723 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
3D Models of Radiatively Driven Colliding Winds In Massive O+O Star Binaries: I. Hydrodynamics
The dynamics of the wind-wind collision in massive stellar binaries is
investigated using three-dimensional hydrodynamical models which incorporate
gravity, the driving of the winds, the orbital motion of the stars, and
radiative cooling of the shocked plasma. In this first paper we restrict our
study to main-sequence O+O binaries. The nature of the wind-wind collision
region is highly dependent on the degree of cooling of the shocked plasma, and
the ratio of the flow timescale of the shocked plasma to the orbital timescale.
The pre-shock wind speeds are lower in close systems as the winds collide prior
to their acceleration to terminal speeds. Radiative inhibition may also reduce
the pre-shock wind speeds. Together, these effects can lead to rapid cooling of
the post-shock gas. Radiative inhibition is less important in wider systems,
where the winds are accelerated to higher speeds before they collide, and the
resulting collision region can be largely adiabatic. In systems with eccentric
orbits, cold gas formed during periastron passage can persist even at apastron,
before being ablated and mixed into its surroundings and/or accelerated out of
the system.Comment: 21 pages, 15 figures, accepted for publication in MNRA
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
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
Tails of the Unexpected: The Interaction of an Isothermal Shell with a Cloud
A new mechanism for the formation of cometary tails behind dense clouds or
globules is discussed. Numerical hydrodynamical models show that when a dense
shell of swept-up matter overruns a cloud, material in the shell is focussed
behind the cloud to form a tail. This mode of tail formation is completely
distinct from other methods, which involve either the removal of material from
the cloud, or shadowing from a strong, nearby source of ionization. This
mechanism is relevant to the cometary tails seen in planetary nebulae and to
the interaction of superbubble shells with dense clouds.Comment: 6 pages, 6 figures, accepted for publication in MNRAS letter
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