985 research outputs found
Rapid neutron capture in supernova explosions
Rapid neutron capture in supernova explosion
Effects of Shocks on Emission from Central Engines of Active Galactic Nuclei. I
In this paper we show that perturbations of the accretion flow within the central engines of some active galactic nuclei (AGNS) are likely to form shock waves in the accreting plasma. Such shocks, which may be either collisional or collisionless, can contribute to the observed high-energy temporal and spectral variability. Our rationale is the following: Observations show that the continuum emission probably originates in an optically thin, hot plasma in the AGN central engine. The flux and spectrum from this hot plasma varies significantly over light crossing timescales. Several authors have suggested that macroscopic perturbations contained within this plasma are the sources of this variability. In order to produce the observed emission the perturbations must be radiatively coupled with the optically thin hot matter and must also move with high velocities. We suggest that shocks, which can be very effective in randomizing the bulk motion of the perturbations, are responsible for this coupling. Shocks should form in the central engine, because the temperatures and magnetic fields are probably reduced below their virial values by radiative dissipation. Perturbations moving at Keplerian speeds, or strong non-linear excitations, result in supersonic and super-Alfvenic velocities leading to shock waves within the hot plasma. We show that even a perturbation smaller than the emitting region can form a shock that significantly modifies the continuum emission in an AGN, and that the spectral and temporal variability from such a shock generally resembles those of radio-quiet AGNS. As an example, the shock inducing perturbation in our model is a small main-sequence star, the capturing and eventual accretion of which are known to be a plausible process. We argue that shocks in the central engine may also provide a natural triggering mechanism for the "cold" component of Guilbert & Rees two-phase medium and an efficient mecha- nism for angular momentum transfer. Current and future missions, such as ASCA, XTE, XMM, AXAF, and ASTRO-E may determine the importance of shock-related emission from the central engines of AGNS
R-Mode Oscillations in Rotating Magnetic Neutron Stars
We show that r-mode oscillations distort the magnetic fields of neutron stars
and that their occurrence is likely to be limited by this interaction. If the
field is gtrsim 10^{16} (Omega/Omega_B) G, where Omega and Omega_B are the
angular velocities of the star and at which mass shedding occurs, r-mode
oscillations cannot occur. Much weaker fields will prevent gravitational
radiation from exciting r-mode oscillations or damp them on a relatively short
timescale by extracting energy from the modes faster than gravitational wave
emission can pump energy into them. For example, a 10^{10} G poloidal magnetic
field that threads the star's superconducting core is likely to prevent the
ell=2 mode from being excited unless Omega exceeds 0.35 Omega_B. If Omega is
larger than 0.35 Omega_B initially, the ell=2 mode may be excited but is likely
to decay rapidly once Omega falls below 0.35 Omega_B, which happens in lesssim
15^d if the saturation amplitude is gtrsim 0.1. The r-mode oscillations may
play an important role in determining the structure of neutron star magnetic
fields.Comment: 4 pages, 1 postscript figure, uses emulateapj; submitted to ApJ
Letters 1999 Nov 8; accepted 2000 Jan 25; this version is essentially
identical to the original version except that Figure 2 was deleted in order
to fit within the ApJ Letters page limi
Evaporation of Lennard-Jones Fluids
Evaporation and condensation at a liquid/vapor interface are ubiquitous
interphase mass and energy transfer phenomena that are still not well
understood. We have carried out large scale molecular dynamics simulations of
Lennard-Jones (LJ) fluids composed of monomers, dimers, or trimers to
investigate these processes with molecular detail. For LJ monomers in contact
with a vacuum, the evaporation rate is found to be very high with significant
evaporative cooling and an accompanying density gradient in the liquid domain
near the liquid/vapor interface. Increasing the chain length to just dimers
significantly reduces the evaporation rate. We confirm that mechanical
equilibrium plays a key role in determining the evaporation rate and the
density and temperature profiles across the liquid/vapor interface. The
velocity distributions of evaporated molecules and the evaporation and
condensation coefficients are measured and compared to the predictions of an
existing model based on kinetic theory of gases. Our results indicate that for
both monatomic and polyatomic molecules, the evaporation and condensation
coefficients are equal when systems are not far from equilibrium and smaller
than one, and decrease with increasing temperature. For the same reduced
temperature , where is the critical temperature, these two
