Hartigan's Research/Stellar Jets
Young stars are often surrounded by circumstellar accretion disks
that somehow redirect the infalling material into a highly collimated supersonic outflow.
The link between accretion and outflow seems to universal among a diverse collection of
objects, including compact objects, black holes, and active galactic nuclei, but we can
study this phenomenon in great detail around young stars because
unlike many other astrophysical jets, stellar jets radiate.
Hence, it is possible to observe how the emission line ratios and line
profiles (which can be several hundred km/s wide) vary in the flow, and thereby
understand the dominant physics in the interaction of a supersonic flow with its surroundings.
In addition, stellar jets provide a measure of the rate of mass-loss from young stars, and if we trace
the flow back toward the star we can observe how the wind varies with time and get some idea
how the star drives the outflow.
The HST image above illustrates some of the basic features of jets from young stars. An infrared
source located in the lower right corner of the picture is the young star responsible for the outflow.
The jet is moving out of the dark cloud in the direction of the Earth at an angle of about 45 degrees,
and extends about 0.1 pc. A large bow shock in the upper left corner denotes a position where fast
material in the jet overtakes slower material. Radiation is present from both the bow shock, which
accelerates the material ahead of the shock, and a `Mach disk' that decelerates the jet. Weaker shocks
occur along the jet, but these heat the gas enough to make it visible.
The 'wiggles' in the jet seem to come from slight changes in the direction of the ejected material.
My research in this field includes observations and numerical shock models of the
physical conditions such as temperature, electron density, and ionization throughout the flow.
It is possible to connect the observations directly with numerical simulations by
combining together multiple epochs of high-resolution images from the Hubble Space Telescope
into `movies' that show how stellar jets evolve. I've also led several
laboratory experiments where we use intense lasers to
create and control astrophysical analogs of strong shock fronts. These campaigns have
combined astronomical observations with numerical simulations and lab results to clarify
how magnetic fields behave in complex supersonic flows and to ascertain what happens
when strong shock waves intersect, as occurs frequently in astrophysical settings.
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