8,143 research outputs found
Adenine Abundance in a Collapsing Molecular Cloud
A vital ingredient of DNA molecule named adenine may be produced by
successive addition of HCN during molecular cloud collapse and star formation.
We compute its abundance in a collapsing cloud as a function of the reaction
rate and show that in much of the circumstances the resulting amount may be
sufficient to contaminate planets, comets and meteorites. We introduce a
-parameter which may be used to study the abundance where radiative
association takes place.Comment: Six pages and one figure. Accepted for Publication in Indian Journal
of Physics (April 1, 2000 issue
Studies of dissipative standing shock waves around black holes
We investigate the dynamical structure of advective accretion flow around
stationary as well as rotating black holes. For a suitable choice of input
parameters, such as, accretion rate () and angular momentum
(), global accretion solution may include a shock wave. The post shock
flow is located at few tens of Schwarzchild radius and it is generally very hot
and dense. This successfully mimics the so called Compton cloud which is
believed to be responsible for emitting hard radiations. Due to the radiative
loss, a significant energy from the accreting matter is removed and the shock
moves forward towards the black hole in order to maintain the pressure balance
across it. We identify the effective area of the parameter space () which allows accretion flows to have some energy dissipation at
the shock . As the dissipation is increased, the parameter
space is reduced and finally disappears when the dissipation is reached its
critical value. The dissipation has a profound effect on the dynamics of
post-shock flow. By moving forward, an unstable shock whose oscillation causes
Quasi-Periodic Oscillations (QPOs) in the emitted radiation, will produce
oscillations of high frequency. Such an evolution of QPOs has been observed in
several black hole candidates during their outbursts.Comment: 13 pages, 5 figures, accepted by MNRA
Satellite observations of thought experiments close to a black hole
Since black holes are `black', methods of their identification must
necessarily be indirect. Due to very special boundary condition on the horizon,
the advective flow behaves in a particular way, which includes formation of
centrifugal pressure dominated boundary layer or CENBOL where much of the
infall energy is released and outflows are generated. The observational aspects
of black holes must depend on the steady and time-dependent properties of this
boundary layer. Several observational results are written down in this review
which seem to support the predictions of thought experiments based on this
advective accretion/outflow model. In future, when gravitational waves are
detected, some other predictions of this model could be tested as well.Comment: Published in Classical and Quantum Gravity, v. 17, No. 12, p. 2427,
200
Black Hole Accretion: From Quasars to Nano-Quasars
In this review we shall comment on a few recent findings which strengthen the
view that the black hole accretion has substantial amount of sub-Keplerian
component. The manifestation of this component is many fold. We discuss some of
them. A general outline of the complex structure that emerges from the
multitude of observations is presented. A detailed outline of what might be
going on in outburst sources is also discussed. The relationship amount the
spectral and timing properties can be best understood by this picture. We claim
that the sub-Keplerian advective disk paradigm is a complete package. Since
signatures of sub-Keplerian motion is already increasing in the literature, the
whole package must be correct.Comment: 7 Pages, 2 Figures, Proceeding of the 2nd Kolkata Conference on
"Observational Evidence for the Black Holes in the Universe", Published in
AIP, 200
Spectral Properties of Accretion Disks Around Black Holes II -- Sub-Keplerian Flows With and Without Shocks
Close to a black hole, the density of the sub-Keplerian accreting matter
becomes higher compared to a spherical flow due to the presence of a
centrifugal barrier independent of whether or not a standing shock actually
forms. This hot dense flow intercepts soft photons from a cold Keplerian disk
and reprocesses them to form high energy X-rays and gamma rays. We study the
spectral properties of various models of accretion disks where a Keplerian disk
on the equatorial plane may or may not be flanked by a sub-Keplerian disk and
the sub-Keplerian flow may or may not possess standing shocks. From comparison
with the spectra, we believe that the observed properties could be explained
better when both the components (Keplerian and sub-Keplerian) are
simultaneously present close to a black hole, even though the sub-Keplerian
halo component may have been produced out of the Keplerian disk itself at
larger radii. We are able to understand soft and hard states of black hole
candidates, properties of X-ray novae outbursts, and quasi-periodic
oscillations of black hole candidates using these two component models. We fit
spectra of X-ray novae GS1124-68 and GS2000+25 and satisfactorily reproduce the
light curves of these objects.Comment: 15 Latex pages plus 12 figures. Macros included. Astrophysical
Journal (In press
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