534 research outputs found
A Clumping Independent Diagnostic of Stellar Mass-loss Rates: Rapid Clump Destruction in Adiabatic Colliding Winds
Clumping in hot star winds can significantly affect estimates of mass-loss
rates, the inferred evolution of the star and the environmental impact of the
wind. A hydrodynamical simulation of a colliding winds binary (CWB) with clumpy
winds reveals that the clumps are rapidly destroyed after passing through the
confining shocks of the wind-wind collision region (WCR) for reasonable
parameters of the clumps if the flow in the WCR is adiabatic. Despite large
density and temperature fluctuations in the post-shock gas, the overall effect
of the interaction is to smooth the existing structure in the winds. Averaged
over the entire interaction region, the resulting X-ray emission is very
similar to that from the collision of smooth winds. The insensitivity of the
X-ray emission to clumping suggests it is an excellent diagnostic of the
stellar mass-loss rates in wide CWBs, and may prove to be a useful addition to
existing techniques for deriving mass-loss rates, many of which are extremely
sensitive to clumping. Clumpy winds also have implications for a variety of
phenomena at the WCR: particle acceleration may occur throughout the WCR due to
supersonic MHD turbulence, re-acceleration at multiple shocks, and
re-connection; a statistical description of the properties of the WCR may be
required for studies of non-equilibrium ionization and the rate of electron
heating; and the physical mixing of the two winds will be enhanced, as seems
necessary to trigger dust formation.Comment: 4 pages, 3 figures, accepted for publication in ApJ
Non-thermal X-ray and Gamma-ray Emission from the Colliding Wind Binary WR140
WR140 is the archetype long-period colliding wind binary (CWB) system, and is
well known for dramatic variations in its synchrotron emission during its
7.9-yr, highly eccentric orbit. This emission is thought to arise from
relativistic electrons accelerated at the global shocks bounding the
wind-collision region (WCR). The presence of non-thermal electrons and ions
should also give rise to X-ray and gamma-ray emission from several separate
mechanisms, including inverse-Compton cooling, relativistic bremsstrahlung, and
pion decay. We describe new calculations of this emission and make some
preliminary predictions for the new generation of gamma-ray observatories. We
determine that WR140 will likely require several Megaseconds of observation
before detection with INTEGRAL, but should be a reasonably strong source for
GLAST.Comment: 4 pages, 1 figure, contribution to "Massive Stars and High-Energy
Emission in OB Associations"; JENAM 2005, held in Liege (Belgium
Momentum and energy injection by a wind-blown bubble into an inhomogeneous interstellar medium
We investigate the effect of mass-loading from embedded clouds on the
evolution of wind-blown bubbles. We use 1D hydrodynamical calculations and
assume that the clouds are numerous enough that they can be treated in the
continuous limit, and that rapid mixing occurs so that the injected mass
quickly merges with the global flow. The destruction of embedded clouds adds
mass into the bubble, increasing its density. Mass-loading increases the
temperature of the unshocked stellar wind due to the frictional drag, and
reduces the temperature of the hot shocked gas as the available thermal energy
is shared between more particles. Mass-loading may increase or decrease the
volume-averaged bubble pressure. Mass-loaded bubbles are smaller, have less
retained energy and lower radial momentum, but in all cases examined are still
able to do significant work on the swept-up gas. In this latter respect,
the bubbles more closely resemble energy-conserving bubbles than the
momentum-conserving-like behaviour of ``quenched'' bubbles.Comment: 15 pages, 7 figures, accepted by MNRA
The dominant X-ray wind in massive star binaries
We investigate which shocked wind is responsible for the majority of the
X-ray emission in colliding wind binaries, an issue where there is some
confusion in the literature, and which we show is more complicated than has
been assumed. We find that where both winds rapidly cool (typically close
binaries), the ratio of the wind speeds is often more important than the
momentum ratio, because it controls the energy flux ratio, and the faster wind
is generally the dominant emitter. When both winds are largely adiabatic
(typically long-period binaries), the slower and denser wind will cool faster
and the stronger wind generally dominates the X-ray luminosity.Comment: 4 pages, 1 figure, accepted by A&A Letter
Mass-loaded spherical accretion flows
We have calculated the evolution of spherical accretion flows undergoing mass-loading from embedded clouds through either conduction or hydrodynamical ablation. We have observed the effect of varying the ratios of the mass-loading timescale and the cooling timescale to the ballistic crossing timescale through the mass-loading region.
We have also varied the ratio of the potential energy of a particle injected into the flow near the outer region of mass-loading to the temperature at which a minimum occurs in the cooling curve. The two types of mass-loading produce qualitatively different types of behaviour in the accretion flow, since mass-loading through conduction requires the ambient gas to be hot, whereas mass ablation from clumps occurs throughout the flow. Higher ratios of injected to accreted mass typically occur with hydrodynamical ablation, in agreement with previous work on wind-blown bubbles and supernova remnants. We find that mass-loading damps the radiative overstability of such flows, in agreement with our earlier work. If the mass-loading is high enough it can stabilize the accretion shock at a constant radius, yielding an almost isothermal subsonic post-shock flow. Such solutions may be relevant to cooling flows onto massive galaxies. Mass-loading can also lead to the formation of isolated shells of high temperature material, separated by gas at cooler temperatures
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