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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Patrick Hartigan