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Reports from PhysCon November 9, 2012

Posted by admin in : Astronomy and Astrophysics (ASTRO) , trackback Bookmark and Share

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Professor Mercedes Richards gives plenary talk, “The Incredible Tomography Imaging Technique”.

Tomography is the reconstruction of a multidimensional physical field from its integral projections in different directions.  Tomography can be used in astronomy for systems that are rotating, such as stars, binary systems, extrasolar planets and accretion disks.   Rotation of the system allows astronomers to see multiple angles of the system.  By observing light emissions from accretion disks and taking Doppler shift into account and other relativistic effects, it is possible to derive velocity profiles throughout the disk.   It then possible to reconstruct a velocity space “image” into an actual spatial image of the accretion disk.  Examples of systems include binary stars, and stars near black holes where the black hole is stripping off matter from the star.   Tomography can also be used to  study orbital mechanics.

Since the technique of Doppler tomography was introduced 18 years ago, it has been used to provide indirect images of accretion structures in close binaries which cannot be resolved spatially with the largest telescopes.   The more general technique of tomography has been used successfully in medicine, geophysics, archaeology, and oceanography to construct 3D images from 2D pictures or “slices” through the object collected at many positions around the object.  These slices or projections are represented by the Radon transform.   The 3D image is recovered through a summation process called back projection; and the overall image reconstruction procedure is known as tomography.  In astronomy, the technique can be readily applied to eclipsing binaries and rotating stars, which provide changing views of the system, and the process is called Doppler Tomography because the gas motions detected through Doppler shifts provide an image of the accretion flows in velocity coordinates.

Doppler tomography has been used successfully to produce images of accretion flows in a variety of interacting binaries including the cataclysmic variable stars and x-ray binaries.  It would not have been possible to create these images otherwise since these systems and too distant to be directly imaged.  The major highlights of Doppler tomography have been the discovery of spiral structures in accretion disks, and the comparison of gas stream tomography maps to hydrodynamics models.

Professor Mercedes Richards is in the Department of Astronomy and Astrophysics at Penn State University.  Originally from Jamaica, she attended the University of the West Indies where she graduated with special honors in physics.  She attended graduate school in Canada, earning her Ph.D. at the University of Toronto.   She was recently elected president of the Close Binary Stars commission of the International Astronomical Union.   At PSU she founded a 6-week summer research program for high school students.

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