coefficients are higher for LJ dimers and trimers than for monomers, in
contrast to the traditional viewpoint that they are close to unity for
monatomic molecules and decrease for polyatomic molecules. Furthermore, data
for the two coefficients collapse onto a master curve when plotted against a
translational length ratio between the liquid and vapor phase.Comment: revised version, 15 pages, 15 figures, to appear in J. Chem. Phy
Medium Effects in Cooling of Neutron Stars and Neutron Gap
We study the dependence of the cooling of isolated neutron stars on the
magnitude of the neutron gap. It is demonstrated that our ``nuclear
medium cooling'' scenario is in favor of a suppressed value of the
neutron gap.Comment: 18 p., 12 figs., extended version of astro-ph/0501678, accepted in
A&
Thermal X-Ray Pulses Resulting From Pulsar Glitches
The non-spherically symmetric transport equations and exact thermal evolution
model are used to calculate the transient thermal response to pulsars. The
three possible ways of energy release originated from glitches, namely the
`shell', `ring' and `spot' cases are compared. The X-ray light curves resulting
from the thermal response to the glitches are calculated. Only the `spot' case
and the `ring' case are considered because the `shell' case does not produce
significant modulative X-rays. The magnetic field () effect, the
relativistic light bending effect and the rotational effect on the photons
being emitted in a finite region are considered. Various sets of parameters
result in different evolution patterns of light curves. We find that this
modulated thermal X-ray radiation resulting from glitches may provide some
useful constraints on glitch models.Comment: 48 pages, 20 figures, submitted to Ap
An accretion model for the growth of the central black hole associated with ionization instability in quasars
A possible accretion model associated with the ionization instability of
quasar disks is proposed to address the growth of the central black hole
harbored in the host galaxy.The mass ratio between black hole and its host
galactic bulge is a nature consequence of our model.Comment: submitted to ApJ, 15 page
Superfluid Friction and Late-time Thermal Evolution of Neutron Stars
The recent temperature measurements of the two older isolated neutron stars
PSR 1929+10 and PSR 0950+08 (ages of and yr,
respectively) indicate that these objects are heated. A promising candidate
heat source is friction between the neutron star crust and the superfluid it is
thought to contain. We study the effects of superfluid friction on the
long-term thermal and rotational evolution of a neutron star. Differential
rotation velocities between the superfluid and the crust (averaged over the
inner crust moment of inertia) of rad s for PSR
1929+10 and rad s for PSR 0950+08 would account for their
observed temperatures. These differential velocities could be sustained by
pinning of superfluid vortices to the inner crust lattice with strengths of
1 MeV per nucleus. Pinned vortices can creep outward through thermal
fluctuations or quantum tunneling. For thermally-activated creep, the coupling
between the superfluid and crust is highly sensitive to temperature. If pinning
maintains large differential rotation ( rad s), a feedback
instability could occur in stars younger than yr causing
oscillations of the temperature and spin-down rate over a period of . For stars older than yr, however, vortex creep occurs
through quantum tunneling, and the creep velocity is too insensitive to
temperature for a thermal-rotational instability to occur. These older stars
could be heated through a steady process of superfluid friction.Comment: 26 pages, 1 figure, submitted to Ap
Crystalline-Electric-Field Effect on the Resistivity of Ce-based Heavy Fermion Systems
The behavior of the resistivity of Ce-based heavy fermion systems is studied
using a 1/-expansion method a la Nagoya, where is the spin-orbital
degeneracy of f-electrons. The 1/-expansion is performed in terms of the
auxiliary particles, and a strict requirement of the local constraints is
fulfilled for each order of 1/N. The physical quantities can be calculated over
the entire temperature range by solving the coupled Dyson equations for the
Green functions self-consistently at each temperature. This 1/N-expansion
method is known to provide asymptotically exact results for the behavior of
physical quantities in both low- and high-energy regions when it is applied to
a single orbital periodic Anderson model (PAM). On the basis of a generalized
PAM including crystalline-electric-field splitting with a single conduction
band, the pressure dependence of the resistivity is calculated by
parameterizing the effect of pressure as the variation of the hybridization
parameter between the conduction electrons and f-electrons. The main result of
the present study is that the double-peak structure of the -dependence of
the resistivity is shown to merge into a single-peak structure with increasing
pressure.Comment: 37 pages, 22 figure
- …