Astrophysical flows, driven by the injection of mass, momentum and energy
from either individual stars or groups of stars occur over a vast range
of spatial and energy scales. On the smallest scales, hypersonic winds from
massive stars in binary systems collide and generate X-ray emission from the
very hot shocked gas. On larger scales, these winds interact with the local
circumstellar or interstellar medium (ISM) to produce wind-blown bubbles
(WBBs), and supernova ejecta interact with the local ISM or nuclear winds in
the central regions of active galaxies. On the largest scales, multiple
supernova events generate highly pressurized regions which power superwinds
that flow out of galaxies into intergalactic space.
Although these phenomena are astrophysically very distinct, there are
common features in the physics and hydrodynamics that make a broad approach
to their study feasible and appropriate. For example, many of the flows occur
in astrophysical media where cool clumps are embedded in a hotter substrate and
an understanding of mass injection from these clumps is essential. Theoretical
studies of these flows are accompanied by careful comparison with observational
data to verify the complex physical and hydrodynamical modelling used.
Similarly when considering collisions between winds we gain time on X-ray satellite
observatories to analyse gas at temperatures of ten to a hundred million degrees in
superwinds and colliding winds so these can be compared to theory. We also use
space-borne instruments to observe infrared emission in star forming regions.
Mass-loss from Massive Stars
A hydrodynamic simulation of colliding stellar winds.
The most massive stars are central to a host of important
astrophysical problems (e.g., the chemical evolution of the Galaxy,
and the birth of stellar mass black holes). A crucial parameter
critical to their evolution and their environmental impact is the rate
at which they lose mass through their powerful winds and outbursts.
The wind-wind collision in interacting binary stars is a
powerful probe of the mass-loss from the most massive stars. This is
illustrated in the article on Eta Carinae in the "See also..."
section.
Galactic Superwinds
The galactic superwind in NGC 3079.
On large scales, superwinds are created when supernova remnants (SNRs) within
star-forming regions overlap to create highly pressurized superbubbles which
burst out into intergalactic space. Superwinds are important re-distributors
of heavy elements, mass, and momentum, and their role in enriching and heating
the intergalactic medium is attracting much attention.
There is currently great debate on the physical origin of their X-ray emission.
We are addressing this issue using a novel approach based on the ranges of
SNRs evolving in an inhomogeneous medium. Such models should resolve some of the
outstanding issues and will aid the use of X-ray observations as diagnostics of
starburst superwinds.
Shocks
Shock physics becomes relevant in many areas of astronomy; work done on the
overstability of low Mach number shocks is relevant to the evolution of SNRs
interacting with the hot phase of the ISM (e.g., in starbursts), and to shocks in
dense molecular clouds. An important finding is that the stability properties of
low Mach number shocks are dramatically altered if the shocked gas is able to cool
below the temperature of the pre-shock gas. In such circumstances, low Mach number
shocks behave in many ways as if they had higher Mach numbers.
Damping of an overstable shock, see more at the original location on Prof Pittard's personal page here.
Suggested Reading
Blowing Bubbles in the Cosmos
Astronomical Winds, Jets, and Explosions
By T.W. Hartquist, J.E. Dyson, and D.P. Ruffle
Published January 2004, 208 pages
Oxford University Press
Blowing Bubbles in the Cosmos
Astronomical Winds, Jets, and Explosions
By T.W. Hartquist, J.E. Dyson, and D.P. Ruffle
Published January 2004, 208 pages
Oxford University Press