Lebanese Academy of Sciences    

Professor Edward M. Sion

Edward M. Sion was born in the USA as were his parents but
both sets of grandparents immigrated to the USA between 1904 and 1910 from Southern Lebanon. He earned the Ph.D in astrophysics from the University of Pennsylvania in Philadelphia. His doctoral dissertation in theoretical astrophysics was entitled "Multi-modal Non-adiabatic Radial Oscillations and Pulsational Stability of Hot Degenerate Dwarfs".

His research specialization is the formation, structure and evolution of white dwarf stars and cataclysmic variable stars, the physics of mass accretion onto white dwarfs in close interacting binary stars. He has authored or co-authored 200 peer-reviewed scientific publications in the top five highest impact journals in astrophysics (ApJ, AJ, A\&A, MNRAS, PASP), and 501 publications in total. Among his research contributions are the first evolutionary model sequences of massive white dwarfs undergoing accretion and hydrogen thermonuclear shell flashes (Sion et al. 1979, Astrophysical Journal, 220, 636), the cool white dwarf luminosity function, demonstrating a real deficit of cool white dwarfs and hence a young age for the galactic disk (Sion \& Liebert 1977, Astrophysical Journal, 213, 268), the basic spectroscopic classification system of white dwarf stars which is used worldwide (Sion et al. 1983, Astrophysical Journal, 269, 253) and the spectral evolution of cool white dwarfs
He showed empirically that the DA (hydrogen-rich) white dwarfs and non-DA (helium-rich) white dwarfs undergo spectral evolution when deepening helium convection, as a white dwarf cools, mixes hydrogen downward (Sion, E.M.1984, Astrophysical Journal, 282, 612).

With the Hubble telescope, he carried out extensive studies of the physical properties of exposed white dwarf stars in cataclysmic variables (Sion, E.M.1999, PASP, 111, 532) and the pioneering evolutionary calculations of compressional heating of white dwarfs undergoing time variable accretion and showed that long term compressional heating due to accretion explained the observed distribution of white dwarf temperatures in cataclysmic variables (Sion, E.M. 1995, Astrophysical Journal, 438, 876).

He is currently Professor of Astronomy and Astrophysics at Villanova University. He served for six years as an Associate Editor of the Astrophysical Journal, He has held sabbatical appointments at Arizona State University, Chercheur Associe, Centre Nationale de la Recherche Scientifique (CNRS), University of Toulouse (France), and the Hubble Space Telescope Science Institute. He is the recipient of dozens of research grants and spacecraft observing grants from the National Science Foundation and NASA and recipient of the Outstanding Faculty Research Award. He is the author of numerous encyclopedia articles, invited chapters and has edited or written five books. He has served on dozens of scientific organizing committees, editorial boards and advisory boards.

Sample of Academician's Research

Artistic image of a cataclysmic variable binary star in which a sun-like star transfers mass to a swirling disk of gas (accretion disk)
surrounding a white dwarf which is a compact star composed of electron-degenerate matter. The extremely dense white dwarf
contains the roughly mass of the sun crushed into a volume comparable to that of the Earth.

Among Prof. Sion's many contributions to the field of white dwarfs are his observational studies with the Hubble Space Telescope of how mass accretion affects the white dwarf's temperature, spin rate and chemical abundances and evolution and his theoretical studies of the
physics of the accretion process and the explosive phenomena on the white dwarf triggered by that process.













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Lebanese Academy of Sciences