62,956 research outputs found
Exploring wind-driving dust species in cool luminous giants I. Basic criteria and dynamical models of M-type AGB stars
This work is part of an ongoing effort aiming at identifying the actual
wind-drivers among the dust species observed in circumstellar envelopes. In
particular, we focus on the interplay between a strong stellar radiation field
and the dust formation process. To identify critical properties of potential
wind-driving dust species we use detailed radiation-hydrodynamical models which
include a parameterized dust description, complemented by simple analytical
estimates to help with the physical interpretation of the numerical results.
The adopted dust description is constructed to mimic different chemical and
optical dust properties in order to systematically study the effects of a
realistic radiation field on the second stage of the mass loss mechanism. We
see distinct trends in which combinations of optical and chemical dust
properties are needed to trigger an outflow. Dust species with a low
condensation temperature and a NIR absorption coefficient that decreases
strongly with wavelength will not condense close enough to the stellar surface
to be considered as potential wind-drivers. Our models confirm that metallic
iron and Fe-bearing silicates are not viable as wind-drivers due to their
near-infrared optical properties and resulting large condensation distances.
TiO2 is also excluded as a wind-driver due to the low abundance of Ti. Other
species, such a SiO2 and Al2O3, are less clear-cut cases due to uncertainties
in the optical and chemical data and further work is needed. A strong candidate
is Mg2SiO4 with grain sizes of 0.1-1 micron, where scattering contributes
significantly to the radiative acceleration, as suggested by earlier
theoretical work and supported by recent observations.Comment: 15 pages, 12 figure
A photon breeding mechanism for the high-energy emission of relativistic jets
We propose a straightforward and efficient mechanism for the high-energy
emission of relativistic astrophysical jets associated with an exchange of
interacting high-energy photons between the jet and the external environment.
Physical processes playing the main role in this mechanism are
electron-positron pair production by photons and the inverse Compton
scattering. This scenario has been studied analytically as well as with
numerical simulations demonstrating that a relativistic jet (with the Lorentz
factor larger than 3--4) moving through the sufficiently dense, soft radiation
field inevitably undergoes transformation into a luminous state. The process
has a supercritical character: the high-energy photons breed exponentially
being fed directly by the bulk kinetic energy of the jet. Eventually particles
feed back on the fluid dynamics and the jet partially decelerates. As a result,
a significant fraction (at least 20 per cent) of the jet kinetic energy is
converted into radiation mainly in the MeV -- GeV energy range. The mechanism
maybe responsible for the bulk of the emission of relativistic jets in active
galactic nuclei, microquasars and gamma-ray bursts.Comment: 10 pages, 9 figures; MNRAS, in pres
The rp Process Ashes from Stable Nuclear Burning on an Accreting Neutron Star
We calculate the nucleosynthesis during stable nuclear burning on an
accreting neutron star. This is appropriate for weakly magnetic neutron stars
accreting at near-Eddington rates in low mass X-ray binaries, and for most
accreting X-ray pulsars. We show that the nuclear burning proceeds via the
rapid proton capture process (rp process), and makes nuclei far beyond the iron
group. The final mixture of nuclei consists of elements with a range of masses
between approximately A=60 and A=100. The average nuclear mass of the ashes is
set by the extent of helium burning via (alpha,p) reactions, and depends on the
local accretion rate.
Our results imply that the crust of these accreting neutron stars is made
from a complex mixture of heavy nuclei, with important implications for its
thermal, electrical and structural properties. A crustal lattice as impure as
our results suggest will have a conductivity set mostly by impurity scattering,
allowing more rapid Ohmic diffusion of magnetic fields than previously
estimated.Comment: To appear in the Astrophysical Journal (33 pages, LaTeX, including 11
postscript figures
Development of a software tool for the evaluation of the shading factor under complex boundary conditions
12th Conference of International Building Performance Simulation Associatio
Cygnus X-3 in outburst : quenched radio emission, radiation losses and variable local opacity
We present multiwavelength observations of Cygnus X-3 during an extended
outburst in 1994 February - March. Intensive radio monitoring at 13.3, 3.6 &
2.0 cm is complemented by observations at (sub)millimetre and infrared
wavelengths, which find Cyg X-3 to be unusually bright and variable, and
include the first reported detection of the source at 0.45 mm. We report the
first confirmation of quenched radio emission prior to radio flaring
independent of observations at Green Bank. The observations reveal evidence for
wavelength-dependent radiation losses and gradually decreasing opacity in the
environment of the radio jet. We find that the radiation losses are likely to
be predominantly inverse Compton losses experienced by the radio-emitting
electrons in the strong radiation field of a luminous companion to the compact
object. We interpret the decreasing opacity during the flare sequence as
resulting from a decreasing proportion of thermal electrons entrained in the
jet, reflecting a decreasing density in the region of jet formation. We
present, drawing in part on the work of other authors, a model based upon
mass-transfer rate instability predicting gamma-ray, X-ray, infrared and radio
trends during a radio flaring sequence.Comment: LaTeX, 11 pages, 6 figures. Submitted to MNRA
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