to working in unhealthy working conditions for a long period of time